US2824986A - Increasing contrast of the image intensifier - Google Patents

Description

M. ROME 2,824,986

INCREASING CONTRAST OF THE IMAGE .INTENSIFIER Feb. 25', 1958 Filed April 19, 1954 Fig. l.

Fig. 3.

hotoelectric Phoioconductive Material Conduciive Coating Transparent l n e C 5 e r. O U

X- RAY Intensity INVENTOR Martin Rome WITNESSES MZM ATTORNEY MKGZM invention.

United States Patent lNCREA SING. CONTRAST OF THE IlVIAGE INTENSIFIER Martin Rome, Elmira Heights, N. Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 19, 1354, Serial No. 424,073

3 Olaims. (Cl. 313-65) ducing tube which, like the Mason and Coltman device,

embodies an electron image producing means as an agency in its operation.

One object of my invention is accordingly to provide an improved type of image reproducing tube;

Another object is to provide means for regulating and improving the contrast between difierent portions of the output image in an image reproducing tube.

Another object is to provide'a device in which an image embodied in radiation of one type is transformed into an image in radiation of another type with means for regulating and improving'the contrasts, or gamma, of different portions of the image.

Still another-object is to provide an improved type of tube for transforming X-ray images into visible replicas thereof.

Other objects of my invention will become apparent upon reading the following description taken in connection with the drawings in which:

Figure 1 is a schematic view partly in cross-section of an image intensifier tube of the general type described in the above-mentionedMason and Coltman patent;

Fig. 2 is a schematic cross-sectional view of a portion of the input screen embodying my invention for use in the Fig. 1 tube; and

Fig. 3 is a graphical plot employed in explaining my Referring to the drawings in detail, the image intensifieftube comprises a vacuum tight container l which may be of glass provided with an input-screen 2 consisting of a glass-plate 3 of general watch-glass form having on its convex surface a layer 4 of zinc sulphide or other fluorescent material. The concave surface of the plate 3 is coated with a transparent conductive coating 5 such as tin oxide, and the latter is, faced with a layer 6 of some photoconductivematerial such as selenium, arsenic trisulphide, antimony trisulphide, or cadmium sulphide. The concave'face of thelayer 6 is coated witha layer 7 of cesiate d antimony or other suitable photoelectric emitter. V 7

At the opposite end of container 1 is an output screen 8 which mayc'omprise a glass plate 9 coated with a layer 11 of an electron-phosphor sandwiched between glass plate 9. and a layer 12 of thin enough to be pen/ions topelectrons'made to bombard its surface in the nianner about to'be described; An electron lens system 13 envelopes, the space between input screen 2 and output screens. i

In operation of this image tube an X-ray image is pro- 2,824,986 Patented Feb. 25,

jected onto the fluorescent layer 4 where it generates light which passes through glass plate 3, conductive coatingS and photoconductive coating 6 to generate at the free surface of photoelectron emissive coating 7 an electron image which is a replica in space-distribution of the X-ray field incident on layer 4. An electron-lens system 13 of any suitable type energized by source 14 which connects layers 5 and 12 focuses this electron image into incidence on the layer llthrough metal layer 12, and generates in layer :11 a light image which is a replica '(except as pointed out below), at reduced scale, of the original X-ray image. The electron image, moving from the inside face of screen 2 into incidence upon screen 8 constitutes an electric current which cannot flow, except transiently, if a complete electric circuit for its flow does not exist; and hence current must flow from front to back face from photoelectrically-emissive layer 7 to conductive layer 5 through the photoconductive layer 6. The number of electrons which can flow, as part of the electron image, from a point on the concave face of layer 7 thus depends on the resistance of the portion of the photoconductive layer which underlies it. The resistance of the photoconductive layer 6, point-by-point over its area, is determined by the intensityof the light projected through it by the light-image generated on the fluorescent layer 4. Thus, at a point where this light image is bright, the resistance of the photoconductive layer 6 is low, and this permits a large number of photoelectrons to be emitted per second from the photoelectrically emissive layer 7 at that point on the screen. On the other hand, at a point where the X-rayv image is of low intensity, the photoconductive layer 6 is of high resistance and this permits only a relatively few electrons to be emitted per second from the-1ayer7 at that point of the image. In short, the photoconductive layer 6 may be considered .to embody a conductivity-image corresponding in distribution over the screen face to the distribution of the X-ray image, and this conductivity irnage modifies and accentuates the contrasts in the electron image which is projected onto the output screen 8. Thus bright areas on the X-ray image are made relatively prominent in the image on output screen 8, while dark areas on input screen 2 are relatively dark on the output screen. i

The changes in resistivity of the photoconductive layer 6 are in general not strictly proportional to changes of intensity in the X-ray field but are likely to be large where the X-ray field is weak and smaller where theX-ray field is intense. The exact relationship of percenage change in resistivity in photoconductive layer 6 to percentage change in X-ray intensity depends of course'on the particular material constituting layer 6. Moreover, the resistance met by the electric current at a given point in passing through layer 6 is only a part of the total resistance in the complete circuit constituting its path through conductive coating 5, coating 6, coating 7, the space-path between coating 7 and output screen 8, and the external circuit leading back to conductive coating '5, and the changes in the electron-current generating light on output-screen 3 are dependent on the percentage change inresistance of this entire path. Thus, the relative magnitude of resistance in layer 6 to these other resistances acts, in conjunction with other factors such as tube 'in Eig; 3 aicurve like A will represent the relation between 'X-ray intensity and output light from screen 8 in an image tube where the thickness of the photoconductive layer 6

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