EP0990189A2

EP0990189A2 - Chromaticity adaptation

Info

Publication number: EP0990189A2
Application number: EP96938447A
Authority: EP
Inventors: Dan Inbar
Original assignee: Smartlight Ltd
Current assignee: Smartlight Ltd
Priority date: 1995-11-24
Filing date: 1996-11-24
Publication date: 2000-04-05
Also published as: EP0990189A4

Abstract

A transparency viewing device including a faceplate adapted for mounting at least one transparency having a particular density at a portion thereof, a source of back illumination for the transparency and a controller which varies the chromaticity of the back illumination based on the particular density. Preferably, the controller varies the chromaticity based on the chromaticity of the transparency. Additionally or alternatively, the controller locally varies the chromaticity.

Description

CHROMATICITY ADAPTATION FIELD OF THE INVENTION

This application relates to viewboxes for viewing x-ray transparencies and in particular to modifying the chromaticity of back-illumination for these viewboxes. BACKGROUND OF THE INVENTION

This application relates to viewboxes for viewing x-ray transparencies and in particular to a class of automated, self-masking viewboxes such as described in a previously filed series of applications for improved display devices for transparencies These applications, the disclosures, claims, annexes, appendices and drawings of which, if any, are incorporated herein by reference are PCT application PCT/EP91/00065 published as WO91/10152, PCT application PCT/EP91/01310 published as WO93/01564, US patent application serial number 07/862,982 titled "Transparency Viewing Apparatus", filed on April 6, 1992, US patent application serial number 08/348,959, titled "Improved Display Device", filed on November 28, 1994, PCT application PCT EP94/03968 published as WO95/14949, PCT application PCT/EP94/03971 published as WO95/14950, PCT application PCT/EP94/04228, published as WO95/16934, PCT application PCT/IL96/00026, titled "Backprojection Viewing System", filed on June 20, 1996, by SmartLight LTD , PCT application PCT/IL96/00023, filed on June 20, 1996. by SmartLight LTD , Israel patent application 1 16,252, titled "Dedicated Mammography Viewer", filed on December 4, 1995 by SmartLight LTD , U S provisional application 60/008,360 titled "Display Device", filed on December 7, 1995, by Dan Inbar, PCT application PCT/EP95/04693 published as WO96/17269 and U S Provisional application number 60/007,522, filed on November 24, 1995, by Dan Inbar et al The PCT applications name, inter alia, the United States of America as a designated state

In the abovementioned patent applications, which were incorporated herein by reference, four major factors in the optimization of viewing conditions are disclosed These are

1) Masking of those areas of the display surface which are not covered by the transparency, or under some embodiments of the device, are not included in an area of the film which is of interest This includes reduction or substantial elimination of lighting of areas which cause glare or which are distracting for other reasons such as their textural or lighting similarities to the region of interest This is referred to as contextual glare

2) Optimization of the intensity of lighting in the portion of the film which is being viewed to provide optimal contrast and spatial resolution by the viewer 3) Optimization of the intensity of ambient lighting to overcome the reduced acuity of the eye under low lighting conditions, where those conditions would otherwise apply

4) Correction of the color changes which are introduced into the back illumination by the liquid crystal devices used in the masking and by the film itself so that the color of the lighting is optimal or near optimal for visual acuity One exemplary method of correction incorporates a camera for viewing the film and determining the color of the light emanating from film and a controller for adjusting the intensity of different lighting sources for adjusting the chromaticity of the back illumination. One way of correcting the chromaticity described in these applications is using specially tinted lamps to adjust the chromaticity of the back-illumination SmartLight LTD of Israel (the assignee of the present application) showed, at a 1995

RSNA conference, on November 27, 1995, a prototype viewbox in which the chromaticity of the back illumination is changed from blue-white to yellow-white when a greater back illumination intensity is used In this prototype viewbox a blue lamp (which corrected for the chromaticity of the LC) was normally used When a higher back illumination intensity was required, additional, white lights could be turned on

WO96/ 17269, cited above, describes a two-intensity viewbox wherein low intensity lighting is provided using a white light source and high intensity lighting is provided by a high efficiency light source which may have a higher color temperature than the low-intensity light source The suggested high intensity light source has a color temperature of 6000 degrees before passing through the LCA and a color temperature of 4000 degrees Kelvin after passing through the LCA A color temperature of 4000 degrees Kelvin is suitable for day vision and not for night vision, as will be described with greater detail below

In many viewboxes known in the art, high density areas may be illuminated using a local spot light Such a spot light is usually white or yellow. It should be appreciated that, warm light is light which has a substantial amount of yellow and red, while cool light has a substantial amount of blue. The color temperature of cool light is higher than the color temperature of warm light. SUMMARY OF THE INVENTION

It is an object of some aspects of the present invention to provide optimal viewing conditions for viewing transparencies on a viewbox Preferably, the viewing conditions are automatically optimized with a minimal requirement of operator intervention

One problem with transparency viewing using a back-illuminated viewbox is that the chromaticity ofthe back-illumination is affected by the chromaticity ofthe transparency

In one preferred embodiment of the invention, the viewbox adjusts the back-illumination to compensate for the chromatic absorbency of the transparency In a preferred embodiment of the invention, the transparency type is inputted to the viewbox by the user and the viewbox calculates the chromaticity correction based on a chromaticity correction table which is stored in a memory Preferably, such a table includes entries for different types of film and/or for various back-illumination and/or masking conditions Preferably, the table may include signals sets for driving the back-illumination under various viewing situations Alternatively, illumination spectra for the various viewing situations may be stored

Alternatively, the viewbox may read the film type from the transparency itself, for example using a bar code reader Alternatively, the chromaticity of the transparency itself may be determined using a color sensor Alternatively, the user may input directly the desired chromaticity correction, such as using a (manual) table of values for different film types In a preferred embodiment of the invention, especially where the film chromaticity is dependent on the film density, the viewbox determines the density of at least a portion of the transparency in order to calculate the desired chromaticity compensation

In a preferred embodiment of the invention, a relatively precise back-illumination chromaticity may be achieved by using a large number of lamps for the back-illumination Each of the lamps has a narrow spectral range, such as red, green or blue When a precise illumination chromaticity is desired, a number of lamps of each spectral range are lighted, so that a relatively exact chromaticity is obtained. Preferably, the relative intensity of each lamp is also controlled to fine tune the chromaticity

In an alternator type viewbox, reading data from the transparency and/or determining transparency chromaticity and/or determining film density is preferably performed while the transparency is being transported from a storage portion of the viewbox to a display portion of the viewbox A transparency, in accordance with another preferred embodiment of the invention, includes a viewbox readable indication of the film type and/or chromaticity Preferably, the indication is a bar-code type indication for ease of reading

In a further preferred embodiment of the invention, a bluer back-illumination is used for transparency portions which are denser and a redder back-illumination is used for transparency portions which are clearer This blue-shifting compensates to some extent for the reduction in visual acuity caused by the low-levels of light which actually pass through the denser portions Preferably, different portions of a single transparency are back-illuminated to transmit light with different chromaticities Preferably, these different portions are illuminated simultaneously with different color light

Preferably, most of the back-illumination is provided by lamps with a bluer output spectrum, with the balance being provided by warmer colored lamps Thus, for clear portions of the film, only a few, warm colored lamps are lighted, while for dense portions o the film, all the lamps are lighted, so that the back-illumination is cooler (blue-shifted) One case where very high film optical densities are encountered is in viewing the skin layer of a breast in a mammography image The skin layer is characterized by very high optical density values, even compared to the rest of the breast, normally with a two order of magnitude difference in density between the skin layer and the rest of the breast

In a preferred embodiment of the invention, the image of a breast in a mammogram is back-illuminated using a masking pattern which back-illuminates only the skin layer of the breast Preferably, the masking pattern has a varying density, so that portions of the skin layer with a higher density are back-illuminated with more intensity than portions with a lower density

Another preferred embodiment of the invention relates to controlling the pupil size of an observer by varying the chromaticity of ambient light In a preferred embodiment of the invention, a blue-tinted ambient light is used so that the operator's pupil decreases in diameter and a better visual acuity is provided, such as required when viewing high detail areas of the transparency When higher light sensitivity is desired, a pink-tinted ambient light is used Preferably, the exact tint used is personalized for the individual operator It should be appreciated that when the pupil size decreases the visual acuity increases because the effect of aberrations in the eye is reduced The decrease in aberrations more than compensates for the reduction in light reaching the retina of the eye Below some pupil size, the increase in acuity is reversed, since at very small pupil sizes not enough light reaches the retina of the eye Thus, an optimal pupil size can be achieved for a particular operator and/or lighting condition Alternatively or additionally, the viewbox automatically chooses the ambient color tint based on the gaze direction of the operator and in particular on the objects of interest in the field of view ofthe gaze direction. It should be appreciated that in radiology what is important is the discernability of lesions and not just acuity, thus, contrast sensitivity is also an important factor in determining the optimal pupil size

There is therefore provided in accordance with a preferred embodiment of the invention, a transparency viewing device including a faceplate adapted for mounting at least one transparency having a particular average density at at least a portion thereof, a source of back illumination for the transparency, and a controller which varies the chromaticity of the back illumination based on the particular average density Preferably, the controller varies the chromaticity based on the chromaticity of the transparency

There is also provided in accordance with a preferred embodiment of the invention a transparency viewing device including a faceplate adapted for mounting at least one transparency having a particular chromaticity thereon, a source of back-illumination for the transparency, and a controller which varies the chromaticity of the back-illumination based on the particular chromaticity of the transparency, where the viewbox does not directly measure the chromaticity of light transmitted through the transparency Preferably, the controller receives an indication of the particular chromaticity from an operator Alternatively, the controller reads an indication of the chromaticity of the transparency from the film Preferably, the indication includes a film type

Alternatively or additionally, the controller varies the chromaticity based on a particular density of at least a portion ofthe transparency In a preferred embodiment of the invention, the controller locally varies the back- illumination chromaticity responsive to local characteristics of the transparency Preferably, different back-illumination chromaticity is provided at at least two different locations In a preferred embodiment of the invention, the controller provides higher color temperature back-illumination at locations of film having higher densities to compensate for a limited ability of the back-illumination to provide sufficient illumination for the higher density locations. Alternatively or additionally, the device includes a masking pattern generator which affects the chromaticity of the back-illumination Preferably, the masking pattern generator includes an LC array Alternatively or additionally, the controller varies the back-illumination based on the effect of the masking pattern generator on the back-illumination chromaticity

Alternatively or additionally, the controller includes a chromaticity/type memory, which memory stores information regarding chromaticity corrections for a plurality of transparency types

Alternatively or additionally, the controller includes a chromaticity/mask memory_', which memory stores information regarding chromaticity corrections for a plurality of mask patterns

Alternatively or additionally, the controller includes a chromaticity/back-illumination memory, which memory stores information regarding chromaticity corrections for a plurality of back-illumination conditions

Alternatively or additionally, the controller includes a chromaticity/density memory, which memory stores information regarding chromaticity corrections for a plurality of transparency densities Alternatively or additionally, the back-illumination source includes a plurality of narrow spectrum lamps, where the controller lights selected ones of said plurality of lamps to generate back-illumination of a desired chromaticity

Alternatively or additionally, the back-illumination source includes at least one high color temperature lamp which provides most of the intensity of said back-illumination and at least one low color temperature lamp which provides a small portion of the intensity of said back-illumination

There is further provided in accordance with a preferred embodiment of the invention, a transparency including a film base, and a computer readable marking on the film indicative ofthe film chromaticity

Preferably, the indication includes a film type Alternatively or additionally, the indication includes a bar-code indication There is also provided in accordance with a preferred embodiment of the invention, a transparency viewing device including a faceplate adapted for mounting at least one transparency thereon; a source of back illumination for the transparency, including a plurality of narrow spectrum lamps of a plurality of spectral bands, and a controller which lights selected ones of the plurality of lamps to generate back- illumination ofa desired chromaticity

Preferably, the plurality of spectral bands includes at least three spectral bands Additionally or alternatively, the plurality of lamps comprises at least 5, more preferably, at least 10 and most preferably, at least 20 lamps

There is also provided in accordance with a preferred embodiment of the invention, a transparency viewing device including a faceplate adapted for mounting at least one transparency thereon, a source of back illumination for the transparency, a source of ambient lighting having a variable hue, and a controller which controls the hue ofthe ambient lighting

Preferably, the controller controls the hue of the ambient light responsive to a characteristic of at least a portion of the transparency Preferably, the characteristic includes the density of the portion of the transparency Alternatively or additionally, the characteristic includes the level of detail of the portion of the transparency

There is also provided in accordance with a preferred embodiment of the mvention, a method of controlling a pupil size of an eye of an operator viewing a transparency on a viewbox, including illuminating the eye with light of a controllable hue, and controlling the hue ofthe light to achieve a desired pupil size

Preferably, pink hued light is used to increase the size of the pupil Alternatively, blue hued light is used to decrease the size of the pupil Preferably, the method includes determining a direction of gaze of the eye and controlling the pupil size includes controlling the pupil size responsive to the direction of gaze Preferably, the hued illumination ofthe eye is achieved using ambient lighting, not direct illumination through a faceplate on which the transparency is usually placed

There is further provided in accordance with another preferred embodiment of the invention a method of scanning a mammogram, including back-illuminating only a portion of a mammogram corresponding to an outer layer of a breast, and subsequently back-illuminating only a second outer layer portion of the breast Preferably, the method includes serially scanning the entire outer layer of the breast in the mammogram

Alternatively or additionally, back-illuminating includes graded back-illuminating, such that denser image parts of the portion receive greater amounts of back-illumination than less dense parts thereof

Preferably, the method includes detecting the outer layer of a breast in a mammogram Preferably, detecting includes detecting the outer layer based on a density analysis of the mammogram Alternatively or additionally, detecting includes detecting the outer layer based on a textural analysis of the mammogram Alternatively or additionally, detecting includes detecting the outer layer based on feature recognition of the breast orientation and location

There is also provided in accordance with a preferred embodiment of the invention a method of automatic scanning of an x-ray transparency of soft tissue disposed near a bone, including masking a low density portion ofthe transparency corresponding to the bone, sequentially back-illuminating at least one substantially only soft tissue portion on both side of the masked bone There is provided in accordance with another preferred embodiment of the invention a method of viewing a transparency including determining at least one characteristic of at least a portion of the transparency, without using color sensing of light passing through the transparency, and back-illuminating the portion with light, where the chromaticity of the back-illumination is dependent on the determined characteristic

Preferably, the at least one characteristic includes chromaticity Preferably, determining includes receiving an input from a user Alternatively, determining includes reading a marking indicating the chromaticity from the transparency Additionally or alternatively, the at least one characteristic includes density In a preferred embodiment of the invention, a second portion of the transparency is simultaneously back-illuminated with light having a different chromaticity from light back- illuminating the first portion Preferably, light transmitted through the first portion has a different chromaticity from light transmitted through the second portion Alternatively or additionally, the chromaticity of the back-illumination of a dense portion of the transparency is ofa high color temperature to at least partially compensate for reduced visual acuity due to low levels of light which are transmitted through the dense portion

There is also provided in accordance with another preferred embodiment of the invention a method of transparency viewing including. back-illuminating a first portion of a transparency to transmit light having a first chromaticity and a first intensity, and back-illuminating a second portion of the transparency to transmit light having a second chromaticity and a second intensity, lower than the first intensity, where the second chromaticity has a higher color temperature than the first chromaticity

BRIEF DESCRIPTION OF THE FIGURES The present invention will be more clearly understood from the following detailed description ofthe invention and from the attached drawings in which

Fig 1 A is a graph showing the relative sensitivity of a human eye at various wavelengths of light at a first lighting condition,

Fig IB is a graph showing the relative sensitivity of a human eye at various wavelengths of light at a second lighting condition,

Fig 2 illustrates the use of different wavelengths of light to back illuminate different portions of a single image, Fig 3 is a partial cut-away drawing of a viewbox in accordance with a preferred embodiment ofthe invention, and

Fig 4 shows a transparency and an ROI thereon in accordance with a scanning method ofthe present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One aspect of the present invention relates to modifying the chromaticity of the back- illumination of a viewbox responsive to the film density. Various studies have shown that for proper viewing of x-ray transparencies, illumination having about 100 nits emanating from the area of interest, provides optimal reading acuity. To achieve an optimal chromaticity at 100 nits the chromaticity of the back-illumination source must generally compensate for wave-length dependent absorbency of the viewbox and/or of the film. It should be understood that it may not be possible, in a practical device to provide both optimum back illumination intensity and chromaticity for all image-carrying film types and densities One particular case where such optimization is not possible arises when viewing very dense portions of film. If such an area has a density of 3 (which is not unusual) the back illumination intensity required for proper viewing is on the order of 100,000 nits, which light intensity is not currently obtainable at a reasonable cost and complexity.

The following papers, the disclosures of which are incorporated herein by reference, describe recent psycho-physical findings which show that at decreased light levels the eye becomes progressively more "blue-sensitive": "Luminance Controlled Pupil Size Affects Word Reading Accuracy", IESNA 1995 conference paper #25, by S M. Berman, G Fein, D. L Jewett, B. Benson, T. Law and A. Myers, pp. 542-554; "Lighting Spectral Effect on Landolt C Performance is Abolished by Mydriasis", IESNA 1995 conference paper #24, by S.M. Berman, G. Fein, D. L. Jewett, M. A. Bullimore, B Benson, T. Law and A. Myers, pp 529-541 , "Energy Efficiency Consequences of Scoptic Sensitivity", by S M. Berman, IESNA 1991, which also appears in Journal of the Illuminating Engineering Society, Winter 1992, pp 3-14; and "Visual Functions and Visual Performance in Mesopic Lighting Levels", by W Adrian, IES annual conference 95, NY, USA, paper #70, pp. 35-44.

The human visual system has two types of photo-detectors, rods and cones. Rods are more sensitive to blue wavelengths (prominent in high color temperature white), while cones are more sensitive to yellow-green wavelengths (prominent in low color temperature white) "Imaging Systems for Medical Diagnostics," edited by Erich Krestel and published by Siemens Aktiengesellschaft, the disclosure of which is incorporated herein by reference, shows on page 36 a graph of the relative threshold intensities of rods and cone to monochromatic illumination at different wavelengths. The studies described in the above cited papers found that for intermediate illumination, both rods and cones are used by the eye for visual tasks Even at typical office lighting levels, the eye is more sensitive to blue-tinted light than to yellow or white light. It is therefore suggested in these papers that, at "office" light levels, bluish light is more efficient than white light and that by using blue-tinted lights lower light intensities (and less energy) can be used for similar visual tasks Alternatively, by blue shifting the light sources, increased visual acuity (and hence productivity) can be achieved in visual tasks, without increasing the illumination levels (or energy)

Fig IA is a graph showing the relative sensitivity of a human eye to various frequencies at a first, high intensity, lighting condition and Fig IB is a graph showing the relative sensitivity of a human eye to various frequencies at a second, low intensity, lighting condition The shift of the maximum visual sensitivity from a long wavelength to a short wavelength is clear from a comparison of the two figures

Fig 2 illustrates the use of different wavelengths of light to back illuminate different portions of a single image A lung portion of the image of Fig 2 is very dense, so blue-shifted (cooler) light may be required to achieve a minimal visual acuity, while a spine portion of the image is not very dense, so only low intensity back-illumination levels are required to achieve light levels where redder (warmer) light is more suitable for higher visual acuity

Fig 3 is a partial cut-through drawing of a viewbox 42 in accordance with a preferred embodiment of the invention Viewbox 42 includes a plurality of back illumination sources 4, a display surface 8 which is back illuminated by light sources 4 and a masking device, preferably at least one LC layer 6, which masks the light from light sources 4 to a transparency 10 placed on display surface 8 In a typical application, LC 6 is controlled to mask out light from sources 4 to most of display surface 8 and, preferably from at least a part of transparency 10 so that only a region of interest (ROI) of transparency 10 is back-illuminated In a preferred embodiment of the invention, the chromaticity of the back illumination is controlled based on the intensity of light emanating from the ROI

In one preferred embodiment of the invention, a sensor 44 senses the intensity of light emanating from transparency 10 and a controller 45 controls the chromaticity of the back- illumination in response to this measured intensity Alternatively, for example in an alternator where transparencies are provided from a storage, the intensity is determined using a scanner which scans transparency 10 during its conveyance from such a storage to display surface 8 Alternatively, the chromaticity is manually changed by an operator based on his perception of the intensity of the lighting It is to be appreciated that the precise wavelength of the illumination is dependent not only on psycho-physical consideration but also on subjective considerations such a color with which the operator feels comfortable Normally, these colors are tinted off-white colors, high color temperature white (which is bluish) or low color temperature white (which is red-green) In one preferred embodiment of the invention, variable amounts of blue-wavelength back-illumination may be provided by using at least one blue light source 5 in addition to light sources 4. The light from light sources 4 and 5 is preferably well mixed prior to reaching display surface 8 Methods of mixing light from a multiplicity of light sources is described in the above referenced applications, in particular, WO96/17269 Alternatively or additionally, a local blue light source, such as a white-blue spot light may be used Light source 5 may be provided with a dimmer to fine tune the amount of blue (or white-blue) light added to the back illumination Alternatively or additionally, several blue lamps 5 may be provided and the amount of blue controlled by illuminating only some of the lamps Further alternatively, a blue filter with a varying density may be provided to change the chromaticity of the back illumination Further alternatively or additionally, variable wave-length light sources, as well known in the art, may be used

In another preferred embodiment of the invention, the chromaticity of the back illumination of an entire transparency is determined based on the average light intensity emanating from the transparency or a portion thereof Alternatively, it is based on the average density of the transparency or a portion thereof and on the intensity of the back illumination

In another preferred embodiment of the invention, different portions of transparency 10 are simultaneously back illuminated with light having different chromaticities Referring back to Fig. 2, the lungs are preferably back-illuminated with light of a shorter wavelength, while the spine is preferably back illuminated with light of a longer wavelength To achieve this type of back-illumination, a backprojection system using an active matrix LCA is preferred, as described in the above PCT application PCT/IL96/00023 The local chromaticity of the back-illumination may also be modified by using local tinted lamps or by using a color LCD

As may be appreciated, the precise desired chromaticity of the back illumination may be dependent on the chromaticity and intensity of the light sources and on the chromaticity and density of transparency 10, LC 6 and display surface 8 If viewbox 42 is physically constrained from achieving the desired chromaticity, such as by the availability of blue light when viewing dense film, a tradeoff between back-illumination intensity and back-illumination chromaticity, such that visual acuity is maximized, may be used It should be appreciated that the term visual acuity, as used herein, includes discernability of lesions on x-ray images.

In one preferred embodiment of the invention, light sources 4 and 5 are chosen to include mainly higher color temperature light sources (which provide a high intensity of light) and some lower color temperature light sources (which provide a lower intensity of light) Fluorescent light sources, which are generally preferred for viewboxes, emit light having three spectral peaks, red, green and blue The color temperature of the light is determined by the relative intensities of the peaks High color temperature light has higher blue peaks, while lower color temperature light has higher red peaks Since the requirement for high intensity back illumination is correlated with the requirement for high color temperature back illumination, it is preferred that most of the potential light output of the viewbox be of a high color temperature (preferably, after correction for the chromaticity of LC 6 and transparency 10) The balance of light output is preferably low color temperature back illumination This selection of light sources is more energy and space efficient than a half-and-half mix When high light intensities are required for viewing of high density images, all the light sources (or at least the high color temperature ones) are used, resulting in a high intensity blue-shifted back-illumination When low light intensities are required, only the low color temperature light sources (and optionally a few of the high color temperature ones, for color balance) need be turned on It should be appreciated that when viewing a low density image, acceptable illumination levels may be obtained by the viewbox without requiring back-illumination levels which are higher than the viewbox can provide Thus, blue-shifting may not be required In preferred embodiment of the invention, low color temperature lighting corresponds to a color temperature of between 4,000 and 5,000 Kelvin, and high color temperature lighting corresponds to a color temperature of between 7,000 and 14,000 Kelvin, preferably, 12,000 Kelvin It should be appreciated that the color temperature is preferably measured after the light exits the transparency, so that the back-illumination chromaticity should also compensate for absorption by the viewbox and the transparency

In another preferred embodiment of the invention a larger number of different lighting sources than described in the above mentioned applications is used In particular, fluorescent or metal halide light sources having peaks at red, blue and green are used and controller 45 can then give a color very close to the desired value by selectively activating certain numbers of lamps at each of the above three narrow spectral bands It should be appreciated that good mixing of the light from the different lamps is desirable and may be achieved as described in the above cited applications, including WO96/ 17269.

In one embodiment of the invention, each lamp has a single narrow spectral peak In another preferred embodiment of the invention, each lamp has more than one narrow spectral peak However, a small number of peaks per lamp is preferred so that it is simpler to determine which combination of lamps will yield the desired back-illumination chromaticity Preferably, at least 5 narrow-band lamps are used, more preferably, more than 10 lamps are used, most preferably, more than 20 lamps are used Using a large number of lamps at at least three different spectral bands, makes it possible to adapt the chromaticity of the back-illumination to a variety of viewing situations, including, different LCA types, different film types and failure and age of the lamps In other embodiments of the invention, when fewer colored sources are used, a less exact overall chromaticity is achieved

It should be understood that it may not be possible, in a practical device, to provide both optimum back-illumination intensity and chromaticity for all film types and intensities Under these circumstances, in a preferred embodiment of the invention, the intensity is optimized and the closest chromaticity to the desired chromaticity possible at the optimum intensity is provided

It should also be noted that the amount of correction for LC selective absorbency of light may be variable For example, if the LC is controlled to have gray-scale attenuation, the ratio between transmission of blue and red shades of light may be dependent on the LC density This variance may apply to other types of light valves and also in certain types of light recycling schemes, such as when a PDLCA (polymer dispersed LCA) with chromaticity dependent scattering is used for light recycling A variable chromaticity correction may be calculated from known characteristics ofthe LC or by using a color sensor The above cited paper entitled "Lighting Spectral Effect on Landolt C Performance is

Abolished by Mydriasis" describes a further effect of the chromaticity of the illumination The chromaticity of background (ambient) illumination affects the pupil size of an eye. even if this ambient light is not directly used for a visual task In a preferred embodiment of the invention, an ambient light source 46 is controlled in a manner to affect the pupil size of an eye of an operator using viewbox 42. Since blue hues tend to make the pupil smaller, which increases the resolution sensitivity of the eye, while red hues tend to make the pupil larger, which increases the light sensitivity of the eye, a sensitivity/resolution sensitivity tradeoff can be achieved by varying the hue of light source 46 In determining the optimal pupil size, it should be appreciated that aberrations in the lens of the eye are a main cause of lesser acuity at large pupil sizes. Reducing the pupil size reduces the effect of these aberrations, however it also reduces the amount of light which reaches the eye's retina It has, however, been shown, in an experiment described in the above article, that the reduction in aberrations more than compensates for the reduction in illumination of the retina, up to a certain point Since absolute sensitivity to light is not usually an issue in radiology, an optimal pupil size for an operator and/or viewing situation is dependent mainly on the amount of light reaching the eye and on the amount of aberrations in the lens ofthe eye ofthe operator

In a preferred embodiment of the invention, the ambient illumination is adjusted for each operator manually, by the operator selecting the ambient lighting which provides him with the greatest ability to discern lesions in an x-ray transparency Alternatively, it can be adjusted automatically, using a camera, or other sensor, as known in the art, which senses the operator's pupil size and/or gaze direction Automatic adjustment may be based on at least one preset value determined manually by the operator A gaze-direction sensor may also require a position determining system, which determines the position and orientation ofthe operator relative to the viewbox

In another preferred embodiment of the invention, ambient lighting chromaticity and/or intensity and back illumination chromaticity and/or intensity are controlled based on pupil size and/or direction ofthe gaze of the operator One situation where viewing of very high density transparencies is clinically important is in mammography In mammography images, the texture and thickness of a skin layer of a breast may yield important clues regarding a carcinoma in the breast

Fig 4 shows a transparency 62 and an ROI 66 thereon in accordance with a scanning method of the present invention In this method, ROI 66 scans along a skin line 68 of a breast 69 (i e , the edge of the breast) in transparency 62 Preferably, this scanning is performed automatically Further preferably, at least two images 62 (either of both breasts and/or of a current and a previous study) are scanned simultaneously Thus, a physician may quickly and easily compare image portions of two breasts (which corresponding portions should look the same) or compare a current mammography to a previous study, to determine time-related changes Alternatively, image 62 is masked so that only skin 68 is back-illuminated (i e , the ROI is not rectangular).

Skin 68 may be automatically detected in several ways One method is based on the uniform configuration of breasts in mammography In this method the breast is assumed to have a generally triangular configuration, with the skin forming the outside of the triangle The skin area may be differentiated from inner portions 70 of the breast based on the density of the skin Alternatively, this differentiation may be based on texture, since inner portions 70 usually have small specks 72, which are shadows of glandular tissue In skin scanning, ROI 66 is preferably not uniform in back-illumination, since even in skin areas a range of an order of magnitude of densities may be expected Rather, a soft-mask, as described in WO96/ 17269, which does not have a sharp border between back-illuminated portions and dark portions, may be used In a preferred soft-mask, the intensity of back-illumination within the ROI is lower where the density of the underlying image is lower and is higher where the density of the underlying image is higher This type of graded masking is preferably achieved using a backprojection system with an active matrix LCA

Skin scanning is generally useful in cases where the image is symmetric, such as in the breast, so that two (supposedly equal) tissue portions may be compared In addition, skin scanning may be applied in other cases where a dense portion of an image is located next to a portion with much lower density One particular example is when searching for a hemorrhage which indicates a fracture in a cervical spine image The glare from the spine itself is preferably masked and the illumination for the adjoining soft tissue (in which a hemorrhage may be located) is optimized The soft tissue on both two sides of the spine may be compared to determine abnormalities Other examples in muscular-skeletal x-ray imaging include viewing soft tissue located near long bones (such as the femur) In a preferred embodiment of the invention, a rectangular back-illuminated ROI sequentially scans an image parallel to a bone or other low density portion in the image Preferably, the glare from the bone is masked out in the scanning ROI Further preferably the extent of the bone in the ROI is determined for each slot position and masked accordingly In one embodiment of the present invention a user specifies the type of film which is used to produce the transparency Controller 45 retrieves the characteristics of the film type from a memory and automatically adjusts the chromaticity of the back-illumination so that the chromaticity ofthe light passing through the film has a desired chromaticity

Alternatively to user specification of the film type, in one preferred embodiment of the invention, the film type of the transparency (and thus its chromaticity) are read off the film by the viewbox One method of reading information from a transparency is disclosed in the above cited WO93/01564, whereby the viewbox reads bar-coded information from the transparency Alternatively or additionally, especially in an alternator, such information is read when the transparency is conveyed from a storage to a display surface

In this embodiment, there is no need for color sensors to sense the color characteristics of the liquid crystal/film combination In general, the liquid crystal chromaticity is measured in the factory or in situ without the film and the correction required for this chromaticity is stored in controller 45 The correction based on the type of film is added to that of the liquid crystal Alternatively, correction factors (intensities of the various chromatic light sources) are stored for the combinations of film and liquid crystal types

In a second embodiment of the invention, in which the chromaticity of the film is dependent on the density of the film, color correction also depends on the density of the film If the color correction is made in the closed loop mode as described in the above-mentioned applications, for example WO96/17269, then color correction will be optimized for the particular film or region of interest of the film which is illuminated However, this type of optimization can be made without any color sensors for determining the actual chromaticity of the light passing through the film

In this embodiment of the invention, the chromaticity characteristics of the film as a function of density is stored in controller 45 As described in the abovementioned patent applications, controller 45 automatically increases the lighting to a desired level based on a measurement of the light intensity passing through the film In accordance with a preferred embodiment of the present invention, the light intensity of the back-illumination source (either as measured by a detector or as determined by commands from controller 45) is compared to the light intensity measured by the camera The ratio between these intensities indicates the density of the film which can then be used to adjust the desired color of the back-illumination source to correct for the overall chromaticity of the liquid crystal and film at the density of the entire transparency or the region of interest being viewed

It should be appreciated, that although the present invention has been described mainly with respect to semi-automated and automated viewboxes, many aspects of the present invention are at least equally applicable to other types of viewboxes, for example, viewboxes with mechanical shutters or even viewboxes with no masking at all, as are in common use in many hospitals

It should be appreciated that the present invention is not limited to what has thus far been described with reference to preferred embodiments of the invention Rather, the scope of the present invention is limited only by the claims which follow

Claims

1. A transparency viewing device comprising a faceplate adapted for mounting at least one transparency having a particular average density at at least a portion thereof; a source of back illumination for the transparency, and a controller which varies the chromaticity of the back illumination based on the particular average density.

2. A device in accordance with claim 1, wherein the controller varies the chromaticity based on the chromaticity ofthe transparency

3 A transparency viewing device comprising a faceplate adapted for mounting at least one transparency having a particular chromaticity thereon; a source of back-illumination for the transparency, and a controller which varies the chromaticity of the back-illumination based on the particular chromaticity ofthe transparency, wherein said viewbox does not directly measure the chromaticity of light transmitted through the transparency

4. A device in accordance with claim 3, wherein the controller receives an indication of the particular chromaticity from an operator.

5. A device in accordance with claim 3, wherein the controller reads an indication of the chromaticity ofthe transparency off the film.

6. A device in accordance with claim 5, wherein the indication comprises a film type

7. A device in accordance with claim 3, wherein the controller varies the chromaticity based on a particular density of at least a portion of the transparency

8 A device in accordance with any of claims 1-7, wherein the controller locally varies the back-illumination chromaticity responsive to local characteristics ofthe transparency

9 A device in accordance with claim 8, wherein different back-illumination chromaticity is provided at at least two different locations

10 A device in accordance with any of claims 1 -7, wherein the controller provides higher color temperature back-illumination at locations of film having higher densities to compensate for a limited ability of the back-illumination to provide sufficient illumination for the higher density locations

1 1 A device in accordance with any of claims 1 -7, comprising a masking pattern generator which affects the chromaticity of the back-illumination

12 A device in accordance with claim 1 1 , wherein the masking pattern generator comprises an LC array

13 A device in accordance with claim 1 1 , wherein the controller varies the back-illumination based on the effect ofthe masking pattern generator on the back-illumination chromaticity

14 A device in accordance with any of claims 1 -7, wherein the controller compnses a chromaticity/type memory, which memory stores information regarding chromaticity corrections for a plurality of transparency types

15 A device in accordance with any of claims 1 -7, wherein the controller comprises a chromaticity/mask memory, which memory stores information regarding chromaticity corrections for a plurality of mask patterns

16 A device in accordance with any of claims 1 -7, wherein the controller comprises a chromaticity/back-illumination memory, which memory stores information regarding chromaticity corrections for a plurality of back-illumination conditions

17. A device in accordance with any of claims 1-7, wherein the controller comprises a chromaticity/density memory, which memory stores information regarding chromaticity corrections for a plurality of transparency densities

18 A device in accordance with any of claims 1-7, wherein the back-illumination source comprises a plurality of narrow spectrum lamps and wherein the controller lights selected ones of said plurality of lamps to generate back-illumination of a desired chromaticity

19 A device in accordance with any of claims 1-7, wherein the back-illumination source comprises at least one high color temperature lamp which provides most of the intensity of said back-illumination and at least one low color temperature lamp which provides a small portion of the intensity of said back-illumination

20 A transparency comprising a film base, and a computer readable marking on the film indicative of the film chromaticity

21 A transparency in accordance with claim 20, wherein the indication comprises a film type

22 A transparency in accordance with claim 20 or claim 21 , wherein the indication comprises a bar-code indication

23 A transparency viewing device comprising a faceplate adapted for mounting at least one transparency thereon, a source of back illumination for the transparency, comprising a plurality of narrow spectrum lamps of a plurality of spectral bands, and a controller which lights selected ones of said plurality of lamps to generate back- illumination of a desired chromaticity

24 A transparency viewing device according to claim 23, wherein the plurality of lamps comprises a plurality of lamps of at least three narrow spectral bands

25. A transparency viewing device according to claim 23 or claim 24, wherein the plurality of lamps comprises at least 5 lamps.

26 A transparency viewing device according to claim 23 or claim 24, wherein the plurality of lamps comprises at least 10 lamps

27 A transparency viewing device according to claim 23 or claims 24, wherein the plurality of lamps comprises at least 20 lamps

28 A transparency viewing device comprising a faceplate adapted for mounting at least one transparency thereon, a source of back illumination for the transparency, a source of ambient lighting having a variable hue, and a controller which controls the hue ofthe ambient lighting

29 A viewing device in accordance with claim 28, wherein the controller controls the hue of the ambient light responsive to a characteristic of at least a portion of the transparency

30 A viewing device in accordance with claim 29, wherein the controller controls the hue responsive to the density of the portion of the transparency

31 A viewing device in accordance with claim 29, wherein the controller controls the hue responsive to the level of detail ofthe portion ofthe transparency

32 A method of controlling a pupil size of an eye of an operator viewing a transparency on a viewbox comprising illuminating the eye with light of a controllable hue, and controlling the hue ofthe light to achieve a desired pupil size

33 A method in accordance with claim 32, wherein pink hued light is used to increase the size ofthe pupil

34 A method in accordance with claim 32, wherein blue hued light is used to decrease the size of the pupil

35. A method in accordance with any of claims 32-34, wherein illuminating comprises illuminating the eye using ambient lighting

36. A method in accordance with any of claims 32-34, comprising determining a direction of gaze of the eye, wherein controlling the pupil size comprises controlling the pupil size responsive to the direction of gaze

37. A method of scanning a mammogram comprising. back-illuminating only a portion of a mammogram corresponding to an outer layer of a breast; and subsequently back-illuminating only a second outer layer poπion of the breast

38 A method in accordance with claim 37, wherein subsequently comprises serially scanning the entire outer layer ofthe breast in the mammogram

39 A method in accordance with claim 37, wherein back-illuminating comprises graded back-illuminating, such that denser image parts of said portion receive greater amounts of back- illumination than less dense parts thereof

40 A method in accordance with any of claims 37-39, comprising detecting the outer layer of a breast in a mammogram

41 A method in accordance with claim 40, wherein detecting comprises detecting the outer layer based on a density analysis of the mammogram

42 A method in accordance with claim 40, wherein detecting comprises detecting the outer layer based on a textural analysis ofthe mammogram

43 A method in accordance with claim 40, wherein detecting comprises detecting the outer layer based on feature recognition of the breast orientation and location

44 A method of automatic scanning of an x-ray transparency of soft tissue disposed near a bone, comprising masking a low density portion ofthe transparency corresponding to the bone, sequentially back-illuminating at least one substantially only soft tissue portion on both side ofthe masked bone 45 A method of viewing a transparency comprising determining at least one characteristic of at least a portion of the transparency, without using color sensing of light passing through the transparency, and back-illuminating the portion with light, wherein the chromaticity of the back- illumination is dependent on the determined charactenstic

46 A method in accordance with claim 45, wherein the at least one charactenstic comprises chromaticity

47 A method in accordance with claim 46, wherein determining comprises receiving an input from a user

48 A method in accordance with claim 46, wherein determining comprises reading a marking indicating the chromaticity from the transparency

49 A method in accordance with claim 45, wherein the at least one characteristic comprises density

50 A method in accordance with claim 49, wherein a second portion of said transparency is simultaneously back-illuminated with light having a different chromaticity from light back- llluminating the first portion

51 A method in accordance with claim 50, wherein light transmitted through the first portion has a different chromaticity from light transmitted through the second portion

52 A method in accordance with any of claims 45-51 wherein the chromaticity of the back- lllumination of a dense portion of the transparency is of a high color temperature to at least partially compensate for reduced visual acuity due to low levels of light which are transmitted through the dense portion

53 A method of transparency viewing comprising back-illuminating a first portion of a transparency to transmit light having a first chromaticity and a first intensity, and back-illuminating a second portion of the transparency to transmit light having a second chromaticity and a second intensity, lower than the first intensity, wherein, the second chromaticity has a higher color temperature than the first chromaticity