US3439212A

US3439212A - Spot counter employing a vidicon tube having a pickup screen with different spectral sensitivities

Info

Publication number: US3439212A
Application number: US687284A
Authority: US
Inventors: Joseph Feinstein
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1967-12-01
Filing date: 1967-12-01
Publication date: 1969-04-15
Anticipated expiration: 1986-04-15

Description

Apnl 15, 1969 y .J. FEINSTEIN 3,439,212 SPOT COUNTER EMPLOYING A VIDICON TUBE HAVING A PICKUP SCREEN WITH DIFFERENT SPECTRAL SENSITIVITIE Filed Dec. 1. 1967 FIG. 2

A. RED SENSITIVE 5 ANTIMONY 5g TRISULFIDE I9 \2 $5 AMSORPHOUS 'E 0 2| mvaw'roa g; 0' 17 JOSEPH FEINSTEIN g 4000 I L 6 ao'oo BY 5% 5 WAVELENGTH g na 9 (ANGSTROMS) ATTORNEY United States Patent US. Cl. 315- 11 Claims ABSTRACT OF THE DISCLOSURE A spot counter method and apparatus is disclosed employing a vidicon tube. The vidicon tube has a pickup screen formed by a first photoconductive layer, hereinafter referred to as the outside layer, facing a source of optical radiant energy for illumination of a spot pattern to be counted with an overlaying second or inside photoconductive layer facing the scanning electron beam of the vidicon tube. The two photoconductive layers of the pickup screen have different spectral sensitivities. The inside photoconductive layer facing the electron beam is scanned by the beam to uniformly charge it. The pickup screen is then illuminated with optical radiant energy containing the spot image pattern to be counted. The optical energy is selected to be within the optical spectral range to render both photoconductive layers conductive in a spot pattern corresponding to the spot pattern to be counted. This causes the charge to be drained from the inside layer of the screen through both photoconductive layers in a pattern conforming to the spot image pattern to be counted. The spectral range of the spot pattern illumination is then altered to cause the outside photoconductive layer to be rendered nonconductive while the inside layer is maintained conductive in accordance with the spot pattern. The scanning electron beam of the vidicon tube is then caused to scan the inside photoconductive layer. When the beam first encounters any portion of a conductive spot in the inside photoconductive layer all portions of that spot are immediately charged to the same potential as the non-spot background portions of the charge pattern on the inside layer. The charging displacement current for charging the spot is detected to produce a spot output count signal. The counted spot is thereby eradicated such that it is not again counted by the scanning electron beam. In this manner, false counts from the spots in the pattern are avoided, independent of spot shape. The spectral range of the spot pattern illumination applied to the pickup screen may be altered by inserting an optical filter or by employing two sources of optical radiation for illuminating the pattern. In the latter case, by turning off one of the sources having a spectral range different from the other source, the spectral range of the illumination is changed. Photoconductive materials having substantially different spectral responses are typically characterized by one of the photoconductive materials having a bandgap energy substantially greater than the bandgap energy of the other photoconductive material.

Description of the prior art Heretofore, vidicon tubes have been employed as spot counters. In such devices, the pickup screen included only a single layer of photoconductive material which was first uniformly charged by the scanning electron beam. The spot image pattern to be counted was then projected onto the pickup screen to render the screen conductive in accordance with the spot pattern to be counted. The scanning electron beam was then scanned over the spot pattern on the pickup screen. A spot count output was obtained each time the beam scanned into the spot to be counted. When counting spots substantially larger than the cross section of the scanning electron beam, each spot produced a multitude of spot count output pulses in a very complicated time sequence. The pulse sequence could then be fed to a computer which correlated the spot count signals with positions on the pickup screen to reduce the spot count pulses to a number which approximated the number of separate spots in a spot pattern to be counted. Such a method, at best, produced only an approximation of the correct number of spots to b counted and, furthermore, the data processing of the count signals required a relatively complex computer program.

It is desired to produce a spot counting method and apparatus which will yield a more correct spot count output and which does not require complex data processing.

Summary of the present invention The principal object of the present invention is the proapparatus.

One feature of the present invention is the provision, in a spot counter apparatus, of a vidicon tube having a pickup screen employing first and second photoconductive layers having substantially different spectral sensitivities such that by altering the spectral range of the illuminated spot pattern, the inside photoconductive layer may remain conductive in accordance with the spot pattern to be counted While the outer photoconductive layer becomes non-conductive such that when the scanning beam encounters any portion of the spot pat-tern to be counted it eradicates the spot and produces only one spot count signal, whereby false spot count signals are eliminated.

Another feature of the present invention is the same as the preceding feature wherein the spectral range of the spot pattern illumination is altered by means of a filter inserted into the spot pattern projecting means between the source of light and pickup screen.

Another feature ofthe present invent-ion is the same as the first feature wherein the means for altering the spectral range of the spot pattern illumination includes a first and second source of optical radiant energy, one of the light sources producing optical energy for rendering one of the photoconductive layers conductive and the other light source providing a second range of optical radiation for rendering the second photoconductive layer conductive, such that by turning off or otherwise eliminating the optical radiation from one of the light sources the outer photoconductive layer is rendered nonconductive while permitting the inside photoconductive layer to remain conductive.

Another feature of the present invention is the same as any one or more of the preceding features wherein the inside photoconductive layer is made of a material having a bandgap energy substantially lower than the bandgap energy of the outside photoconductive layer, whereby white light may be employed for rendering both the inside and the outside photoconductive layers conductive and by inserting a filter for passing only the long wavelength spectral range of the white light to the pickup screen, the inside photoconductive layer may be maintained conductive while rendering the outside layer nonconductive.

Another feature of the present invention is the same as any one or more of the preceding features wherein the inside photoconductive layer has a substantially higher photoconductivity than the outer photoconductive layer,

whereby the counted spots are readily eradicated by conduction of electrons from the electron scanning beam.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of a pulse height or pulse area analyzer for analyzing the spot counting pulses for deriving outputs representative of the spot size distribution.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

Brief description of the drawings FIG. 1 is a schematic perspective view, partly broken away, of a spot counting method and apparatus employing features of the present invention,

FIG. 2 (ad) is a schematic flow diagram depicting the sequential operating steps involving the pickup screen portion of the vidicon tube as taken along sectional line 22 of FIG. 1, and

FIG. 3 is a plot of relative photosensitivity in arbitrary units vs. wavelength in A. and depicting the spectral sensitivity characteristics of the two photoconductive layers in the pickup screen portion of the vidicon tube of FIG. l.

Description of the preferred embodiments Referring now to FIG. 1, there is shown a spot counter apparatus employing features of the present invention. The spot counter includes a vidicon tube 1 having its pickup screen 2 disposed to receive an illuminated spot image pattern from a projector 3. The projector 3 includes a light source 4 which may comprise a. white light lamp 5 or, alternatively, a pair of light sources producing light within two different spectral ranges. In the latter case, a mercury lamp 6 produces light predominantly in the blue range and a neon lamp 7 produces light predominantly in the red spectral range. Light produced by source 4 illuminates a transparency 8 which is a negative of the spot image pattern to be counted. A typical spot image to be counted would be, for example, a biological colony or other planar random spot distribution. A lens 9 picks up the illuminated negative spot image and focuses same onto the pickup screen 2 of the vidicon tube 1. When employing a white light lamp 5, a removable red filter 11 may be employed between the negative transparency 8 and the lamp 5 for altering the spectral distribution of the light pattern illuminating the pickup screen 2. Use of the [filter 11 and the two lamps 6 and 7 for altering the spectral distribution of the light is more fully described below.

The vidicon tube 1 may be of the type described in an article titled, Performance of the Vidicon, a Small Development Television Camera Tube, appearing in the RCA Review of March 1952 at pages 3-10. The vidicon tube 1, as employed herein, is slightly modified compared to the conventional vidicon, described in the cited article, in that the pickup screen 2 includes two photoconductive layers having substantially different spectral sensitivities. A first or outer layer 12 of the photoconductive layers faces the optical spot image projected onto the pickup screen 2. A second or inside photoconduc tive layer 13 overlays the outer layer 12 and faces the scanning beam 14 of the vidicon tube 1. The pickup screen 2 includes a transparent face plate 15 as of glass forming a portion of the vacuum envelope of the vidicon tube 1. A transparent conductive electrode 16 is deposited on the inside surface of the face plate 15. A source of potential 17 is connected between the cathode emitter of the electron gun 18 of the vidicon tube 1 and the conductive electrode 16 of the pickup screen 2. A load resistor 19 is connected in circuit between the conductive electrode 16- and the grounded positive terminal of the potential source 17. A coupling capacitor 21 is connected to one terminal of the load resistor 19 for coupling spot counting pulses developed across load resistor 19 with respect to ground potential to an output terminal 22.

Referring now to FIG. 2, there is shown, in a step-bystep manner, the mode of operation of the specially modified vidicon tube 1 as employed for counting spots in the spot pattern projected onto the pickup screen 2. In the first step of the counting process illustrated at a, the scanning beam 14 scans the surface of the inside photoconductive layer 13 to uniformly charge the surface of the photoconductor 13 to the beam potential, as of 30 volts. The screen 2 is not illuminated with the image to be counted and, thus,

photoconductive layers

13 and 12 are non-conductive and the uniform charge is retained.

In the next step illustrated at b, the pickup screen 2 is illuminated with the spot pattern by light derived from the source 4 and within the spectral range which will render both the inner photoconductive layer 13 and the outer photoconductive layer 12 conductive in the regions of the spots as indicated by region 31. This causes regions of the charge deposited upon the inner photoconductive layer 13 to be drained through

layers

13 and 12 to the conductive electrode 16 producing a charge pattern on the inside surface of photoconductive layer 13 corresponding to the spot pattern to be counted. More particularly, the inside surface of the inside photoconductive layer 13 will be charged to -30 volts in the regions outside of the spots and the spots will have a potential of 0 volts for the applied potentials of the particular example.

Suitable photoconductive materials for

layers

12 and 13 include amorphous selenium and antimony trisulfide having spectral sensitivity characteristics as shown in FIG. 3. The amorphous selenium would form the outside photoconductive layer 12 and the antimony trisulfide would form the inner photoconductive layer 13. Alternatively, Se- Te may be employed for the inside layer 13. When such a sandwich of photoconductive layers is illuminated by white light, as obtained from a white light lamp 5 or by light obtained from a mercury and neon lamp as indicated by lamps 6 and 7, both

layers

12 and 13 are rendered conductive. Such photoconductive materials having different spectral sensitivities are typically characterized by materials having substantially different bandgap energies. Thus, the outside photoconductive layer 12 would have a bandgap energy substantially greater than the bandgap energy for the inside photoconductive layer 13. When white light is used for illuminating the spot pattern projected onto the pickup screen 2, the white light contains optical radiant energy within the spectral range of both

photoconductive layers

12 and 13. It is also preferred that the inside layer 13 have an electrical conductivity substantially greater than the electrical conductivity of the outside layer 12 when the layers are illuminated by light within a spectral range to render both photoconductive layers conductive.

In the next step of the counting method, as illustrated at c, the spectral range of the light employed for illuminating the spot pattern is altered in such a manner as to render the inside photoconductive layer 13 conductive in the original spot pattern region 31 and to exclude optical radiant energy within the spectral range which would render the outside photoconductive layer 12 conductive. Thus, the inside layer 13 retains the spot charge pattern established in step b while the outer photoconductive layer 12 is rendered nonconductive. One way to alter the spectral range of the light which illuminates the spot pattern is to employ a white light lamp 5 and to insert the red filter 11 as indicated in FIG. 1. The red filter permits only red light to pass through the negative 8 for illuminating the screen 2 in the spot pattern. Alternatively, the red filter may be eliminated and, when employing blue and red lamps as indicated by mercury lamp 6 and neon lamp 7 as the light source 4, the blue lamp 6 may be extinguished leaving only the red lamp 7 for illumination of the spot pattern.

In the next step illustrated at d, the charge pattern on the inside surface of the inner photoconductive layer 12 is scanned by the electron scanning beam 14. When the scanning beam 14 first encounters the conductive spot region 31 of the inside photoconductive layer 12, charge flows from the beam into the entire spot region 31 to uniformly charge the entire spot to 30 volts such that the spot is eradicated and in the process a displacement current flows in the circuit including the source 17, load resistor 19 and electrode 16 to produce an output pulse across load resistor 19 which is coupled to output terminal 22 via coupling capacitor 21. This pulse of displacement current corresponds to a count of the spot and is fed to a suitable counter circuit, not shown. Since the spot region 31 is charged by the beam to the voltage corresponding to the non-spot background on the inside surface of the photoconductive layer 13, the spot will not again be counted when it is encountered by the scanning electron beam 14, thereby preventing false counts. After the spot pattern on the screen 2 has been counted, in the manner as above described, the screen 2 may be illuminated with light which will render both inner and outer

photoconductive layers

13 and 12 conductive to erase any remnant of the spot pattern, such that the pickup screen 2 of the vidicon tube 1 is then in a condition to be uniformly charged to initiate a second count of a second spot pattern to be counted.

The advantage of using the vidicon tube 1 with a pickup screen 2 having an inside photoconductive layer 13 and an outside photoconductive layer 12 with differing spectral sensitivities is that it eliminates false counts and permits counting of spot patterns without the necessity of a computer or other complicated device for processing the counting data to obtain an accurate count of the spots in the pattern.

Although a preferred embodiment of the invention has been described which employs an outer photoconductive layer 12 which has a bandgap energy substantially greater than that of the inner photoconductive layer 13 this is not a requirement. It is only required that the two photoconductive layers have substantially different spectral sensitivity characteristics such that the inside layer may be rendered conductive while permitting the outside layer to be nonconductive in accordance with the altered spectral range of the illumination of the spot pattern.

The spot counting output at terminal 22 comprises a train of randomly spaced electrical pulses, as indicated at 41 of FIG. 1. The height of and the area under each counting pulse is proportional to the size of the spot being counted. Thus, in a preferred embodiment of the present invention, a pulse height or pulse area analyzer 42 is connected to the output terminal 22 for analyzing the counting pulses. The pulse height or area analyzer 42 analyzes the pulses and has a series of output channels 43 which provide a readout of the number of counting pulses falling within certain preselected size ranges as determined by the limits of certain pulse height or area windows as established by discriminating circuits within the pulse analyzer 42. In this manner, valuable information about the spot size distribution is obtained.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a spot counter apparatus; means forming an evacuated vidicon image pickup tube employing an electron beam for scanning a pickup screen for counting spots in an optical spot image pattern illuminating said pickup screen; the improvement wherein, said pickup screen includes, an outer photoconductive layer facing the source of illumination for the spot image pattern to be counted, and an inside photoconductive layer overlaying said outer layer and facing the scanning electron beam, said inner layer having a photoconductive sensitivity to optical -radiant energy Within a certain spectral range which is substantially different than the photoconductive sensitivity of said outer photoconductive layer to the same spectral range of optical radiant energy.

2. The apparatus of claim 1 including, means for illuminating said pickup screen of said vidicon tube with an optical spot image pattern to be counted, such illumination being within a spectral range to render both said inner and outer photoconductive layers conductive, and means for altering the spectral range of the optical radiant energy forming the illuminated spot image pattern on said pickup screen to render said outer photoconductive layer nonconductive while rendering said inner photoconductive layer conductive in accordance with spot image pattern to be counted.

3. The apparatus of claim 2 wherein said means for altering the spectral range of the illumination applied to said pickup screen includes, an optical filter for filtering out optical radiant energy Within the spectral range which renders said outer photoconductive layer conductive.

4. The apparatus of claim 2 wherein said means for illuminating said pickup screen to render both said inner and said outer layers conductive includes first and second light sources producing optical radiant energy within substantially different optical spectral ranges.

5. The apparatus of claim 1 wherein the photoconductive material of said outer photoconductive layer has a bandgap energy greater than the bandgap energy of the photoconductive material of said inner photoconductive layer.

-6. The apparatus of claim 1 wherein the photoconductivity of said inner layer is substantially greater than the photoconductivity of said outer layer.

7. In a method for counting spots the steps of, presenting the pickup screen of a vidicon tube to a source of an optical spot image pattern to be counted, such a vidicon pickup tube having outer and inner photoconductive pickup screen layers of differing spectral sensitivity in the optical spectral range, the outer layer facing the source of the optical image and the inner layer facing the scanning electron beam of the vidicon tube, causing the beam to uniformly charge the beam side of the inner layer, illuminating the pickup screen with optical radiant energy containing the spot image pattern to be counted, such optical energy being within the optical spectral range to render both inner and outer photoconductive layers conductive in a spot pattern corresponding to the spot pattern to be counted, causing the charge to be drained from the face of the inner layer through both photoconductive layers in a pattern conforming to the spot image to be counted, altering the spectral range of the spot pattern illumination applied to the screen to render the outer layer nonconductive and the inner layer conductive in accordance with the spot pattern, scanning the inner photoconductive layer with the scanning electron beam of the vidicon tube, and detecting changes in the current flowing to charge the inner photoconductive layer to produce a spot count output.

8. The method of claim 7 wherein the step of altering the spectral range of the spot pattern illumination includes the step of, filtering out the optical illumination of the spot pattern optical radiant energy which renders the outer photoconductive layer conductive.

9. The method of claim 7 wherein the step of illuminating the pickup screen with optical radiant energy having a spectral range to render the inner and outer layers conductive comprises the steps of, illuminating the pickup screen with illumination from first and second light sources, one of the light sources causing the inner layer to be conductive and the other source causing the References Cited outer layer to be conductive. UNITED STATES PATENTS 10. The method of claim wherein the step of aliering the spectral range of the spot pattern illumination in- 2,927,212 3/1960 Young 250*217 XR eludes the step of, substantially removing that portion of gi sgigg 25 th t tt '11 ct' odu d b 0 f th 1 1 i fj fg 1 ummnon p1 C6 y O e 3,289,024 11/1966 De Haan et a1. 313- 94 XR 11. The apparatus of claim 1 wherein the output of said vidicon tube is a train of spot counting pulses, and RODNEY BENNETT P'lmmy Exammer including means forming a pulse analyzer for analyzing JEFFREY P. MORRIS, Assistant Examiner. the spot counting pulses to derive an Output representative of the spot size distribution in the spot image pattern X- being countfid- 88-14; 1786; 235-4 2; 250-417; 31365, 96