CA1064746A - Speckle minimization in projection displays by reducing spatial coherence of the image light - Google Patents
Speckle minimization in projection displays by reducing spatial coherence of the image lightInfo
- Publication number
- CA1064746A CA1064746A CA252,583A CA252583A CA1064746A CA 1064746 A CA1064746 A CA 1064746A CA 252583 A CA252583 A CA 252583A CA 1064746 A CA1064746 A CA 1064746A
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- CA
- Canada
- Prior art keywords
- image
- light
- screen
- illuminating
- projection screen
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/12—Picture reproducers
- H04N9/31—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM]
- H04N9/3141—Constructional details thereof
- H04N9/315—Modulator illumination systems
- H04N9/3161—Modulator illumination systems using laser light sources
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/48—Laser speckle optics
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/12—Picture reproducers
- H04N9/31—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM]
- H04N9/3102—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM] using two-dimensional electronic spatial light modulators
- H04N9/3105—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM] using two-dimensional electronic spatial light modulators for displaying all colours simultaneously, e.g. by using two or more electronic spatial light modulators
- H04N9/3108—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM] using two-dimensional electronic spatial light modulators for displaying all colours simultaneously, e.g. by using two or more electronic spatial light modulators by using a single electronic spatial light modulator
Abstract
SPECKLE MINIMIZATION IN PROJECTION DISPLAYS
BY REDUCING SPATIAL COHERENCE OF THE IMAGE LIGHT
ABSTRACT OF THE DISCLOSURE
An optical system for minimizing the speckle observed in a projected image, the image being produced by a projection display device. Coherent or non-coherent light illuminates the projection display device image and an image thereof is formed at an intermediate image plane. A narrow angle diffuser is positioned at the intermediate image plane and produces an enlarged illuminating aperture at the Fourier transform plane.
The relatively large angle subtended by the illuminating aperture at a second image plane reduces the spatial coherence of the image formed thereat.
BY REDUCING SPATIAL COHERENCE OF THE IMAGE LIGHT
ABSTRACT OF THE DISCLOSURE
An optical system for minimizing the speckle observed in a projected image, the image being produced by a projection display device. Coherent or non-coherent light illuminates the projection display device image and an image thereof is formed at an intermediate image plane. A narrow angle diffuser is positioned at the intermediate image plane and produces an enlarged illuminating aperture at the Fourier transform plane.
The relatively large angle subtended by the illuminating aperture at a second image plane reduces the spatial coherence of the image formed thereat.
Description
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B~CKGROU~D OF THE INVEN~XO~
Image~ which have been formed on a viewing screen (of either the front~projection or rear~projection type) by projection ~rom projection display devices, such as the one disclosed in U.S. Patent No. 3,853,614, may suffer from small scale scintil}ation , or speckle, which appear to move only when the viewer of the image moves his head. ~hi~
effect is more objectionable in rear-projection screens upon whi~h the image formed on ~he aforementioned display is pro-jected because the light passing through a typical rear-projection screen retain~ more of its partial coherence than does the same light when reflected from a typical ~ront-pro~ection screen. Front projection displays are often u~ed ~or this reason notwithstanding the more complex, less unified ~ystem arrangement which is required. The compact system geometries po~sible with rear-projection systems makas it ;~- highly advantageous to ~ind solutions to the speckle probl~m associated with its use.
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SUMMARY OF THE P~ESE~T IMVE~TIO~
The present invention provides an optical system for reducing the speckle observed in a projected Lmage, the image b~ing produced by a projection display device. In-coherent light illuminates the projection disp~ay device Lmage and an ~mage thereof is formed at a first image plane~
A narrow angle diffuser i~ positioned at ~he ~irst image plane and produces an enlarged illuminating aperture at the Fourier trans~orm plane. The relatively large angle su~-tended by the illwminating aperture at a second image plane, reduces the spatial coherence of the image formed thereatO
.
''~
B~CKGROU~D OF THE INVEN~XO~
Image~ which have been formed on a viewing screen (of either the front~projection or rear~projection type) by projection ~rom projection display devices, such as the one disclosed in U.S. Patent No. 3,853,614, may suffer from small scale scintil}ation , or speckle, which appear to move only when the viewer of the image moves his head. ~hi~
effect is more objectionable in rear-projection screens upon whi~h the image formed on ~he aforementioned display is pro-jected because the light passing through a typical rear-projection screen retain~ more of its partial coherence than does the same light when reflected from a typical ~ront-pro~ection screen. Front projection displays are often u~ed ~or this reason notwithstanding the more complex, less unified ~ystem arrangement which is required. The compact system geometries po~sible with rear-projection systems makas it ;~- highly advantageous to ~ind solutions to the speckle probl~m associated with its use.
~ ~ .
SUMMARY OF THE P~ESE~T IMVE~TIO~
The present invention provides an optical system for reducing the speckle observed in a projected Lmage, the image b~ing produced by a projection display device. In-coherent light illuminates the projection disp~ay device Lmage and an ~mage thereof is formed at a first image plane~
A narrow angle diffuser i~ positioned at ~he ~irst image plane and produces an enlarged illuminating aperture at the Fourier trans~orm plane. The relatively large angle su~-tended by the illwminating aperture at a second image plane, reduces the spatial coherence of the image formed thereatO
.
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In accordance with one aspect of this invention there is provided a system for reducing the spatial coherence of an image projected on a projection screen, the image being formed on the surface of an image bearing member comprising: an image bearing member having an image formed on a surface thereof, means for illuminating said image surface with light, a first lens for focusing the light reflected from said image surface to orm an image thereof at a first plane, a diffusing screen positione`d at said first plane whereby the light image incident thereon 1,5 diffused as it passes therethrough, a second lens for collecting the diffused light image and for projecting the diffused image, said second lens forming an illuminating aperture at a distance spaced therefrom, and a projection screen spaced : from said 1lluminating aperture for receiving said projected image, said diffusing screen being selected to increase the : diameter of said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is : reduced.
In accordance with another aspect of this invention there is provided a method of reducing the spatial coherence of an image projected on a projection screen, the image being formed on the surface of an image bearing member comprising the steps o~; illuminating the image surface of an image bearing`member with light, focusing the light reflected from said image surface to form an image thereof at a first plane, positioning a diffusing screen at said first plane whereby the light image incident thereon is diffused as it passes there-through, collecting and projecting the diffused image via an illuminating aperture, and receiving said diffused image at a projection screen spaced from said illuminating aperture, said diffusing screen being selected to increase the diameter of l~
c : - ~ 3 -B
; ~647~6 said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
It is an object of an aspect of the present invention to provide method and apparatus for minimizing speckle observed in viewing an image projected onto a screen, the i~age being formed on a projection display device.
~ ~ It is an object of an aspect of the present invention ; to provide method and apparatus for minimizing speckle observed in an image projected on a rear or front screen, th- image being formed on a projection display device.
It is an object of an aspect of the present invention to provide method and apparatus for increasing the apparent size of the aperture illuminating a projection screen with ~- image light thereby minimizing speckle observed in ~iewing the image.
; It is an object of an aspect of the pres~nt invention to provide method and apparatus for increasing the ~pparent size of the aperture illuminating a projection scre2n with image light from an image formed OD a projection display device, by utili~ing a narrow angle diffuser positioned at ~n inter-mediate image plane! thereby minimizing speckle obs~rved in viewing the image.
DESCRIPTION OF THE DRAWINGS
._ For a better understanding of the inventi~n as well as other objects and further features thereof, refe-ence is made to the following description which is to be re_d in con-junction with the accompanying,drawings wherein:
Figure 1 illustrates a system for illumin-ting a rear projection screen;
~ - 3a -Figure 2 is apparatus incorporating the teachings of the present invention; and Figure 3 illustrates a narrow angle diffusing scr~en which may be utilized in the practice of the preferred :. embodiment of the present invention.
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DESCRIP~IO~ OF THE P~EFERRED ~MBO~IMENT
An Lmage buffer and display system, for storing and displaying an opticaL imas2 is shown in Fig~re 1. In particular, an image buffer 10, such as ~hat disclosed in ~.S. Patent ~oO 3,853,614, i~ utilized to store optical information. The teachings of Patent ~o. 3,853,614, which are necessary for purposes of this disclosure, are incor-porated herein ~y reference. The optical informa~ion is ~: recorded on the image buffer in the manner described in the aforementioned patent. The image ~tored on the surface of Lmage buffer 10 is readout by illuminating the surface thereof with relatively ~right illumination genexated by source 12, su~h as that produ~ed by a source of incoherent light, typically a polychromatic tungsten light source, or coherent light produced by a laser. .The light rays 14 and 16 generated - by source 12 are directed to ].ens 18 and focused thereby into : a front sur~ace mirror 20. ~he light rays reflecked from mirror 20 are directed to image buffer 10 via lens 22. The mage bearing ~urface o~ image bufer 10 scatters light from the ~ro~ted area of the image and enters lens 22 and i9 pro-jected onto ~creen 24 via illuminating aperture 26 of diameter d (typically 4 mm in diameter) formed between diagonal mirror 20 a~d 8tQp member 28. Image bu~er 10 is separated from le~s 22 by di-~tance s, the distance ~eing selected such that rear projection rsen 24 is at the conjugate imaye plane o image buffer 10. The aperture 26 pas~es the zeroth-order ligh~ from the unfrosted areas on image buffer 10 while blocking all higher order re1ections from the frosted areas.
The aperture 26 is positioned at the Fourier transform plane Fl~ which i~ spaced a dis~ance ~ equal to the rear focal length of lens 22. The light passed by aperture 26 is projactad onto rear screen 24, an observer 30 viewing the image projected ~g6~7~6 thereon~ The partial spatial coherence of the illuminating light transmitted by aperture 2~ causes the speckle in the image when viewed by the observer 30. For example, if the distance Sl from transfo~m pla~e Fl to rear projection scree~
24 is 0.9 meters, for a 4 millimeter apexture the source (assuming the aperture 26 as the illumination light source) .subtends a /225 cone at the projection screen 24.
As set forth hereinabove, although the system shown in Figure 1 can readout the image formed on image buffer 10, the image ormed on rear projection screen 24 as viewed by observer 30 has an unusually high degree of speckle due prLmarily to the small size of the illuminating aperture 26.
It should be noted that the embodiment shown in Figure 1 (and Figure 2 described hereinater) is a zeroth order display ystem whereby the zeroth order reflection i5 laterally off-sa~ in the Fourier transform plane Fl from the image of the source 12 formed by condenser lens 18 by means of a small tilt of image buffer 10.
The present inventic)n Lmproves upo~ the system shown ~ - in Figure 1 by in~reasing the apparent SiZ2 of the illuminating apertuxe by using a unique dif~using screen abricated to have a suitably narrow scattering distribution and low scatter~ing at large angles, particularly in the backward direction. The diffu~ing screen and a technique for its fabrication is des-cribed hereinafter with reference to Figure 3. Figure 2 shows the optical system in which the diffusing screen is utilized.
Referring now to Figure 2, image buffer 40, such as that disclosed in aforementioned Patent ~o. 3,853,614, has an image formed thereon which is to be viewed by an observer 41. In particular, it is desired to project the image formed ~L~!647~6 on image buffer 40 onto rear projection scxeen 42. It should be noted that the invention could be adap ed to reduce speckle in a front projection screen by utilizing screen 42 as a front projection screen and positioning observer 41 on the side of ~: 5 screen 42 opposite to that shown in the figure. In order to readout the image, image buffer 40 i5 illuminated hy light produced by a source of iLlumination 44 which~ in the pre-ferred mode, is incoherent light. The light emitted therefrom is condensed by lens 46 onto tilted mirror 48 which directs the rays onto image buffer 40 via lens 50, len~ 50 being spaced rom image buffer 10 ~y a distance fl equal to its front focal length. The light rays reflected from image buffer 40 are focuse~ via lens 50 at the Fourier transform plane F
via the aperture formed between mirror 48 and stop 49 to ~hereafter form an image at diffusing screen 54, described in ; more detail hereinafter with :reference to Figure 3, having passed through and been coll~nated by lens 52. Fourier trans-form plane Fl is spaced from lens 50 a distance fl e~ual to its rear focal length.
: 20 Although in the preferred mode of operation the screen is stationary, it may be rotated about its axis at a prede~ermined angular frequency by motor 56 via shaf~ 58.
~he image which is projected through screen 54 is difuse, the difuse light ~ays indicated by reference numerals 60, 62 and ~1, 63 being incident upon projection lens 64 as shown.
The emerging ligh~ ray~ 66 and 68, ~orresponding to rays 60 and 62, respectively and rays 77 and 79, corresponding to rays 61 and 63, respectively, are imaged at points 70 and 80 on screen 42, as shown. With di~fusing screen 54 in the system light rays 72 and 74 (and the rays lying at angles be~ween rays 60, 62 and 61, 63) exit from lens 52. Without diffusing screen 54, all rays lie very close to the single rays 72 and 74 as shown. In this sense, rays 72 and 74 are representative rays. In this latter case, incident light rays 72 and 74 are focused by lens 64, em~rging rays 76 and 78 tcorresponding to rays 72 and 74, respecti~ely) being projacted onto projection screen 42 at points 70 and 80 as shown. If the diffuser was not present, the light rays 72 and 74 would be focused at a distance f3 from lens 64, forming an image thereat of the light source 44, and then pass on to be imaged on screen 42 as ~hown. With ~he di~fusing screen 54 in place, the light rays 66 and 68 (and 77 and 79) define an increased apparent size of the illuminating aperture a at Fourier trans~er plane F2; that is, there i~ an increase in the width of the light intensity distribution at plane F2.
The optical sy3tam shown in Figurè 2 is sLmilar to that shown in Figure 1 through Fourier tran~form plane Fl, except that the Lmage buffer 40 is positioned at a distance ~rom len~ S0 equal to the front focal leng~h thereof. A
COhd leng 52 i8 po~itioned to provide a ~irst image o~ the store~ imaga o~ image ~uffer 40 at its ~a~k focal plane, a distance equal to~the focal length ~2 f the lens 52, i.e.
the imag~ appea~s at the plane of diffu~ing screen 54. ~ens ~ pro~cts the diffuse image from diffusing screen 54 onto the rear projection screen 42. The diffusing Ccreen 54 has a very narrow angular scattering distribu~ion, descr~bed herainater in more detail, with a typical full angle at half intensït~ being approximately 4 to 8. Lens 64 has an entrance pupil large enough to accept a substantial fraction of the scattered rays ~rom each point on the image to project the di~fuse ~mage on screen 42.
When the diffusing screen 54 is removed, the illu-minating aperture (i.e. the beam diameter or area occupied by the light distribution at that point) at plane F2 is the sama ~L~6~746 size as that at plan~ Fl (typicallx 4 mm), assuming f2 equals f3, and ths beam coherence is therefore as high as that produced by the system shown in Figure l. With a di~fusing screen in place, however, the illwminating aperture a at plane F2 ~defined by the broken line rays) is much Larger, typically 40 millimetersO This increase by a factor of approximately lO times the illuminating aperture corresponds to a ~/number reduction from approximately f/225 to approxi-mately f/22 and hencP a correspondingly large reduction in the magnitude o~ ~peckle in the image light scattered from . screen 42 to observer 41. As the f/numher of the light beam decreases ~i.e., as the beam becomes wider), ~he ~patial co-herence o the beam decreases.
Re~erring now to Figure 3, a typical diffusing screen arrangement which may ~e utilized in the present in-~ention is illustrated. A glass disc 90, corresponding to diffu~er 54 shown in Figure 20 ground and etched Oll side 92 in a mannex as set forth hereinafter, has a center hole 94 ~or mounting di~c 90 on motor shaf~ 58 (Figure 2). A narrow light beam 96 ~rom lens 52 i5 incident normal to the ~ur~ace 98 of disc 90. A set of rectangular coordinate axis with origin at poin~ lO0 where beam 96 is incident ontv surface 92 i.a shown, the z-axis being perpendicular to sur~ace 92 of disc 90. A polar plot 102 of the far-field distribu~ion o~ -the scattered light intensity is il~ustrated, the i~tensity ; scattered in the forward direction being a maxLmum (Io) on the z-axis. The angles where the in~ensity falls to Io/2 de~ine the scattering "half-angle", 0 l/2 which is typically in the range from 4 to 8. It should ~e noted that there is little or no scattering at large angles or in the backward direction. The angular width of the diffusing screen scatter-ing distribution should be chosen to be as large as possible 4~7~6 consistent with retaining most of the ~cattered light within the entrance pupil of lens 64~ As set forth hereinabove, the typical width i9 approxLmately 4 to 8. The smaller value generally provides good speckle reduction with negli-gible light loss while the higher value provides better speckle reduc ion with moderate light loss.
A suitable dif~u~ing screen with the desired angular width is provided in the following manner. One surace of a glass plate (disc) is ground with a fine grit (typically #600 grade grit) a~d then etched with a diluted solution o~ hydro-fluoric acid (~ypically l~/o hydrofluoric acid and gO% water) for a length of time found empirically to give the desired an~ular wldtho In general, the longar the etching time, the narrower the distribution becomes. A typical etch time is about 15 minutes (for angular width equal to 8) to one hour (for angular width of 4) but this depends on the type of glass utilized and the nature of the initial ground surface.
It is to be noted that'~600 grit" is a generic texm deæcrib~
ing a ~ine grit, typically the finest of a series used in sequence (from coarse to finer to very ~ine~ when preparing conventional polished glass surfaces and is typically the final grinding compound use~ before polishingO ~n the etching process reerred to hareinabo-ve, the glass is placed in the : dilute acid, the glass is agitated relative to the acid tomaintain ~resh acid at the surface and the glass i~ removed ~rom the acid at the appropriate time. The other flat glass face could be protected during the acid etch ~y temporarily coating it with plastic or a photore~ist layer which is re-moved after the etching process. The resulting diffuser is a sur~ace with small, concave, nearly spherical scallops randomly located over its sur~ace and has the desired angular scattering distribution. In the case where the average size ,_r. _ 9 _ .
of the scallops may be larger than the required image (picture) element size, the di~using screen can be rotated so as to render su~h scallops not noticable by ~irtue of their motion and persistance of vision e~fects in the observer by energizing motor 56~ In the ~ituation where rotation is desired, diffusing screen 54 is shaped as a disc. In the non-rotating situation, diffusing screen 54 obviou~ly may be shaped in alternate forms~ $uch as a rectangularl~ ~haped glasæ plate Whil~ the invention has been described with refer-e~ce to itæ preferred embodLment, it will be undarstood by those skilled in the art that vari~us changes may ~e made ; and equivalents may be substituted for elements thereof without departing from the true cpirit and scope of the in-vention. In addition, many modifications may ~e made to ; adapt a particular situation or material to the teachings ; of the in~ention without departing from its essential teachi~g : 25
In accordance with one aspect of this invention there is provided a system for reducing the spatial coherence of an image projected on a projection screen, the image being formed on the surface of an image bearing member comprising: an image bearing member having an image formed on a surface thereof, means for illuminating said image surface with light, a first lens for focusing the light reflected from said image surface to orm an image thereof at a first plane, a diffusing screen positione`d at said first plane whereby the light image incident thereon 1,5 diffused as it passes therethrough, a second lens for collecting the diffused light image and for projecting the diffused image, said second lens forming an illuminating aperture at a distance spaced therefrom, and a projection screen spaced : from said 1lluminating aperture for receiving said projected image, said diffusing screen being selected to increase the : diameter of said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is : reduced.
In accordance with another aspect of this invention there is provided a method of reducing the spatial coherence of an image projected on a projection screen, the image being formed on the surface of an image bearing member comprising the steps o~; illuminating the image surface of an image bearing`member with light, focusing the light reflected from said image surface to form an image thereof at a first plane, positioning a diffusing screen at said first plane whereby the light image incident thereon is diffused as it passes there-through, collecting and projecting the diffused image via an illuminating aperture, and receiving said diffused image at a projection screen spaced from said illuminating aperture, said diffusing screen being selected to increase the diameter of l~
c : - ~ 3 -B
; ~647~6 said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
It is an object of an aspect of the present invention to provide method and apparatus for minimizing speckle observed in viewing an image projected onto a screen, the i~age being formed on a projection display device.
~ ~ It is an object of an aspect of the present invention ; to provide method and apparatus for minimizing speckle observed in an image projected on a rear or front screen, th- image being formed on a projection display device.
It is an object of an aspect of the present invention to provide method and apparatus for increasing the apparent size of the aperture illuminating a projection screen with ~- image light thereby minimizing speckle observed in ~iewing the image.
; It is an object of an aspect of the pres~nt invention to provide method and apparatus for increasing the ~pparent size of the aperture illuminating a projection scre2n with image light from an image formed OD a projection display device, by utili~ing a narrow angle diffuser positioned at ~n inter-mediate image plane! thereby minimizing speckle obs~rved in viewing the image.
DESCRIPTION OF THE DRAWINGS
._ For a better understanding of the inventi~n as well as other objects and further features thereof, refe-ence is made to the following description which is to be re_d in con-junction with the accompanying,drawings wherein:
Figure 1 illustrates a system for illumin-ting a rear projection screen;
~ - 3a -Figure 2 is apparatus incorporating the teachings of the present invention; and Figure 3 illustrates a narrow angle diffusing scr~en which may be utilized in the practice of the preferred :. embodiment of the present invention.
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- 3b -.
DESCRIP~IO~ OF THE P~EFERRED ~MBO~IMENT
An Lmage buffer and display system, for storing and displaying an opticaL imas2 is shown in Fig~re 1. In particular, an image buffer 10, such as ~hat disclosed in ~.S. Patent ~oO 3,853,614, i~ utilized to store optical information. The teachings of Patent ~o. 3,853,614, which are necessary for purposes of this disclosure, are incor-porated herein ~y reference. The optical informa~ion is ~: recorded on the image buffer in the manner described in the aforementioned patent. The image ~tored on the surface of Lmage buffer 10 is readout by illuminating the surface thereof with relatively ~right illumination genexated by source 12, su~h as that produ~ed by a source of incoherent light, typically a polychromatic tungsten light source, or coherent light produced by a laser. .The light rays 14 and 16 generated - by source 12 are directed to ].ens 18 and focused thereby into : a front sur~ace mirror 20. ~he light rays reflecked from mirror 20 are directed to image buffer 10 via lens 22. The mage bearing ~urface o~ image bufer 10 scatters light from the ~ro~ted area of the image and enters lens 22 and i9 pro-jected onto ~creen 24 via illuminating aperture 26 of diameter d (typically 4 mm in diameter) formed between diagonal mirror 20 a~d 8tQp member 28. Image bu~er 10 is separated from le~s 22 by di-~tance s, the distance ~eing selected such that rear projection rsen 24 is at the conjugate imaye plane o image buffer 10. The aperture 26 pas~es the zeroth-order ligh~ from the unfrosted areas on image buffer 10 while blocking all higher order re1ections from the frosted areas.
The aperture 26 is positioned at the Fourier transform plane Fl~ which i~ spaced a dis~ance ~ equal to the rear focal length of lens 22. The light passed by aperture 26 is projactad onto rear screen 24, an observer 30 viewing the image projected ~g6~7~6 thereon~ The partial spatial coherence of the illuminating light transmitted by aperture 2~ causes the speckle in the image when viewed by the observer 30. For example, if the distance Sl from transfo~m pla~e Fl to rear projection scree~
24 is 0.9 meters, for a 4 millimeter apexture the source (assuming the aperture 26 as the illumination light source) .subtends a /225 cone at the projection screen 24.
As set forth hereinabove, although the system shown in Figure 1 can readout the image formed on image buffer 10, the image ormed on rear projection screen 24 as viewed by observer 30 has an unusually high degree of speckle due prLmarily to the small size of the illuminating aperture 26.
It should be noted that the embodiment shown in Figure 1 (and Figure 2 described hereinater) is a zeroth order display ystem whereby the zeroth order reflection i5 laterally off-sa~ in the Fourier transform plane Fl from the image of the source 12 formed by condenser lens 18 by means of a small tilt of image buffer 10.
The present inventic)n Lmproves upo~ the system shown ~ - in Figure 1 by in~reasing the apparent SiZ2 of the illuminating apertuxe by using a unique dif~using screen abricated to have a suitably narrow scattering distribution and low scatter~ing at large angles, particularly in the backward direction. The diffu~ing screen and a technique for its fabrication is des-cribed hereinafter with reference to Figure 3. Figure 2 shows the optical system in which the diffusing screen is utilized.
Referring now to Figure 2, image buffer 40, such as that disclosed in aforementioned Patent ~o. 3,853,614, has an image formed thereon which is to be viewed by an observer 41. In particular, it is desired to project the image formed ~L~!647~6 on image buffer 40 onto rear projection scxeen 42. It should be noted that the invention could be adap ed to reduce speckle in a front projection screen by utilizing screen 42 as a front projection screen and positioning observer 41 on the side of ~: 5 screen 42 opposite to that shown in the figure. In order to readout the image, image buffer 40 i5 illuminated hy light produced by a source of iLlumination 44 which~ in the pre-ferred mode, is incoherent light. The light emitted therefrom is condensed by lens 46 onto tilted mirror 48 which directs the rays onto image buffer 40 via lens 50, len~ 50 being spaced rom image buffer 10 ~y a distance fl equal to its front focal length. The light rays reflected from image buffer 40 are focuse~ via lens 50 at the Fourier transform plane F
via the aperture formed between mirror 48 and stop 49 to ~hereafter form an image at diffusing screen 54, described in ; more detail hereinafter with :reference to Figure 3, having passed through and been coll~nated by lens 52. Fourier trans-form plane Fl is spaced from lens 50 a distance fl e~ual to its rear focal length.
: 20 Although in the preferred mode of operation the screen is stationary, it may be rotated about its axis at a prede~ermined angular frequency by motor 56 via shaf~ 58.
~he image which is projected through screen 54 is difuse, the difuse light ~ays indicated by reference numerals 60, 62 and ~1, 63 being incident upon projection lens 64 as shown.
The emerging ligh~ ray~ 66 and 68, ~orresponding to rays 60 and 62, respectively and rays 77 and 79, corresponding to rays 61 and 63, respectively, are imaged at points 70 and 80 on screen 42, as shown. With di~fusing screen 54 in the system light rays 72 and 74 (and the rays lying at angles be~ween rays 60, 62 and 61, 63) exit from lens 52. Without diffusing screen 54, all rays lie very close to the single rays 72 and 74 as shown. In this sense, rays 72 and 74 are representative rays. In this latter case, incident light rays 72 and 74 are focused by lens 64, em~rging rays 76 and 78 tcorresponding to rays 72 and 74, respecti~ely) being projacted onto projection screen 42 at points 70 and 80 as shown. If the diffuser was not present, the light rays 72 and 74 would be focused at a distance f3 from lens 64, forming an image thereat of the light source 44, and then pass on to be imaged on screen 42 as ~hown. With ~he di~fusing screen 54 in place, the light rays 66 and 68 (and 77 and 79) define an increased apparent size of the illuminating aperture a at Fourier trans~er plane F2; that is, there i~ an increase in the width of the light intensity distribution at plane F2.
The optical sy3tam shown in Figurè 2 is sLmilar to that shown in Figure 1 through Fourier tran~form plane Fl, except that the Lmage buffer 40 is positioned at a distance ~rom len~ S0 equal to the front focal leng~h thereof. A
COhd leng 52 i8 po~itioned to provide a ~irst image o~ the store~ imaga o~ image ~uffer 40 at its ~a~k focal plane, a distance equal to~the focal length ~2 f the lens 52, i.e.
the imag~ appea~s at the plane of diffu~ing screen 54. ~ens ~ pro~cts the diffuse image from diffusing screen 54 onto the rear projection screen 42. The diffusing Ccreen 54 has a very narrow angular scattering distribu~ion, descr~bed herainater in more detail, with a typical full angle at half intensït~ being approximately 4 to 8. Lens 64 has an entrance pupil large enough to accept a substantial fraction of the scattered rays ~rom each point on the image to project the di~fuse ~mage on screen 42.
When the diffusing screen 54 is removed, the illu-minating aperture (i.e. the beam diameter or area occupied by the light distribution at that point) at plane F2 is the sama ~L~6~746 size as that at plan~ Fl (typicallx 4 mm), assuming f2 equals f3, and ths beam coherence is therefore as high as that produced by the system shown in Figure l. With a di~fusing screen in place, however, the illwminating aperture a at plane F2 ~defined by the broken line rays) is much Larger, typically 40 millimetersO This increase by a factor of approximately lO times the illuminating aperture corresponds to a ~/number reduction from approximately f/225 to approxi-mately f/22 and hencP a correspondingly large reduction in the magnitude o~ ~peckle in the image light scattered from . screen 42 to observer 41. As the f/numher of the light beam decreases ~i.e., as the beam becomes wider), ~he ~patial co-herence o the beam decreases.
Re~erring now to Figure 3, a typical diffusing screen arrangement which may ~e utilized in the present in-~ention is illustrated. A glass disc 90, corresponding to diffu~er 54 shown in Figure 20 ground and etched Oll side 92 in a mannex as set forth hereinafter, has a center hole 94 ~or mounting di~c 90 on motor shaf~ 58 (Figure 2). A narrow light beam 96 ~rom lens 52 i5 incident normal to the ~ur~ace 98 of disc 90. A set of rectangular coordinate axis with origin at poin~ lO0 where beam 96 is incident ontv surface 92 i.a shown, the z-axis being perpendicular to sur~ace 92 of disc 90. A polar plot 102 of the far-field distribu~ion o~ -the scattered light intensity is il~ustrated, the i~tensity ; scattered in the forward direction being a maxLmum (Io) on the z-axis. The angles where the in~ensity falls to Io/2 de~ine the scattering "half-angle", 0 l/2 which is typically in the range from 4 to 8. It should ~e noted that there is little or no scattering at large angles or in the backward direction. The angular width of the diffusing screen scatter-ing distribution should be chosen to be as large as possible 4~7~6 consistent with retaining most of the ~cattered light within the entrance pupil of lens 64~ As set forth hereinabove, the typical width i9 approxLmately 4 to 8. The smaller value generally provides good speckle reduction with negli-gible light loss while the higher value provides better speckle reduc ion with moderate light loss.
A suitable dif~u~ing screen with the desired angular width is provided in the following manner. One surace of a glass plate (disc) is ground with a fine grit (typically #600 grade grit) a~d then etched with a diluted solution o~ hydro-fluoric acid (~ypically l~/o hydrofluoric acid and gO% water) for a length of time found empirically to give the desired an~ular wldtho In general, the longar the etching time, the narrower the distribution becomes. A typical etch time is about 15 minutes (for angular width equal to 8) to one hour (for angular width of 4) but this depends on the type of glass utilized and the nature of the initial ground surface.
It is to be noted that'~600 grit" is a generic texm deæcrib~
ing a ~ine grit, typically the finest of a series used in sequence (from coarse to finer to very ~ine~ when preparing conventional polished glass surfaces and is typically the final grinding compound use~ before polishingO ~n the etching process reerred to hareinabo-ve, the glass is placed in the : dilute acid, the glass is agitated relative to the acid tomaintain ~resh acid at the surface and the glass i~ removed ~rom the acid at the appropriate time. The other flat glass face could be protected during the acid etch ~y temporarily coating it with plastic or a photore~ist layer which is re-moved after the etching process. The resulting diffuser is a sur~ace with small, concave, nearly spherical scallops randomly located over its sur~ace and has the desired angular scattering distribution. In the case where the average size ,_r. _ 9 _ .
of the scallops may be larger than the required image (picture) element size, the di~using screen can be rotated so as to render su~h scallops not noticable by ~irtue of their motion and persistance of vision e~fects in the observer by energizing motor 56~ In the ~ituation where rotation is desired, diffusing screen 54 is shaped as a disc. In the non-rotating situation, diffusing screen 54 obviou~ly may be shaped in alternate forms~ $uch as a rectangularl~ ~haped glasæ plate Whil~ the invention has been described with refer-e~ce to itæ preferred embodLment, it will be undarstood by those skilled in the art that vari~us changes may ~e made ; and equivalents may be substituted for elements thereof without departing from the true cpirit and scope of the in-vention. In addition, many modifications may ~e made to ; adapt a particular situation or material to the teachings ; of the in~ention without departing from its essential teachi~g : 25
Claims (23)
1. A system for reducing the spatial coherence of an image projected on a projection screen, the image being formed on the surface of an image bearing member comprising:
an image bearing member having an image formed on a surface thereof, means for illuminating said image surface with light, a first lens for focusing the light reflected from said image surface to form an image thereof at a first plane, a diffusing screen positioned at said first plane whereby the light image incident thereon is diffused as it passes therethrough, a second lens for collecting the diffused light image and for projecting the diffused image, said second lens forming an illuminating aperture at a distance spaced there-from, and a projection screen spaced from said illuminating aperture for receiving said projected image, said diffusing screen being selected to increase the diameter of said illuminat-ing aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
an image bearing member having an image formed on a surface thereof, means for illuminating said image surface with light, a first lens for focusing the light reflected from said image surface to form an image thereof at a first plane, a diffusing screen positioned at said first plane whereby the light image incident thereon is diffused as it passes therethrough, a second lens for collecting the diffused light image and for projecting the diffused image, said second lens forming an illuminating aperture at a distance spaced there-from, and a projection screen spaced from said illuminating aperture for receiving said projected image, said diffusing screen being selected to increase the diameter of said illuminat-ing aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
2. The system as defined in Claim 1 wherein said illuminating light is incoherent.
3, The system as defined in Claim 1 wherein said diffusing screen has a narrow angular distribution.
4. The system as defined in Claim 3 further including means for rotating said diffusing screen.
5. The system as defined in Claim 1 wherein the image on said projection screen is observed on the side opposite to 11 .
that upon which the image is projected.
that upon which the image is projected.
6. A system for minimizing the speckle observed in an image projected onto a projection screen by reducing the spatial coherence of the image light, the image being formed on the surface of an image bearing member comprising:
an image bearing member having an image formed on a surface thereof, means for illuminating said image surface with light, a first lens for focusing the light reflected from said image surface to form an image thereof at a first plane, a diffusing screen positioned at said first plane whereby the light image incident thereon is diffused as it passes therethrough, a second lens for collecting the diffused light image and for projecting the diffused image, said second lens forming an illuminating aperture at a distance spaced there-from, and a projection screen spaced from said illuminating aperture for receiving said projected image, said diffusing screen being selected to increase the diameter of said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
an image bearing member having an image formed on a surface thereof, means for illuminating said image surface with light, a first lens for focusing the light reflected from said image surface to form an image thereof at a first plane, a diffusing screen positioned at said first plane whereby the light image incident thereon is diffused as it passes therethrough, a second lens for collecting the diffused light image and for projecting the diffused image, said second lens forming an illuminating aperture at a distance spaced there-from, and a projection screen spaced from said illuminating aperture for receiving said projected image, said diffusing screen being selected to increase the diameter of said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
7. The system as defined in Claim 6 wherein said illuminating light is incoherent.
8. The system as defined in Claim 6 wherein said diffusing screen has a narrow angular distribution.
9. The system as defined in Claim 8 further including means for rotating said diffusing screen.
10. The system as defined in Claim 6 wherein the image on said projection screen is observed on the side opposite to that upon which the image is projected.
11. The system as defined in Claim 8 further including a third lens interposed between said first lens and said diffusing screen for collimating the light image formed by said first lens.
12. The system as defined in Claim 9 wherein said diffusing screen comprises a glass disc, one surface of which diffuses light incident thereon.
13. A method of reducing the spatial coherence of an image projected on a projection screen, the image being formed on the surface of an image bearing member comprising the steps of:
illuminating the image surface of an image bearing member with light, focusing the light reflected from said image surface to form an image thereof at a first plane, positioning a diffusing screen at said first plane whereby the light image incident thereon is diffused as it passes therethrough, collecting and projecting the diffused image via an illuminating aperture, and receiving said diffused image at a projection screen spaced from said illuminating aperture, said diffusing screen being selected to increase the diameter of said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
illuminating the image surface of an image bearing member with light, focusing the light reflected from said image surface to form an image thereof at a first plane, positioning a diffusing screen at said first plane whereby the light image incident thereon is diffused as it passes therethrough, collecting and projecting the diffused image via an illuminating aperture, and receiving said diffused image at a projection screen spaced from said illuminating aperture, said diffusing screen being selected to increase the diameter of said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
14. The method as defined in Claim 13 wherein the reduction of the spatial coherence of the image formed at the projection screen minimizes the speckle observed in the image formed thereat.
15. The method as defined in Claim 13 wherein said illuminating light is incoherent.
16. The method as defined in Claim 13 wherein said diffusing screen has a narrow angular distribution.
17. The method as defined in Claim 16 further including the step of rotating said diffusing screen.
18. The method as defined in Claim 13 wherein the image on said projection screen is observed on the side opposite to that upon which the image is projected.
19. A method of minimizing the speckle observed in an image projected onto a projection screen by reducing the spatial coherence of the image light, the image being formed on the surface of an image bearing member comprising:
illuminating the image surface of an image bearing member with light, focusing the light reflected from said image surface to form an image thereof at a first plane, positioning a diffusing screen at said first plane whereby the light image incident thereon is diffused as it passes therethrough, collecting and projecting the diffused image via an illuminating aperture, and receiving said diffused image at a projection screen spaced from said illuminating aperture, said diffusing screen being selected to increase the diameter of said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
illuminating the image surface of an image bearing member with light, focusing the light reflected from said image surface to form an image thereof at a first plane, positioning a diffusing screen at said first plane whereby the light image incident thereon is diffused as it passes therethrough, collecting and projecting the diffused image via an illuminating aperture, and receiving said diffused image at a projection screen spaced from said illuminating aperture, said diffusing screen being selected to increase the diameter of said illuminating aperture whereby the spatial coherence of the image formed on said projection screen is reduced.
20. The method as defined in Claim 19 wherein said illuminating light is incoherent.
21. The method as defined in Claim 19 wherein said diffusing screen has a narrow angular distribution.
22. The method as defined in Claim 21 further including the step of rotating said diffusing screen.
23. The method as defined in Claim 19 wherein the image on said projection screen is observed on the side opposite to that upon which the image is projected.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/590,250 US4035068A (en) | 1975-06-25 | 1975-06-25 | Speckle minimization in projection displays by reducing spatial coherence of the image light |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1064746A true CA1064746A (en) | 1979-10-23 |
Family
ID=24361481
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA252,583A Expired CA1064746A (en) | 1975-06-25 | 1976-05-14 | Speckle minimization in projection displays by reducing spatial coherence of the image light |
Country Status (3)
| Country | Link |
|---|---|
| US (1) | US4035068A (en) |
| CA (1) | CA1064746A (en) |
| GB (1) | GB1551756A (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US1780969A (en) * | 1925-03-05 | 1930-11-11 | Brunner Rudolf | Process and apparatus for producing artistic designs |
| US3262359A (en) * | 1963-12-30 | 1966-07-26 | Bausch & Lomb | Optical system |
| US3850515A (en) * | 1970-07-16 | 1974-11-26 | Texas Instruments Inc | Coherent light array for use in image projection systems |
| DE2315658C3 (en) * | 1973-03-29 | 1980-11-20 | Philips Patentverwaltung Gmbh, 2000 Hamburg | Method and device for reducing or eliminating the granulation occurring in laser beam projections |
| US3904289A (en) * | 1975-01-10 | 1975-09-09 | Us Navy | Flight simulator visual display system |
-
1975
- 1975-06-25 US US05/590,250 patent/US4035068A/en not_active Expired - Lifetime
-
1976
- 1976-05-14 CA CA252,583A patent/CA1064746A/en not_active Expired
- 1976-06-24 GB GB26282/76A patent/GB1551756A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB1551756A (en) | 1979-08-30 |
| US4035068A (en) | 1977-07-12 |
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