US3350594A - Image intensifier having continuous conducting layer between porous metallic coating and luminescent layer - Google Patents

Image intensifier having continuous conducting layer between porous metallic coating and luminescent layer Download PDF

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US3350594A
US3350594A US385664A US38566464A US3350594A US 3350594 A US3350594 A US 3350594A US 385664 A US385664 A US 385664A US 38566464 A US38566464 A US 38566464A US 3350594 A US3350594 A US 3350594A
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photo
target
layer
electrons
image
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US385664A
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Davis Gordon Peter
Benbow Henry Alfred
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EMI Ltd
Electrical and Musical Industries Ltd
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EMI Ltd
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Priority claimed from GB3067863A external-priority patent/GB1090774A/en
Priority claimed from GB30678/64A external-priority patent/GB1025505A/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J43/00—Secondary-emission tubes; Electron-multiplier tubes
    • H01J43/04—Electron multipliers
    • H01J43/06—Electrode arrangements
    • H01J43/08—Cathode arrangements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
    • H01J29/02—Electrodes; Screens; Mounting, supporting, spacing or insulating thereof
    • H01J29/10—Screens on or from which an image or pattern is formed, picked up, converted or stored
    • H01J29/18—Luminescent screens
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
    • H01J29/02—Electrodes; Screens; Mounting, supporting, spacing or insulating thereof
    • H01J29/10—Screens on or from which an image or pattern is formed, picked up, converted or stored
    • H01J29/18—Luminescent screens
    • H01J29/28—Luminescent screens with protective, conductive or reflective layers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J31/00—Cathode ray tubes; Electron beam tubes
    • H01J31/08—Cathode ray tubes; Electron beam tubes having a screen on or from which an image or pattern is formed, picked up, converted, or stored
    • H01J31/50—Image-conversion or image-amplification tubes, i.e. having optical, X-ray, or analogous input, and optical output
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2231/00—Cathode ray tubes or electron beam tubes
    • H01J2231/50—Imaging and conversion tubes
    • H01J2231/50057—Imaging and conversion tubes characterised by form of output stage
    • H01J2231/50063—Optical

Definitions

  • This invention relates to photo-electron emissive discharge devices in which image intensification can be achieved by accelera-ting the photo-electrons and causing them to impinge on a target with a high velocity.
  • the invention is applicable for example to image intensiliers, photo-multiplier devices or'television or other image pick-up devices.
  • the object of the present invention is to provide an improved photo-electron emissive discharge device in which image intensification can be achieved 'by accelerating the photo-electrons and causing them to impinge on a target with a high velocity, with a view to reducing the liability for spurious signals to -be produced.
  • an image intensifier positive ions released from the surface of the luminescent screen in which the intensified light image is produced may impinge on the photocathode with energies of l0 kilovolts or more and each ion may produce several secondary electrons.
  • a photo-electron emissive discharge device comprising a photo-electron emissive cathode for converting incident light into an electron stream, a target for said elect-ron stream, said target being arranged to receive a high voltage so that electrons of said electron stream can impinge on the target with a high velocity and wherein to reduce the release of ions the surface of the ltarget on which the photo-electrons impinge is provided with a porous coating ⁇ formed by evaporation of metal onto the surface of the target in a gaseous atmosphere, which coating tends to trap such ions.
  • the porous coating is effective because the ions remain trapped for a sufficient time to enable their cha-rge to be neutrolised.
  • the porous coating is a metal coating formed by evaporating a suitable metal in a gaseous atmosphere.
  • Any other electrode at which when the device is operating the electric field intensity is suicient for release of ions may also be provided with a porous coating.
  • an accelerating elect-rode or more than one accelerating electrode disposed between the photo-cathode and the target may be provided with a porous coating.
  • a further spurious noise signal may be set up in a photo-electron emissive discharge device by reflected light feedback.
  • the photo-emissive cathode of the device is semi-transparent, as in the case of an antimony photo-cathode sensitised with caesium or with a plurality of alkali metals, some incident light, particularly at the long wave end of the visual spectrum may pass through said photo-cathode and subsequently be refiected back to said photo-cathode by other electrodes in the device or by glass or aluminised surfaces for example.
  • the above mentioned porous coating is of such a material as to provide an intense, black, non-reflecting surface so as to reduce said light reflection.
  • FIGURE 1 shows one -form of photo-electron emissive discharge devices according -to an embodiment of the invention
  • FIGURE 2 is a detail of FIGURE 1 on an enlarged scale.
  • a single stage image intensifier which comprises a substantially cylindrical envelope 1 having on one end wall a photo-electron emissive cathode 2 which may by way of example be of antimony sensitised with caesium or a plurality of alkali metals.
  • a luminescent screen 3' On the other end Wall of the envelope 1 there is formed a luminescent screen 3'.
  • an annular accelerating electrode 4 and during operation of the device, the envelope 1 4will be surrounded by a focussing coil and the electrode 4 and screen 3 wlill be maintained at high positive potentials with respect to the photo-cathode 2.
  • the luminescent screen 3 is shown in greater detail in FIGURE 2 and comprises a layer 5 of luminescent material which may be formed ⁇ on the end wall of envelope 1 by gravity deposition or electrophoresis.
  • the layer 5 is covered with a layer 6 of aluminum which is evaporated in a high vacuum, for example Iat a pressure of 2 105 millimetres of mercury, the layer 6 having a thickness between 250 A. and 1,000 A.
  • the layer 6 is then covered by a porous coating 7 which may also be of aluminium evaporated in an atmosphere of an inert gas such as argon at a pressure between 0.2 and 2 millimetres of mercury.
  • a coating 7 of suitable thickness may be formed by evaporating 10 milligrams of aluminium at a distance of 31/2 inches from the surface on which it is to be deposited, the area of the surface being about 20 sq. cms.
  • the porous coating 7 of aluminium provides a black non-refiecting surface, and a similar coating may, if desired be provided on the electrode 4.
  • a similar coating may, if desired be provided on the electrode 4.
  • the porous coating 7 should not substantially reduce the energy of penetrating electrons and for this purpose a low density material such as aluminium for the porous coating is particularly desirable.
  • other materials may be used to form the porous coating. Examples of such materials are gold and silver.
  • the device which is illustrated in FIGURES 1 and 2 When the device which is illustrated in FIGURES 1 and 2 is in operation it is connected to an external circuit in such a way that the accelerating electrode 4 is maintained at va positive potential of 5 kilovolts with respect to the photo-cathode 2 whilst the aluminium layer 6 on the luminescent screen 5 is maintained at a positive potential of kilovolts, also with respect to the photo-cathode 2.
  • the intensifier is moreover disposed inside a focussing solenoid which produces the magnetic eld parallel to the axis of the intensier.
  • the invention is also applicable to image intensifiers having a plurality of stages, each with a photocathode, in which case one or more of the luminescent screens may include a porous layer.
  • the invention can also be applied to other photo-electron emissive devices such as image orthicons or other television or image pick-up tubes or photo-multiplier devices.
  • One type of television or image pick-up tube to which the invention is especially applicable is a tube of the type in which the target comprises a layer of insulating material which exhibits electron bombardment induced conductivity.
  • the photo-electrons from the photo-cathode impinge on the target with a very high velocity, in the order of l0 kilo electron volts.
  • the image electrons are therefore particularly liable to liberate positive ions from the surface of the target on which they impinge.
  • the conductivity induced by the image electrons causes charges to be accumulated on different elemental areas of the target. These charges are discharged periodically by scanning the target with a low velocity electron beam, for example. If electron beam scanning is employed, the scanning section of the tube is generally of a similar construction to the scanning section of an image orthicon.
  • a photo-electron emissive discharge device comprising a photo-electron emissive cathode for converting incident light into an electron stream, a target for said electron stream, said target being arranged to receive a high voltage so that electrons of said electron stream can impinge on the target with a high velocity and wherein to reduce the release of ions the surface of the target on which the photo-electrons impinge is provided with a porous coating formed by evaporation of metal onto the surface of the target in a gaseous atmosphere, which coating tends to trap such ions, said device constituting an image intensier and in which said cathode converts an incident light image into an electron image which in operation of the device impinges on said target, land said target comprises a luminescent layer having a conducting metallic layer formed on the surface of the luminescent layer facing said cathode by evaporation in a high vacuum, said porous layer being provided on said conducting layer.

Description

Oct. 3l, 1967 G P. DAvls ET AL 3,350,594
IMAGE INTENSIFIER HAVIG CONTINUOUS CONDUCTING LAYER BETWEEN POROUS METALLIC COATING AND LUMINESCENT LAYER Filed July 28, 1964 United States Patent 3,350,594 IMAGE INTENSIFIER HAVING CONTINUOUS CONDUCTING LAYER BETWEEN POROUS ME- TALLIC CATING AND LUMINESCENT LAYER Gordon Peter Davis, Sunhury-on-Thames, Middlesex, and Henry Alfred Benbow, Amersham, England, assignors to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Filed July 28, 1964, Ser. No. 385,664 Claims priority, application Great Britain, Aug. 2, 1963, 30,678/ 63 4 Claims. (Cl. 313-94) This invention relates to photo-electron emissive discharge devices in which image intensification can be achieved by accelera-ting the photo-electrons and causing them to impinge on a target with a high velocity. The invention is applicable for example to image intensiliers, photo-multiplier devices or'television or other image pick-up devices.
It has been found that in photo-electron emissive discharge devices in which image intensification is achieved by accelerating photo-electrons and causing them to impinge on a target with a high velocity, spurious` noise signals are liable to be produced, which signals cannot be accounted for by t-he more usual causes of spurious signals.
The object of the present invention is to provide an improved photo-electron emissive discharge device in which image intensification can be achieved 'by accelerating the photo-electrons and causing them to impinge on a target with a high velocity, with a view to reducing the liability for spurious signals to -be produced.
It has been found by the inventors that the spurious signals above referred to arise lbecause positive ions can be produced at or near high voltage electrodes in discharge devices in which the photo emissive electrons are accelerated to a high velocity by the electric field in the device. Such ions may be liberated especially from the surface of the target on which the photo-electrons impinge Vand the ions so liberated may be accelerated by the electric field which acts on the electrons, but in the opposite direction to the electrons so that they arrive at the photo-electron emissive cathode with sufficient energy to release secondary electrons. These in turn travel to the target and give rise to the spurious signals. For example in an image intensifier positive ions released from the surface of the luminescent screen in which the intensified light image is produced may impinge on the photocathode with energies of l0 kilovolts or more and each ion may produce several secondary electrons.
Having regard to the cause of the spurious signals found by the inventors, there is provided, according to the present invention, a photo-electron emissive discharge device comprising a photo-electron emissive cathode for converting incident light into an electron stream, a target for said elect-ron stream, said target being arranged to receive a high voltage so that electrons of said electron stream can impinge on the target with a high velocity and wherein to reduce the release of ions the surface of the ltarget on which the photo-electrons impinge is provided with a porous coating `formed by evaporation of metal onto the surface of the target in a gaseous atmosphere, which coating tends to trap such ions.
It is lbelieved that the porous coating is effective because the ions remain trapped for a sufficient time to enable their cha-rge to be neutrolised. The porous coating is a metal coating formed by evaporating a suitable metal in a gaseous atmosphere.
Any other electrode at which when the device is operating the electric field intensity is suicient for release of ions may also be provided with a porous coating. For example an accelerating elect-rode or more than one accelerating electrode disposed between the photo-cathode and the target may be provided with a porous coating.
A further spurious noise signal may be set up in a photo-electron emissive discharge device by reflected light feedback. For example if the photo-emissive cathode of the device is semi-transparent, as in the case of an antimony photo-cathode sensitised with caesium or with a plurality of alkali metals, some incident light, particularly at the long wave end of the visual spectrum may pass through said photo-cathode and subsequently be refiected back to said photo-cathode by other electrodes in the device or by glass or aluminised surfaces for example. In one embodiment of the invention the above mentioned porous coating is of such a material as to provide an intense, black, non-reflecting surface so as to reduce said light reflection.
In order that the invention may be clearly understood and readily carried into effect it will now be more fully described with reference to the accompanying drawings of which:
FIGURE 1 shows one -form of photo-electron emissive discharge devices according -to an embodiment of the invention, and
FIGURE 2 is a detail of FIGURE 1 on an enlarged scale.
The invention will be `described with reference to the drawing, Iby way of example, as applied to a single stage image intensifier which comprises a substantially cylindrical envelope 1 having on one end wall a photo-electron emissive cathode 2 which may by way of example be of antimony sensitised with caesium or a plurality of alkali metals. On the other end Wall of the envelope 1 there is formed a luminescent screen 3'. Between the photo-cathode 2 and luminescent screen 3 there is provided an annular accelerating electrode 4 and during operation of the device, the envelope 1 4will be surrounded by a focussing coil and the electrode 4 and screen 3 wlill be maintained at high positive potentials with respect to the photo-cathode 2.
The luminescent screen 3 is shown in greater detail in FIGURE 2 and comprises a layer 5 of luminescent material which may be formed `on the end wall of envelope 1 by gravity deposition or electrophoresis. The layer 5 is covered with a layer 6 of aluminum which is evaporated in a high vacuum, for example Iat a pressure of 2 105 millimetres of mercury, the layer 6 having a thickness between 250 A. and 1,000 A. The layer 6 is then covered by a porous coating 7 which may also be of aluminium evaporated in an atmosphere of an inert gas such as argon at a pressure between 0.2 and 2 millimetres of mercury. By way of example a coating 7 of suitable thickness may be formed by evaporating 10 milligrams of aluminium at a distance of 31/2 inches from the surface on which it is to be deposited, the area of the surface being about 20 sq. cms.
The porous coating 7 of aluminium provides a black non-refiecting surface, and a similar coating may, if desired be provided on the electrode 4. In the case of the luminescent screen of an image intensifier it is desirable that the porous coating 7 should not substantially reduce the energy of penetrating electrons and for this purpose a low density material such as aluminium for the porous coating is particularly desirable. In other cases where density is not so important as on the electrode 4, for example, other materials may be used to form the porous coating. Examples of such materials are gold and silver.
When the device which is illustrated in FIGURES 1 and 2 is in operation it is connected to an external circuit in such a way that the accelerating electrode 4 is maintained at va positive potential of 5 kilovolts with respect to the photo-cathode 2 whilst the aluminium layer 6 on the luminescent screen 5 is maintained at a positive potential of kilovolts, also with respect to the photo-cathode 2. The intensifier is moreover disposed inside a focussing solenoid which produces the magnetic eld parallel to the axis of the intensier.
In the particular image intensier described and illustrated only one accelerating electrode is provided, but if desired two or more such electrodes may be provided, any or all of which can be provided with a porous coating. Moreover, the invention is also applicable to image intensifiers having a plurality of stages, each with a photocathode, in which case one or more of the luminescent screens may include a porous layer. The invention can also be applied to other photo-electron emissive devices such as image orthicons or other television or image pick-up tubes or photo-multiplier devices.
One type of television or image pick-up tube to which the invention is especially applicable is a tube of the type in which the target comprises a layer of insulating material which exhibits electron bombardment induced conductivity. In a tube of this type the photo-electrons from the photo-cathode impinge on the target with a very high velocity, in the order of l0 kilo electron volts. The image electrons are therefore particularly liable to liberate positive ions from the surface of the target on which they impinge. It will be understood that in a tube of this type the conductivity induced by the image electrons causes charges to be accumulated on different elemental areas of the target. These charges are discharged periodically by scanning the target with a low velocity electron beam, for example. If electron beam scanning is employed, the scanning section of the tube is generally of a similar construction to the scanning section of an image orthicon.
It will be understood that the invention is also applicable to image intensiers which are constructed integrally with television pick-up tubes.
What we claim is:
1. A photo-electron emissive discharge device comprising a photo-electron emissive cathode for converting incident light into an electron stream, a target for said electron stream, said target being arranged to receive a high voltage so that electrons of said electron stream can impinge on the target with a high velocity and wherein to reduce the release of ions the surface of the target on which the photo-electrons impinge is provided with a porous coating formed by evaporation of metal onto the surface of the target in a gaseous atmosphere, which coating tends to trap such ions, said device constituting an image intensier and in which said cathode converts an incident light image into an electron image which in operation of the device impinges on said target, land said target comprises a luminescent layer having a conducting metallic layer formed on the surface of the luminescent layer facing said cathode by evaporation in a high vacuum, said porous layer being provided on said conducting layer.
2. A device according to claim 1 in which said porous coating comprises aluminium.
3. A device according to claim 2 in which said porous layer is produced by evaporating aluminium in an inert gas at a pressure between 0.2 and 2 millimetres of mercury.
4. A device according to claim 1 in which said porous layer is black.
References Cited UNITED STATES PATENTS 2,155,465 4/1939 Behne et al 313-92 X 2,303,563 12/1942 Law 313-92 2,544,754 3/1951 Townes 313-65 2,616,057 10/1952 Coltman 313-92 2,898,499 8/1959 Sternglass et al 313-103 2,942,130 6/ 1960 Sheldon 313-92 X 2,960,416 11/1960 Reed 313-92 X 2,994,798 8/1961 Krieger et al. 313-92 X 3,073,989 1/1963 Amsterdam 313-92 X FOREIGN PATENTS 651,728 4/ 1951 Great Britain. 668,854 3/ 1952 Great Britain.
ROBERT SEGAL, Primary Examiner.

Claims (1)

1. A PHOTO-ELECTRON EMISSIVE DISCHARGE DEVICE COMPRISING A PHOTO-ELECTRON EMISSIVE CATHODE FOR CONVERTING INCIDENT LIGHT INTO AN ELECTRON STREAM, A TARGET FOR SAID ELECTRON STREAM, SAID TARGET BEING ARRANGED TO RECEIVE A HIGH VOLTAGE SO THAT ELECTRONS OF SAID ELECTRON STREAM CAN IMPINGE ON THE TARGET WITH A HIGH VELOCITY AND WHEREIN TO REDUCE THE RELEASE OF IONS THE SURFACE OF THE TARGET ON WHICH THE PHOTO-ELECTRONS IMPINGE IS PROVIDED WITH A POROUS COATING FORMED BY EVAPORATION OF METAL ONTO THE SURFACE OF THE TARGET IN A GASEOUS ATMOSPHERE, WHICH
US385664A 1963-08-02 1964-07-28 Image intensifier having continuous conducting layer between porous metallic coating and luminescent layer Expired - Lifetime US3350594A (en)

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GB3067863A GB1090774A (en) 1963-08-02 1963-08-02 Improvements in or relating to photo-electron emissive discharge devices
GB30678/64A GB1025505A (en) 1963-07-22 1964-07-24 Coating process

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3449582A (en) * 1966-02-02 1969-06-10 Westinghouse Electric Corp Electron multiplier device having an electrically insulating secondary emission control surface
US3774038A (en) * 1971-04-16 1973-11-20 Philips Corp Imaging device comprising an image-intensifying tube
US3973157A (en) * 1974-01-07 1976-08-03 S.A.E.S. Getters S.P.A. Charged-particle trapping electrode
US4251748A (en) * 1976-04-12 1981-02-17 U.S. Philips Corporation Camera tube having photocathode absorptive of shorter wavelength and filter absorptive of longer wavelength light

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NL7611593A (en) * 1976-10-20 1978-04-24 Optische Ind De Oude Delft Nv METHOD OF APPLYING A LIGHT-SORTABLE ELECTRONIC PENETRATION LAYER INTO AN IMAGE AMPLIFIER TUBE.
DE3315011A1 (en) * 1983-04-26 1984-10-31 Licentia Patent-Verwaltungs-Gmbh, 6000 Frankfurt ELECTRON TUBES WITH LUMINAIRE

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US2155465A (en) * 1937-01-12 1939-04-25 Firm Fernseh Ag Fluorescent screen for cathode ray tubes
US2303563A (en) * 1941-05-09 1942-12-01 Rca Corp Cathode ray tube and luminescent screen
US2544754A (en) * 1947-12-04 1951-03-13 Bell Telephone Labor Inc Electron camera tube
GB651728A (en) * 1939-02-14 1951-04-11 Alfred De Quervain Improvements in or relating to cathode-ray tubes
GB668854A (en) * 1948-05-08 1952-03-26 Westinghouse Electric Int Co Improvements in or relating to image intensifiers
US2616057A (en) * 1950-05-20 1952-10-28 Westinghouse Electric Corp Black screen television cathode-ray tube
US2898499A (en) * 1956-05-23 1959-08-04 Westinghouse Electric Corp Transmission secondary emission dynode structure
US2942130A (en) * 1957-02-25 1960-06-21 Corning Glass Works Aperture mask coating to prevent cathode poisoning
US2960416A (en) * 1952-07-29 1960-11-15 Rauland Corp Method of manufacturing screens for electron-discharge devices
US2994798A (en) * 1946-12-26 1961-08-01 Gardner L Krieger High voltage image tube
US3073989A (en) * 1960-04-18 1963-01-15 Michael F Amsterdam Image converter device

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DE733611C (en) * 1937-11-10 1943-03-30 Ig Farbenindustrie Ag Device for making corpuscular rays visible
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NL164276B (en) * 1951-09-27 Sandoz Ag PROCESS OF PREPARING A 1,3-DIPHENYL PYRAZOLINE, OF A PREPARATION FOR OPTICAL BLEACHING OF FIBER MATERIALS BY DISSOLVING ONE OR SOME OF THESE PYRAZOLINS IN WATER AND PROCESS FOR OPTICAL BLEACHING OF FIBER MATERIALS 1,3 -DIPHENYLPYRAZOLINS AND OPTICALLY BLEACHED FIBER MATERIALS OBTAINED BY APPLICATION OF LATEST METHOD.
DE1055711B (en) * 1957-08-05 1959-04-23 Siemens Ag Albis Image converter tube
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US2155465A (en) * 1937-01-12 1939-04-25 Firm Fernseh Ag Fluorescent screen for cathode ray tubes
GB651728A (en) * 1939-02-14 1951-04-11 Alfred De Quervain Improvements in or relating to cathode-ray tubes
US2303563A (en) * 1941-05-09 1942-12-01 Rca Corp Cathode ray tube and luminescent screen
US2994798A (en) * 1946-12-26 1961-08-01 Gardner L Krieger High voltage image tube
US2544754A (en) * 1947-12-04 1951-03-13 Bell Telephone Labor Inc Electron camera tube
GB668854A (en) * 1948-05-08 1952-03-26 Westinghouse Electric Int Co Improvements in or relating to image intensifiers
US2616057A (en) * 1950-05-20 1952-10-28 Westinghouse Electric Corp Black screen television cathode-ray tube
US2960416A (en) * 1952-07-29 1960-11-15 Rauland Corp Method of manufacturing screens for electron-discharge devices
US2898499A (en) * 1956-05-23 1959-08-04 Westinghouse Electric Corp Transmission secondary emission dynode structure
US2942130A (en) * 1957-02-25 1960-06-21 Corning Glass Works Aperture mask coating to prevent cathode poisoning
US3073989A (en) * 1960-04-18 1963-01-15 Michael F Amsterdam Image converter device

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3449582A (en) * 1966-02-02 1969-06-10 Westinghouse Electric Corp Electron multiplier device having an electrically insulating secondary emission control surface
US3774038A (en) * 1971-04-16 1973-11-20 Philips Corp Imaging device comprising an image-intensifying tube
US3973157A (en) * 1974-01-07 1976-08-03 S.A.E.S. Getters S.P.A. Charged-particle trapping electrode
US4251748A (en) * 1976-04-12 1981-02-17 U.S. Philips Corporation Camera tube having photocathode absorptive of shorter wavelength and filter absorptive of longer wavelength light

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