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Title:
SINGLE SCREEN/FILM STRUCTURE WITH GREATLY IMPROVED RADIOLOGICAL PROPERTIES
Document Type and Number:
WIPO Patent Application WO/1993/001522
Kind Code:
A1
Abstract:
An X-ray film/screen combination which will produce an image with very high sharpness is described. This combination comprises a screen comprising a UV emitting phosphor with the major emission peak between 300-390 nm dispersed in a binder and coated on a conventional support. The film element is a single-side coated silver halide element. This combination can be used for any type of radiological evaluation where very high sharpness is required and presents a considerable advance over the prior art.

Inventors:
BEUTEL, Jacob MICKEWICH, Daniel, J.
Application Number:
PCT/US1992/005443
Publication Date:
January 21, 1993
Filing Date:
July 02, 1992
Export Citation:
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Assignee:
DU PONT DE NEMOURS AND COMPANY.
International Classes:
A61B6/00; C09K11/00; C09K11/77; C09K11/78; C09K11/86; G03C1/00; G03C1/035; G03C5/17; G21K4/00; (IPC1-7): G03C5/17; G21K4/00
Foreign References:
EP0482603A2
US5017440A
Download PDF:
Claims:
What is claimed is:
1. A radiographic element having improved resolution and decreased noise comprising: (a) an Xray intensifying screen comprising an ultraviolet emitting Xray phosphor having.a peak emission between 300 and 390 nm with the proviso that at least 80% of the ultraviolet emission of said phosphor is between 300 and 390 nm, wherein said phosphor is dispersed in a binder, said binder absorbing no more than 10% of said ultraviolet emission from said phosphor, and wherein the mean free path for absorption of a photon emitted by said phosphor when excited by X radiation is less than 2 cm, and wherein said phosphor dispersed in said binder is coated on a support; and, (b) a coated, gelatino silver halide element in operative association therewith wherein a silver halide coating is present only on one side of a support.
2. The element of Claim 1 wherein said mean free path for absorption of a photon emitted by said phosphor when excited b Xradiation is less than 1 cm.
3. The element of Claim 1 wherein said phosphor has a peak emission between 310 and 360 nm, does not emit more' than 10% of its light below 310 or and above 360 nm and said binder absorbs less than 5% of any ultraviolet light emitted therefrom.
4. The element of Claim 1 wherein said silver halide emulsion is a tabular gelatino silver halide emulsion with an average grain thickness of between .21 and .30 microns and said aspect ratio is between 4.0 and 5.5:1.
5. The element of Claim 1 wherein said Xray intensifying phosphor is selected from the group comprising yttrium tantalate, yttrium tantalate 20 activated with gadolinium, and lanthanum oxybromide activated with gadolinium.
6. The element of Claim 1 wherein said Xray intensifying phosphor is YTa0 and said binder is a mixture of acrylic resins with an average molecular weight of about 100,000 to about 300,000.
7. The element of Claim 1 wherein said tabular silver halide is taken from the group consisting of silver bromide, silver chloride, silver iodide or mixtures thereof.
8. The element of Claim 1 wherein said film element further comprises an auxiliary layer coated on said support opposite to said silver halide element.
Description:
SINGLE SCREEN/FILM STRUCTURE WITH GREATLY IMPROVED RADIOLOGICAL PROPERTIES FIELD OF THE INVENTION

This invention relates to the field of radiology and more specifically to a film/screen structure that can greatly improve image sharpness and resolution. Even more specifically, this invention relates to single ultraviolet emitting screens in conjunction with single- side coated silver halide elements.

BACKGROUND OF THE INVENTION Although X-ray intensifying screens used to increase the ability of a photographic element to capture the image produced thereon are well-known in the art, these prior art screens generally have been used in pairs with double-side coated photographic elements. These intensifying screen elements are generally created from a phosphor dispersion in a binder coated on a suitable support. Many phosphors are in commercial use in intensifying screens. However, since most of these phosphors emit visible blue to green light (in the wavelength range from 400 to 550 nm) spectrally sensitized silver halide elements are used to match the film's spectral response characteristics to the phosphor's emission. In an effort to increase the sharpness of the image produced by these prior art screens which emit visible light it is known that dyes or pigments may be added to the screen's phosphor layer to absorb some of the emitted light which is scattered within the screen structure. To be effective such dyes or pigments must be dispersed uniformly within the screen's phosphor layer. This results in a sharper

image, but at the cost of screen speed, i.e., in the number of light photons emitted at the surface of the screen facing the film, per incident x-ray quantum. This decrease in speed must then be compensated by increasing the screen pair's coating weight, which in turn reduces the sharpness. However, in practice it is difficult to disperse dyes or pigments uniformly and reproducibly within the phosphor layer.

Another way to increase the sharpness of double- side coated film/two-screen systems is to decrease print-through, i.e., to prevent light emanating from the front screen from exposing the photographic emulsion on the back side of the double coated film and vice versa, by placing a layer under the photographic emulsions on both sides of the film, which layer absorbs the actinic light emanating from the screens and prevents this light from penetrating to the other side of the film. However, this applies only to double-side coated film/two-screen systems. Some prior art references describe elements that can be employed to reduce this undesirable print- through. For example, Van Stappen in U.S. 3,923,515, December -2, 1975 teaches the use of an X-ray film element which is double-side coated with both a fine grain and coarse grain emulsions. In this element, the fine grain emulsion is coated in each case nearest to the support layer and the coarse grain emulsion supra thereto. Thus, the fine grain emulsion tends to absorb the radiation that might otherwise penetrate to the opposite side causing print-through. This is a complicated structure and although the print-through is significantly reduced, the image quality suffers from many other deficiencies.

The use of ultraviolet emitting screens (hereafter called "UV screens") is not common in the prior art. This is because, when UV screens are used with conventional silver halide films, they produce a maximum optical density (Dmax) which is significantly lower than that produced on exposure to visible light (having a wavelength greater than 400 nm) , and this loss of Dmax leads to a significant distortion of the film's response cure observed as a decrease in the film's contrast behavior. This decrease in Dmax occurs because the silver halide crystals in the film absorb UV light so strongly that only the top layer of silver halide crystals is exposed while the crystals below this top layer remain unexposed. In a copending application (USSN 07/520,285, filed May 7, 1990) there is described a particular two-side coated film/two-screen system utilizing UV light that is reported to produce good sensitometry, excellent image quality and reduced print- through. However, there is no teaching within this invention to the use of single-side coated silver halide elements which can be used to produce high quality radiographic images such as those required for mammographic or extremity evaluations of the human body. No advantage was expected for the application of these UV screens in single screen/one-side coated film applications since print-through does not apply in one¬ sided systems.

Van Stappen, U.S. 3.912,933, October 14, 1975 does teach a particular element that can be used for mammography, for example. This element requires a single-side coated silver halide element with a particular grain size, an antihalation layer coated on the film element and a high speed X-ray screen in operative contact therewith. Although this system was

one of the first successful elements designed specifically for mammographic and extremity X-ray evaluations, the structure is complex, requiring a dischargeable antihalation dye layer. Additionally, this Van Stappen element was designed specifically for mammographic and extremity evaluations.

SUMMARY OF THE INVENTION

It is an object of this invention to prepare a film/screen system in which the film comprises a single- side coated silver halide element, i.e., a support with a silver halide emulsion coated on one side of a support. It is also an object of this invention to produce a film/screen system that has very high image quality. This and yet other objects are achieved in a radiographic element having improved resolution comprising:

(a) an X-ray intensifying screen comprising an ultraviolet emitting X-ray phosphor having a peak emission between 300 and 390 nm with the proviso that at least 80% of the ultraviolet emission of said phosphor is between 300 and 390 nm, wherein said phosphor is dispersed in a binder, said binder absorbing no more than 10% of said ultraviolet emission from said phosphor, and wherein the mean free path for absorption of a photon emitted by said phosphor when excited by X- radiation is less than 2 cm, and wherein said phosphor dispersed in said binder is coated on a support; and, (b) a single side, gelatino silver halide coated element in operative association therewith.

BRIEF DESCRIPTION OF THE DRAWING

The Figure is a drawing which diagrammatically compares the resolution obtainable using the film/screen

system of this invention with some prior art systems by showing the spreading of the light in systems with different mean free paths for light absorption.

DETAILS OF THE INVENTION

There are many well-known X-ray phosphors which emit in the ultraviolet (UV) when exposed to X- radiation. Although these phosphors may also produce improved image quality, it is also well-known that X-ray intensifying screens prepared from these UV emitting phosphors, produce very low contrast and Dmax on conventional silver halide elements used therewith.

This produces a significant decrease in contrast without increasing the film's dynamic range, thus rendering the recorded image less usable. Typical of these UV emitting phosphors are, for example, Taθ 4 , either unactivated or activated with gadolinium, bismuth, lead, cerium or mixtures of these activators; LaOBr activated with gadolinium or gadolinium and thulium; and La 2 θ 2 S activated with gadolinium, among others. Most of these phosphors emit mainly in the UV, e.g., 300 to 390 nm, although some small amount of light, e..g., up to 20% and preferably less than 10%, may also be emitted below 300 and above 390 nm. For the purpose of this invention, UV emitting phosphors will emit in the range of 300 to 390 nm and preferably in the range of 310 to 360 nm. For the phosphors of this invention to be applicable in practical X-ray imaging systems, the conversion efficiency of the phosphor, i.e., the efficiency with which the energy carried by an X-ray quantum absorbed by this phosphor, is converted to light photons emitted by the phosphor, should be higher than 5%.

These phosphors may be prepared as is well-known in the prior art and then mixed with a suitable binder before coating on a suitable support. Once prepared in this manner, this element is conventionally known as an X-ray intensifying screen and is eminently suitable for radiological evaluations. There are a host of commercially available X-ray intensifying phosphors that do not function within the metes and bounds of this invention. These include the following:

Peak Emission

Remarks Not a UV phosphor

it ti it More than 20% in the visible BaFX:Eu (X=halide) 380

If

LaOBr : Tm 370 & 470 "

Conventionally, a screen of the type encompassed by the phosphors described in this invention will comprise a support, an intensifying phosphor layer, and a topcoat or protective layer therefor. A reflective layer, such as a whitener, e.g., Tiθ 2 dispersed in a suitable binder, may also be added to the screen structure.

Commonly, this reflective layer is interposed between the phosphor layer and the support, or, alternatively, the whitener may be dispersed directly into the support. The reflective layer generally increases the light

output of the intensifying screen during use. The protective layer is important to protect the phosphor layer against mechanical damage. The protective layer should generally also be UV transparent so that the flow of UV light from the phosphor is not decreased. Those layers that are known to absorb a great deal of UV light, e.g., polyethylene terephthalate films, for example, are not particularly μseful within this invention. In operation, the intensifying screen absorbs X-rays that impinge thereon and emits energy having a wavelength that is readily captured by the photographic silver halide X-ray film associated therewith. Effective X-ray intensifying phosphors based on yttrium, gadolinium or lutetium tantalate are known. These particular phosphors, with the monoclinic M' phase, is particularly effective in capturing X-rays. Some of these tantalate phosphors are also efficient emitters of UV light and are particularly preferred within the metes and bounds of this invention. They are generally prepared according to the methods of Brixner, U.S. Pat. No. 4,225,653, and the information contained in this reference is incorporated herein by. reference thereto. • These phosphors are those which emit at least 80% of the light within the range of 300 to 390, and ' preferably within 310 to 360 nm and are generally manufactured by mixing the various oxides and firing in a suitable flux at elevated temperatures. After firing, deagglomerating and washing, the phosphor is mixed with a suitable binder in the presence of a suitable solvent therefore and coated on a support, with the proviso that said binder absorbs less than 10% of any UV light emitted from said phosphor, a so-called "transparent" binder. All of these steps are described in the aforementioned Brixner reference and all are well-known

in the prior art. A protective topcoat may also be applied over this phosphor coating, in fact it is so preferred.

Other limitations relative to the binder used within the ambit of this invention are that the mean free path for adsorption of a photon emitted by the phosphor of this invention will be defined as the average distance which a light photon emitted by the phosphor travels within the structure of the screen before this photon is absorbed therein. Light which is emitted at an angle with respect to that light emitted perpendicular to the screen's surface is more likely to be absorbed when the mean free path is short. Thus, within the ambit of this invention, the mean free path for light absorption should be less than 2 cm, and preferably less than 1 cm, in order to properly function. Light emitted at an angle as defined above, which is not so adsorbed, will exit the screen and expose the film creating a less sharp image. This unwanted light significantly reduces the sharpness of the resultant image and is extremely undesirable. Representative of binders which will function within this invention are those that are well-known in the art and which absorbs no more than 10% of the UV light emitted by the phosphor. These include resinous materials such as poly(methyl methacrylate) , poly(n- butyl methacrylate), poly(isobutyl methacrylate, copolymers of n-butyl methacrylate and isobutyl methacrylate, among others. The Carboset® Acrylic resins manufactured by B. F. Goodrich, Cleveland, OH, e.g., Carboset® 525, average molecular weight 260,000, Acid No. 76-85; Carboset® 526, average molecular weight 200,000, Acid No. 100; Carboset® XL- 27, average

molecular weight 30,000, Acid No. 8, etc. may also be mentioned.

In combination with the UV screen of this invention, we will employ a single-side coated, silver halide element. In the practice of this invention, the silver halide element we prefer may be comprised of spherical or cubic silver chloride grains wherein the chloride represents at least 50 mole percent of the emulsion, or tabular silver halide grains wherein said tabular grains are silver bromide, silver chloride, silver iodide or mixtures thereof and at least 50% of these grains are tabular grains with a grain thickness of less than 0.5 microns, preferably between 0.21 and 0.30 microns, and an average aspect ratio of at least 2:1 (preferably an aspect ratio of between 4.0 and

5.5:1) . These elements are also well-known in the prior art and the preparation of grains of this type are also known and taught therein. The grains are generally made into an emulsion using a binder such as gelatin, and are sensitized with gold and sulfur, for example. Other adjuvants such as antifoggants, wetting and coating aides, dyes, hardeners etc. may also be present if necessary. The emulsions may also be formed from conventional shaped silver bromoiodide grains made by balanced double jet or splash procedures provided that their Dmax does not decrease significantly on exposure to UV light in the wavelength range from 300 to 390 nm compared to exposure to visible light (wavelengths greater than 400 nm) . After preparing the emulsion, it is coated on one side of a conventional photographic support such as a dimensionally stable polyethylene terephthalate film suitably coated with a resin sub followed by a gel sub supra thereto. These are well-known silver halide

support elements and also may be precoated with any of the conventional antistatic subbing layers. Auxiliary layers supra to the emulsion and on the opposite side of the support may also be employed to provide protection from scratches, curl and the like.

Additionally, the film support for the photographic emulsion layer may contain a dye to impart a tint therein, e.g., a blue tint, in fact it is so preferred. Since the emulsions useful within the ambit of this invention are generally UV sensitive in and of themselves, it may not be required to add any kind of sensitizing or desensitizing dye thereto. However, if required, a small amount of a sensitizing dye might advantageously be added. Additionally, it is also conventional to add a sensitizing dye to tabular emulsions in order to increase their ability to respond to light.

To further distinguish the photographic elements useful within this invention, i.e., photographic emulsions containing tabular AglBr or AgBrCl crystals, we can say that when these emulsions are exposed to the UV light which emanates from the screens of this invention, the maximum optical density which they can achieve, e.g., Dmax, will not be lower than 10% of the Dmax obtained when the same system is exposed to visible light. This can be represented by the formula:

Dmax (vis) - Dmax (UV) = to or < than 0.10 Dmax (vis)

Tabular grain silver halide products are well-known in the prior art and present the user with some considerable advantages over conventional grain

products, e.g., semi-spheroidal grains, for example. The tabular products can usually be coated at a much lower coating weight without loss of covering power. They can be hardened with smaller amounts of conventional hardeners, presenting quite a significant advantage over the conventional grains. Tabular chloride emulsions are also well-known and are described by Maskasky in U.S. 4,400,463, 8/23/83 and also by Wey, U.S. 4,399,205. Some other references which describe the manufacture and use of tabular grain elements are Dickerson, U.S. 4,414,304; Wilgus et al., U.S. 4,434,226; Kofron et al., U.S. 4,439,520; and Tufano and Chan, U.S. 4,804,621.

In a particularly preferred embodiment, a single X- ray intensifying screen is made by dispersing YTa0 4 phosphor made as described above, in a mixture of acrylic resins using a solvent. This mixture is then coated on a polyethylene terephthalate support containing a small amount of anatase Tiθ 2 whitener dispersed therein. The phosphor may be coated to a coating weight of ca. 15 to 110 mg of phosphor per cm 2 . A topcoat of sty . rene/acrylonitrile copolymer is coated thereon and dried. The film element is a single-side coated, gelatino silver halide element conventionally prepared as is well-known to those of normal skill in the art, preferably, a tabular emulsion containing a small amount of a blue sensitizing dye therein. This emulsion will also have been raised to its optimum level of sensitivity by the addition of gold and sulfur as well as by the addition of antifoggants and the like. Wetting and coating agents will also be present. The coating weight of this element may be between 60 and 100 mg of silver/dm 2 , for example.

In the practice of this invention, the single-side coated, gelatino silver halide element is placed in a conventional cassette with the X-ray intensifying screen described above. This element is then placed in proximity to the object which is to be examined, e.g., a human patient. The preferred geometric arrangement is to have the front, i.e., the phosphor side of the screen facing the x-ray beam and the emulsion coating of the film facing the front of the screen. X-rays are generated from a source, pass through the object, and are absorbed by the intensifying screens. UV light given off as a result of X-ray absorption, will expose the film element contained therein. A high quality image which has high detail can thus be obtained. The extremely sharp image which is obtained is significantly sharper than an image which would be obtained with a screen having the same coating weight but which emits visible light or which emits ultraviolet light whose mean free path for light absorption in the screen is longer than 2 cm. The elements of this invention may advantageously be employed for any radiological evaluation and excellent image quality and image sharpness will be observed. Where fine detail is a requisite, e.g., mammography and extremity evaluations, these elements are eminently suitable.

This invention will now be illustrated by the following specific examples in which Example 1 is considered to represent the best mode thereof. All parts and percentages are by weight unless otherwise indicated.

EXAMPLE 1 An X-ray intensifying screen structure was made using the following procedures:

The Binder Solution:

The following ingredients were prepared:

Ingredient Ampynt (g)

n-Butyl acetate 43 . 13 n-Propanol 34 . 00 Carboset® 525 < 1 ) 10 . 00 Carboset® 526 < 2 > 10 . 00

Polymeric organic silicone fluid 0.07 Zelec® 2457E <3> 0.40 Aerosol® OT-100 < > 0.40 Pluronic® 31R1 < 5 > 2.00

(1) Acrylic resin; ave. mol. wt. 260,000; acid no. 76-85; B. F. Goodrich Co., Cleveland, OH

(2) Acrylic resin; ave. mol. wt. 200,000; acid no. 100; B. F. Goodrich Co., Cleveland, OH

(3) Anionic antistatic agent of mixed mono and dialkyl phosphates of the general structure R 2 HP0 , where R is C8 to CIO alkyl; E. I. du Pont de Nemours & Co., Wilmington, DE

(4) Sodium dioctyl sulfosuccinate per U.S. 2,441,341

(5) Ethylene oxide/propylene oxide block copolymer; ave. mol. wt. 3,200; BASF Wyandotte; Wyandotte, MI

B. The X-ray Phosphor:

The following ingredients were thoroughly mixed in a paint shaker for about 2 hours before charging to an alumina crucible: Ingredient AmPVn. (?)

Y 2 0 3 101.46

Ta2θ 5 198.54

Li 2 S0 4 150.00

The crucible was then placed in a standard, commercial, high temperature furnace and fired at about 1200°C for about 8 hours and then at about 1250°C for about 16 hours. The f rnace was then allowed to cool and the contents of the crucible weighed and washed thoroughly with water to remove the flux. This material was then added to the binder Solution A using about 200 g of phosphor/60 g of binder solution. This mixture was placed in a plastic container along with about 85 g of 1 cm diameter corundum balls (ca. 15 balls) and this mixture was then ball milled for about 12 to 16 hours at room temperature with a rotation speed of about 60 rpm. After this step, the ball milled suspension was filtered through a 75 mesh Nylon bag and coated onto a suitable support. The support used was 0.010 inch thick, dimensionally stable polyethylene terephthalate film containing a small amount of a whitener, e.g., anatase Tiθ 2 , dispersed therein.

This whitener will give the support some opacity to visible light, e.g., optical density of ca. > 1.7. The coating weight of the phosphor dispersion placed thereon is in the range from about 10 to about 100 mg of phosphor per cm 2 .

C. The Overcoat Layer:

An overcoat layer is prepared from the following solutions:

1) Ingredient Amount (a)

Acetone 67.00

Methanol 9.00 n-Butyl acetate 4.80

Tyril® 100<->•> 12.70 Carboset® XL-27< 2 > 9.00

(1) Styrene/acrylonitrile copolymer resin; Dow Chemical Co., Midland, MI (2) Acrylic resin; ave. mol. wt. 30,000; acid no. 80, B. F. Goodrich Co., Cleveland, OH

A gel solution is prepared by mixing the following ingredients until a thick gel forms:

2)

(1) Acrylic resin thickener; B. F. Goodrich Co., Cleveland, OH

This solution is filtered and a mixture is prepared s follows:

This solution is filtered and a mixture is prepared s follows:

3) Ingredient roQ .lt (?)

Solution 1 50.00

Gel Solution 2 12.19

This mixture is coated on top of the phosphor coating using a doctor knife with a 0.004 inch gap. The resulting top-coat is air dried for 12-16 hours at 40°C. For comparison purposes two additional screens were prepared. The phosphor of these controls was made as above but with an activator present, e.g., 2-4% niobium. The control screens were prepared with a coating weight of 15 mg/cm 2 and 30 mg/cm 2 and were coated and overcoated similar to the screen of this invention as described above. The control screens have a major emission peak at about 400 nm and thus are not considered to be UV emitters within the ambit of this invention.

D. The Film Element: A conventional, tabular grain, blue sensitive X-ray emulsion was prepared as well-known to one of normal skill in the art. This emulsion had tabular silver iodo bromide grains. After precipitation of the grains, the average aspect ratio was determined to be about 5:1 and the thickness about 0.2 μm. The procedures for making tabular grains of this nature are fully described in Nottorf, U.S. 4,772,886 and Ellis, U.S. 4,801,522, the contents of which are incorporated herein by reference. These grains were dispersed in photographic grade gelatin (about 117 grams gelatin/mole of silver iodo bromide) and a suspension of 200 mg of 5-(3-methyl-2- benzothiazolinylidene)-3-carboxy-methylrohdanine sensitizing dye dissolved in 25 ml of methanol added to achieve 133 mg of dye per mole of silver halide. At

this point, the emulsion was brought to its optimum sensitivity with gold and sulfur salts as is well-known to those skilled in the art. The emulsion was then stabilized by the addition of 4-hydroxy-6-methyl- l,3,3a,7-tetraazaindene and l-phenyl-5- mercaptotetrazole. The usual wetting agents, antifoggants, coating aides and hardeners were added and this emulsion was then coated on a dimensionally stable, 7 mil polyethylene terephthalate film support which had first been coated with a conventional resin-sub followed by a thin substratum of hardened gelatin applied supra thereto. These subbing layers were present on both sides of the support. The emulsion was coated on one side of this support at a coating weight of about 3 g/m 2 . A thin abrasion layer of hardened gelatin was applied over the emulsion layer. After drying, samples of this film were used with each of the X-ray intensifying screens made as described above.

E. Film/Screen Exposure & Results:

One of each of the aforementioned screens were used to expose samples of X-ray film elements made above. The screen was placed in a vacuum bag along with the single-side coated X-ray film element and given an exposure to a 60 KVP X-ray source with a tungsten cathode. After exposure, the films were developed in a standard X-ray developer formulation, fixed, washed and dried.

The following results were obtained:

Coating

Sample Weight (mg/dm 2 ) Sharpness< 1 '

Control 30 0.45

Control 15 0.68

Of This Invention 30 0.65

(1) MTF at 4 line pairs/mm

As can be seen from this example, it would be required to coat a very thin conventional screen to achieve the sharpness that can be achieved with the screen of this invention. Preparation of very thin coating weight screens present some real problems in the commercial manufacture and use thereof. Such very thin screens are difficult to coat, are slower than thicker screens, and will produce more structural noise than thicker screens. Moreover, the thicker screen absorbs a higher fraction of the incident x-ray beam and therefore has a higher signal-to-noise ratio as measured by its Detective

Quantum Efficiency (DQE) , so that the quantum noise of the resulting image is reduced significantly. Thus, the screen of this invention had excellent speed, sharpness and reduced noise over that of either control. In referring to Fig. 1 it is evident that this improved image quality is the result of reduced light spreading from the UV screen in which the mean free path for the absorption of light is below 2 cm. Most of the previously reported UV phosphors will not perform in this manner because of the presence of more than 20% visible light above 390 nm.