To minimize the adverse effects of "flare" in reading-out stimulable phosphor recording elements, the light-collecting face of a photodetector is provided with a multiplicity of sharp, light-transparent, optical projections. By a combination of reflections and refractions, such optical projections serve to absorb substantially all incident photons thereby preventing phosphor-stimulating photons from being reflected by the photodetector's light-collecting face and exciting non-addressed regions of the recording element. Preferably, the optical projections take the form of a sawtooth array of optical wedges; however, pyramid-shaped and cone-shaped projections are also disclosed.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of computed radiography. More 
particularly, it relates to improvements in apparatus for recovering the 
image information contained by a previously exposed radiographic recording 
element comprising a stimulable phosphor. 
U.S. Pat. No. 3,859,529 (Re. No. 31,847) to G. W. Luckey discloses the 
basic concept of using stimulable phosphors to record x-ray patterns and 
of recovering (reading-out) such patterns by scanning such phosphors with 
a beam of radiation adapted to stimulate luminescence (fluorescence or 
phosphorescence) from the x-ray exposed portions of the phosphor. During 
the scan-stimulation process, the phosphor luminescence is detected by a 
photodetector (e.g., a photomultiplier tube) to provide an electrical 
signal representative of the recorded x-ray pattern. Such signal may, if 
desired, be digitized and processed, through suitable algorithms, to 
reduce noise, enhance edges, increase contrast, etc. The processed 
electrical signal may then be used, for example, to intensity-modulate a 
scanning laser beam to record the x-ray pattern on a photosensitive film 
or the like. 
In using the Luckey process for the purpose of diagnostic radiography 
(e.g., for mammography) relatively low levels of x-ray radiation are used 
to record the desired x-ray pattern. Since the stimulated luminescence 
intensity is proportional to the intensity of x-radiation received by the 
image-storing phosphor, it will be appreciated that the stimulated 
luminescence level can be very low, especially for such diagnostic 
radiographic applications. To produce useful images, a major portion of 
the stimulated luminescence must be collected for use in producing the 
image signal. 
U.S. Pat. No. 4,346,295 issued to Tanaka et al. discloses an optical 
arrangement for efficiently collecting a maJor portion of the luminescence 
given off by the image-storing phosphor during the scan-stimulation step. 
In the Tanaka et al. reference, a laser beam scans the phosphor, point by 
point, along a rectilinear image line. A light guide member, made of a 
transparent sheet having smooth surfaces for effecting total internal 
reflection, is arranged so that a flat, linear surface thereof is closely 
spaced with respect to the scan line on the stimulable phosphor. Such flat 
surface is typically 5-8 mm in width and is sufficiently long to span the 
phosphor plate (about 100-400 mm). As the scanning laser beam stimulates 
the phosphor, the stimulated luminescence enters the light guide through 
the flat end thereof and is internally reflected toward a distal end which 
is curled to form an annulus. A photomultiplier tube is optically coupled 
to the annular end of the light guide to detect the luminescence 
transmitted by the light guide. 
While the luminescence-detecting apparatus disclosed by Tanaka et al is 
relatively efficient at collecting a large percentage of the photons 
constituting the luminescence given off by the stimulated phosphor, a 
significant percentage of these photons are reflected by the flat surface 
of the light-guide member and, thus, are not detected by the 
photomultiplier tube. More significantly, however, is the fact that the 
stimulating radiation, which can be 10.sup.8 times more intense than the 
stimulated luminescence, also gets reflected by the flat surface of the 
light-guide. A major portion of this reflected stimulating radiation is 
returned to the phosphor plate and acts to prematurely stimulate 
luminescence from non-addressed regions of the plate, i.e., those regions 
not directly irradiated by the scanning laser beam. Such reflected 
stimulating radiation, known as "flare," has the adverse effects of adding 
background noise to the system, thereby reducing the signal-to-noise ratio 
of the read-out signal. Moreover, it gives rise to false signals and ghost 
images which substantially degrade the quality of the ultimate image. 
SUMMARY OF THE INVENTION 
In view of the foregoing discussion, an object of this invention is to 
minimize the production of "flare" during read-out of a latent 
image-stored by a stimulable phosphor plate. According to the present 
invention, there is provided a low reflectance, high transmission optical 
surface which is particularly adapted for use in image read-out apparatus 
of the above type, for reducing the amount of radiation reflected from the 
input face of the luminescence-detection device. Such surface comprises a 
multiplicity of sharp, tapered projections of a light-transparent material 
which, by way of multiple reflections, serves to trap incident photons and 
thereby couple such photons into the light-transparent material thereof. 
Preferably, such projections are in the form of wedges, pyramids, or 
cones, each having an aspect ratio, defined by the ratio of the height to 
base dimensions, of at least 3:1. The projections may be formed on the 
light-collecting surface of the aforementioned light guide. Alternatively, 
they may be formed directly on the light-collecting face of an elongated 
photomultiplier tube or the like. 
The invention and its various advantages will become more apparent to those 
skilled in the art from the ensuing detailed description of preferred 
embodiments, reference being made to the accompanying drawings wherein 
like reference characters denote like parts.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawings, FIGS. 1 and 2 schematically illustrate a 
known apparatus for reading-out a latent, x-ray produced, image-stored in 
a stimulable phosphor plate 10 disposed on a support 11. Such apparatus is 
disclosed, for example, in the aforementioned U.S. Pat. No. 4,346,295. It 
includes a continuous wave laser L for producing a radiation beam B, and a 
motor-controlled deflector 12 which operates, in a well known manner, to 
repeatedly scan the beam through an angle .theta., whereby the beam scans 
the phosphor plate along a linear scan line S as the plate is advanced in 
a perpendicular direction, as indicated by the arrow A. The radiation beam 
produced by laser L is of a wavelength adapted to excite the phosphor of 
plate 10 and thereby cause the plate to luminesce (fluoresce or 
phosphoresce) in those regions which were previously imagewise exposed to 
x-radiation. To detect the photons constituting the luminescence 
stimulated from plate 10, there is provided a photodetector, usually in 
the form of a photomultiplier tube PMT, which is optically coupled to such 
luminescence by means of a light-guide member 14 comprising a transparent 
sheet of acrylic resin or the like. The light-guide has a flat, linear and 
elongated face 14A which is positioned in close proximity to the scan line 
S. Face 14A is typically 5 to 8 mm in width and 300-400 mm in length. An 
opposing end 14B of the light-guide is curled in the manner shown to 
provide a flat, annular surface which is optically coupled to the 
photodetector. 
In operation, deflector 12 functions to scan laser beam B across the 
phosphor plate causing luminescence photons to be emitted in all 
directions from the scan line S. Most of these photons will strike the 
light-guide face 14A, either directly or indirectly (after being reflected 
by mirror 16) and be optically coupled to the photodetector by multiple 
internal reflections within the light-guide. A small percentage (e.g., 
less than 5%) of the luminescing photons will strike face 14A and be 
reflected back toward the phosphor plate, as indicated by the dashed paths 
shown in FIG. 2. These photons go undetected and represent a loss to the 
detection system. But not being of the phosphor-stimulating wavelength, 
these reflected luminescence photons have no significant effect on the 
read-out of the phosphor plate. This is not the case, however, for the 
unabsorbed photons of the stimulating beam B which, upon being reflected 
by the phosphor plate, travel the same paths as the luminescing photons. 
Upon being reflected from light-guide face 14A, these phosphor-stimulating 
photons again strike the phosphor, get absorbed, and cause the phosphor to 
luminesce in locations spaced from the point of incidence of the 
stimulating beam. The result is the aforementioned "flare" problem and the 
false results it produces. As indicated above, an obJect of this invention 
is to minimize flare in optical read-out apparatus of the type shown in 
FIGS. 1 and 2. 
Referring to FIGS. 3-5, the above-noted object of the invention is achieved 
by the provision of a low reflectance, high transmission optical surface 
20 at the air/light-guide interface 22. Such surface serves to 
substantially eliminate the flare problem by more efficiently optically 
coupling the reflected phosphor-stimulating radiation into the 
light-guide, thereby preventing such radiation from being redirected back 
to the phosphor plate. Moreover, face 20 more efficiently couples the 
luminescence photons stimulated by the scanning laser beam B with 
light-guide 14 and, hence, with the photodetector (PMT). According to a 
preferred embodiment, surface 20 comprises a multiplicity of optical 
proJections 24, each extending outwardly from the light-guide's elongated 
face 14A and tapering to a sharp edge or point. The proJections are made 
of a material which is highly transmissive to the stimulated luminescence 
so as to transmit incident photons to the light-guide member. Preferably, 
such material highly absorbs the wavelength of the phosphor-stimulating 
radiation so that such radiation is not detected by the PMT. 
Alternatively, a bandpass filter 30 may be inserted in the optical path 
between the light-guide and PMT to absorb such phosphor-stimulating 
radiation. Projections 24 may be integral with the light-guide, being 
formed by a stamping or molding process, or may be a separate cast part 
which is bonded, by a suitable optical adhesive, to face 14A. 
In the embodiment illustrated in FIGS. 3-5, projections 24 are in the form 
of two-sided optical wedges. The apex angle of each wedge element is 
chosen so as to be sufficiently small that substantially all in:ident 
photons will either be refracted by the wedge material, and thereby 
immediately coupled into the light-guide, or, alternatively, reflected 
toward another wedge element, whereupon it will have multiple chances to 
be refracted by the wedge material. Photons which are reflected toward the 
boundaries of the projection array are redirected toward the projections 
by reflectors 26A-26D which surround surface 20. To assure that 
substantially all incident photons are optically coupled into the 
light-guide, the height-to-height base aspect ratio of each projection 
should be at least 3:1. Such an aspect ratio assures that the wedge faces 
are sufficiently steep as to reflect incident photon deeper into the 
projection array (i.e., toward the bases of the wedges). 
Since most of the photons entering the light guide do so at a relatively 
large angle of incidence, they will undergo a larger number of internal 
reflections prior to arriving at the photodetector. To enhance the 
coupling efficiency of these photons with the photodetector, it is 
preferred that the edges 14B of the light-guide be provided with a highly 
reflective coating. As noted above, a suitable optical filter 30 prevents 
photons of the phosphor-stimulating wavelength from being detected by 
photodetector. 
While the low-reflectance, high transmission optical surface 20 may be 
formed on the input face of a light-guide, as described above, it may be 
applied directly to the input face of a suitable photomultiplier tube. In 
FIG. 6A, there is illustrated an elongated photomultiplier tube 40 having 
an elongated window 42 (e.g., 8 mm in width and 400 mm in length) for 
transmitting incident photons to a photocathode. Such a photomultiplier 
tube is disclosed in IEEE Transactions on Nuclear Science, Vol. 36, No. 1, 
February 1989. In accordance with another embodiment of the invention, the 
input window of such a device is provided with a low-reflectance, high 
transmission optical surface comprising a plurality of tapered optical 
projections. As shown in FIG. 6B, such projections may take the shape of 
four-sided pyramids 44, each terminating in a sharp point and having an 
aspect ratio of at least 3:1. Here, the optical surface can be produced by 
molding or casting techniques to form a discrete part which can be 
cemented, using a transparent adhesive of suitable refractive index, to 
window 42. An optical bandpass filter 46 is positioned to absorb photons 
of the phosphor-stimulating wavelength and to transmit the shorter 
wavelength luminescence photons. Obviously, the optical projections from 
window 42 may take other forms, such as the aforementioned wedge shaped 
projections, the six-sided pyramids 50 shown in FIG. 7, or the cone-shaped 
proJections, as indicated in FIGS. 8 and 9. 
Compared to the pyramidal and wedge-shaped projections described above, the 
cone-shape projections cannot be packed together so tightly as to 
eliminate flat spaces 38 between adJacent cones. Of course, such spaces 
are disadvantageous in that incident photons can reflect from them and 
produce flare. The area of such surfaces can be minimized, however, by 
arranging the cones in the hexagonal close pick array pattern shown in 
FIG. 8, rather than the more orderly square pattern shown in FIG. 9 in 
which the cone axes are arranged at all intersections of the mutually 
perpendicular lines (X and Y) along which the cone axes are located. 
Alternatively, to avoid any flat spaces at all between adjacent cones, the 
plane which cuts the cones at the location shown in FIGS. 8 and 9, could 
be further displaced from the cone vertices, to a location at which all of 
the cone bases merge together. 
In FIG. 10, light-guide 14 is shown in the form of a bundle of optical 
fibers 40 having tapered ends 40a. A photon entering the bundle along path 
P can either be refracted or reflected by an individual fiber, and various 
possible optical paths are shown. Once the photon has entered the fiber, 
it is thereafter totally internally reflected until it arrives at the 
photodetector. Such tapered fibers can be produced by a heating and 
stretching process. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention.