Patent Number: 062947893
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 2, an intensifying screen 100 made according to the present invention includes a first radiation absorbing, luminescent layer 110 which is formed from a first luminescing material capable of producing spectral emissions of known varying wavelengths in response to incident radiation. The first luminescing material also has a known maximum emission wavelength. Preferably, the first luminescing material is a phosphor material. The intensifying screen also includes a second radiation absorbing, luminescent layer 112 which is formed from a second luminescing material capable of producing spectral emissions of known varying wavelengths in response to incident radiation. The second luminescing material also has a known maximum emission wavelength which is either greater than or less than the known maximum emission wavelength of the first luminescing material. Preferably, the second luminescing material is a phosphor. Table 1 below lists some of the phosphors and their emission spectrum ranges that can be used in the present invention. Any combination of two phosphors with spectral emissions not completely overlapped can be used in the present invention. Ideally, in the present invention, the first and second luminescing materials should be selected to ensure that the wavelengths of their respective spectral emission maximums are well separated TABLE 1 Light Emission Spectrum Phosphor Range (nm) Calcium Tungstate (CaWO4) 340-540 Terbium activated Gadolinium Oxysulfide (Gd.sub.2 O.sub.2 S:Tb) 400-650 Europium activated Gadolinium Oxysulfide (Gd.sub.2 O.sub.2 S:Eu) 570-700 Terbium activated Yttrium Oxysulfide (Y.sub.2 O.sub.2 S:Tb) 400-650 Europium activated Yttrium Oxysulfide (Y.sub.2 O.sub.2 S:Eu) 570-700 Terbium activated Lanthanum Oxysulfide (La.sub.2 O.sub.2 S:Tb) 400-650 Europium activated Lanthanum Oxysulfide (La.sub.2 O.sub.2 S:Eu) 570-700 Lanthanum Oxybromide (LaOBr) 360-620 Barium Strontium Sulfate (BaSO.sub.4 :Eu) 330-430 Barium Fluorochloride (BaFCl:Eu) 350-450 Barium Lead Sulfate (BaPbSO.sub.4) 300-500 Zinc Cadmium Sulfide ((Zn,Cd)S:Ag) 450-650 Terbium activated Zinc Sulfide (ZnS:Tb) 400-650 Europium activated Zinc Sulfide (ZnS:Eu) 530-700 Terbium activated Yttria (Y.sub.2 O.sub.3 :Tb) 400-650 Europium activated Yttria (Y.sub.2 O.sub.3 :Eu) 570-700 FIGS. 3A and 3B illustrate the spectral emission wavelength ranges and spectral emission maximums of terbium activated yttria (Y.sub.2 O.sub.3 :Tb) and europium activated yttria (Y.sub.2 O.sub.3 :Eu), two phosphors which may be used together as the first and second radiation absorbing, luminescent layers 110, 112 in the present invention. In addition to the two luminescent layers, the intensifying screen 100 also has a reflective-transmissive layer 114 which is disposed between the first and second luminescent layers 110, 112. The reflective-transmissive layer 114 may be formed from glass or a transparent polymer having either a long pass or short pass optical coating (i.e., filter) thereon. Long and short pass optical filters which are glass or transparent polymer materials with such coatings are commercially available and therefore well known by those skilled in the art. A long pass filter transmits (or passes) a wide spectral band of long wavelength emissions (light) while reflecting short wavelength emissions. A short pass filter transmits (or passes) a wide spectral band of short wavelength emissions while reflecting long wavelength emissions. A long wave pass or short wave pass filter is characterized by a sharp transition from the wavelength region of maximum transmission to maximum reflection. FIG. 3C illustrates the sharp transition of a short pass filter. The type of filter utilized depends upon the wavelength ranges for the materials used to form the two luminescent layers 110, 112, which is typically adjustable in the visible to infrared wavelength range by design. For example, if the material forming the first and second luminescent layers 110, 112 are chosen so that the first luminescent layer 110 has a spectral emissions wavelength range which is generally less than the spectral emissions wavelength range for the second luminescent layer 112, then the reflective-transmissive layer 114 is formed so that it is a long pass filter which is capable of reflecting incident spectral emissions of varying wavelengths that emanate from the first luminescent layer 110 which are less than the cutoff wavelength of the long pass filter material. Also, the long pass filter material forming the reflective-transmissive layer 114 is selected so that it is capable of transmitting incident spectral emissions of varying wavelengths that emanate from the second luminescent layer 112 which are greater than the cutoff wavelength of the long pass filter material. Alternatively, if the material forming the first and second luminescent layers 110, 112 are chosen so that the first luminescent layer 110 has a spectral emissions wavelength range which is generally higher than the spectral emissions wavelength range for the second luminescent layer 112, then the reflective-transmissive layer 114 is formed as a short pass filter which is capable of reflecting incident spectral emissions of varying wavelengths that emanate from the first luminescent layer 110 which are greater than the cutoff wavelength of the short pass filter material. Also, the short pass filter material forming the reflective-transmissive layer 114 in this alternative embodiment is selected so that it is capable of transmitting incident spectral emissions of varying wavelengths that emanate from the second luminescent layer 112 which are less than the cutoff wavelength of the short pass filter material. Preferably, in either embodiment described above, the material selected for the first and second luminescent layers 110, 112 and the reflective-transmissive layer 114 are chosen so that the cutoff wavelength is between the emission maximum wavelengths of the materials forming the two luminescent layers 110, 112. Ideally, the emission maximums of the two luminescent materials should be as far separated as possible, but a minimum of 20 nanometer wavelength separation will work. Optionally, in addition to the first and second luminescent layers and the reflective-transmissive layer, the intensifying screen 100 also includes a backing layer 116 which is disposed adjacent to the second luminescent layer. The screen may also optionally include a secondary reflective layer 118 which is disposed between the second luminescent layer 112 and the backing layer 116. The optional secondary reflective layer 118 is adapted to reflect spectral emissions of varying wavelengths emanating from the second luminescent layer 112. Any glass or polymeric material having a suitable reflective coating may be used. Alternatively, a reflective coating may be applied on the backing layer. Also, a protective layer 120 may be applied over the first luminescent layer 110 in a conventional manner to provide resistance to screen surface abrasion. All of the layers of the intensifying screen of the present invention can be held together in any conventional manner. Some examples of intensifying screens made according to the present invention are described below. EXAMPLE 1 The first radiation absorbing, luminescent layer was composed of terbium activated yttria (Y.sub.2 O.sub.3 :Tb) phosphor. FIG. 3A shows the wavelength distribution of spectral emissions from Y.sub.2 O.sub.3 :Tb phosphor. The spectral emissions from this material have wavelengths ranging from about 400 to 650 nm and a maximum or peak emission wavelength at about 545 nm. The second radiation absorbing, luminescent layer was composed of europium activated yttria (Y.sub.2 O.sub.3 :Eu) phosphor. FIG. 3B shows the wavelength distribution of spectral emissions of Y.sub.2 O.sub.3 :Eu phosphor. As shown in FIG. 3B, the spectral emissions have wavelengths ranging from about 570 to 700 nm with a maximum or major peak emission wavelength at 620 nm. The reflective-transmissive layer was a long pass filter with a cutoff wavelength at about 580 nm. FIG. 3C shows the percent (%) reflectance of the long pass filter as a function of wavelength for this first reflective layer. The secondary reflective layer was formed as a thin (.about.20 .mu.m thick) coating of magnesium oxide, or alternatively titanium dioxide, on the backing layer which reflects the Y.sub.2 O.sub.3 :Eu phosphor spectral emissions. The backing layer was a thin polyester sheet. The protective layer was a thin (approximately 1 to 2 .mu.m thick) transparent film to resist screen surface abrasion. EXAMPLE 2 The first radiation absorbing, luminescent layer was composed of europium activated barium fluorochloride (BaFCl:Eu) phosphor. Its emission spectrum ranges from 350 to 450 nm and peaks at about 380 nm. BaFCl:Eu is the phosphor material found in a commercially available screen sold under the trademark Dupont Quanta II. The second radiation absorbing, luminescent layer was composed of terbium activated gadolinium oxysulfide (Gd.sub.2 O.sub.2 S:Tb) phosphor. Its emission spectrum ranges from about 400 to 650 nm with a major peak at 545 nm. Gd.sub.2 O.sub.2 S:Tb is used in many commercially available screens, e.g., Eastman Kodak Lanex.TM. screens. The reflective-transmissive layer was a long pass filter with a cutoff wavelength at about 500 nm. The secondary reflective layer, backing layer and protective layers were the same as in Example 1. FIG. 4 shows the present invention used in a digital imaging system. In this embodiment, the image intensifying screen 200 is formed by a first radiation absorbing, luminescent layer 210 which is directly deposited on top of an optical fiber plate 212 containing a plurality of optical fibers which optically couples the intensifier 200 to a charge-coupled device (CCD) 214. The CCD 214 converts the spectral emissions or light signals into electronic signals which are then processed to form a final image. CCD devices are well known in the art and therefore not described herein. In this embodiment, a reflective-transmissive layer 216 is provided between the first luminescent layer 210 and a second luminescent layer 218. An optional secondary reflective layer 220 is disposed between the second luminescent layer 218 and a backing layer 222. As illustrated by FIG. 2, incident X-ray absorption at locations A and B in the radiation absorbing, luminescent layers 110, 112 produces spectral emissions in the layers which emit isotropically. Spectral emissions in the form of visible light A', B' emitted towards the protective layer have a relatively short path before emerging from the intensifier, thereby producing image information with relatively high spatial resolution. One of the differences between the prior art conventional intensifier depicted in FIG. 1 and the intensifier of the present invention is that the spectral emissions B' from the second luminescent layer 112 travel through the reflective-transmissive layer 114 prior to emerging from the intensifier. These spectral emissions also have a different wavelength than the spectral emissions A' from the first layer. Another difference is that the spectral emissions A" from the first luminescent layer 110 emitted toward the reflective-transmissive layer may be reflected by the reflective-transmissive layer 114 and, if so, then exit the intensifier from the protective layer side. As compared to the reflected spectral emissions A" in the prior art intensifier illustrated in FIG. 1, the reflected spectral emissions A" in the intensifier of the present invention travel a shorter path towards the protective layer and therefore generally have a shorter lateral dispersion from emission site. This increases the spatial resolution of the intensifier for a given screen speed. It will thus be seen that the objects and advantages set forth above and those made apparent from the preceding descriptions, are efficiently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that the matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.