JPH0560311B2 - - Google Patents

Description

Detailed Description of the Invention (Description of the Technical Field) The present invention provides a method in which the interval between the generation of photoelectrons excited by photons and emitted from the photocathode can be resolved into units of time corresponding to the pixels of a television imaging device. The present invention relates to an imaging device that captures relatively weak images.

(Description of conventional technology) SIT (SILICON INTENSIFIED TARGET)
The lower limit of the illuminance of an image that can be captured with an imaging device using 10 -3 Lux is about 10 ^-3 Lux.

The following devices have been proposed as imaging devices for the above-mentioned weak images that cannot be captured by such high-sensitivity imaging devices.

Photons from the aforementioned weak image are made incident on the photocathode of the image intensifier.

Each photoelectron excited by a photon is multiplied, causing a corresponding position on the fluorescent surface corresponding to the position where the photoelectron was generated to emit light.

Bright spots appear and disappear on the fluorescent surface in response to the generation of photoelectrons.

This image is captured on television at a framing speed such that individual images (bright spots) corresponding to one photoelectron do not overlap.

Prepare a two-dimensional image memory (frame memory) with addresses corresponding to each position on the fluorescent surface,
Add 1 to the address corresponding to the bright spot for each frame period.

By capturing a large number of frames, each address in the frame memory stores the number of occurrences of bright spots on the fluorescent surface corresponding to that address.

By reading out the contents of this two-dimensional image memory and displaying it on the screen of a monitor, the two-dimensional weak image can be reconstructed as a visible image.

However, when the image is weak, the following problems arise.

The output of an image intensifier having a photocathode, a microchannel plate, and a fluorescent surface is considered to be the sum of the noise generated by the image intensifier, photoelectrons excited by photons, and the signal output of the microchannel plate.

Thermionic electrons generated regardless of the photons incident on the photocathode, thermionic electrons generated by the microchannel plate, etc. are multiplied by the microchannel plate and output.

Therefore, when imaging is carried out for a long time, noise is accumulated in the same way as the signal, impairing the resolution of the reproduced image.

When a single photoelectron is multiplied by a microchannel plate, it spreads out and enters the fluorescent surface, so the bright spot that should ideally correspond to one pixel may correspond to many pixels. This causes a decrease in resolution.

Next, the imaging device has a problem in that the imaging state (for example, the focusing state of the optical system) cannot be evaluated until after a relatively long period of time has elapsed.

(Description of Object) The main object of the present invention is to provide an imaging device for capturing weak images that can solve the problems of the conventional imaging device for capturing weak images described above.

(Description of Configuration) In order to achieve the above object, an imaging device for capturing weak images according to the present invention includes an image intensifier having a photocathode, a microchannel plate, and a fluorescent surface, and a fluorescent light of the image intensifier. A television imaging device that captures an image formed on a light surface, and a video signal included in a video signal of the television imaging device, with a central portion of a signal waveform caused by a single electron generated by the photocathode as significant. a binarization circuit having a threshold value for extraction, a frame memory having an address corresponding to a pixel of a television image, and a first digitization circuit in which photons from a photon source to be measured are incident on the photocathode of the image intensifier; A significant part of the output of the binarization circuit is added to the corresponding address of the frame memory for a period of time, and a second It consists of a control circuit that subtracts the significant portion of the output of the binarization circuit from the corresponding address during the period, and an image reproducing device that outputs the calculation result.

(Description of Examples) Hereinafter, the present invention will be described in more detail with reference to the drawings and the like.

FIG. 1 is a block diagram showing an embodiment of a two-dimensional weak image capturing apparatus according to the present invention.

The embodiment device described in detail below has two modes: weak video imaging in units of one photoelectron (hereinafter referred to as digital imaging mode), and normal television imaging mode (hereinafter referred to as analog imaging mode) that precedes the digital imaging mode. Both modes can be implemented.

The digital imaging mode includes the following steps.

(Image Capturing Step) The image capturing step refers to a step in which the output of the image intensifier is imaged and recorded for a certain period of time while the image intensifier receives photons from a weak image to be imaged.

In the data recorded at this time, noise not caused by the photons, such as noise caused by thermoelectrons generated by the photocathode, is also recorded at the same time.

(Background imaging step) What is the background imaging step?
This refers to the step of capturing and recording the output of the image intensifier for a certain period of time while blocking photons from weak images.

(Calculation step) Refers to the step of performing calculations between the records obtained in the image capturing step and the background image capturing step to remove the background.

A photocathode 1a is provided on the inner surface of the vacuum container of the image intensifier 1.

Photons from the photon source O (image to be measured) are guided to the photocathode 1a by an imaging optical system 21.

The imaging optical system 21 captures the image of the photon source O on the fluorescent surface 1a.
It is provided so that it can be adjusted so as to form an image.

Each electron (including thermoelectrons not caused by photons) generated by the photocathode 1a is brought to a corresponding position on the surface of the microchannel plate 1c by the electron lens 1b.

In this embodiment, three microchannel plates 1c are stacked to increase the multiplication factor.

One electron generated by the photocathode 1a is multiplied by the microchannel plate 1c, and a large number of electrons are made to collide with the fluorescent surface 1d of the image intensifier. On the other hand, the thermoelectrons generated from the three microchannel plates 1c themselves are also multiplied by themselves. However, in this case, the number of electrons incident on the fluorescent surface 1d is smaller than when electrons are emitted from the photocathode.

This image is formed via the optical system 2 on the photocathode of the vidicon 3, which has low afterimage characteristics.

The image intensifier 1 and the vidicon 3 are supplied with an operating voltage by a drive power supply circuit 4 and a drive power supply circuit 5, respectively.

An image of a bright spot on the fluorescent surface 1d of the image intensifier 1 formed on the photocathode of the vidicon 3 is taken out by television scanning.

Horizontal scanning frequency 15.75KHz, vertical scanning frequency 60
Hz, interlace 2:1, and screen aspect ratio 3:4.

The video signal output from the video controller 3 is amplified by a video signal amplification circuit 6 and connected to a switching circuit 8.

The switching circuit 8 connects the amplified video signal to the binarization circuit 7 in the digital imaging mode, and connects the amplified video signal to the television monitor 12 in the monitor mode.

Here, the bright spots of the image intensifier 1 and the video signals corresponding to the bright spots in the digital imaging mode will be explained.

FIG. 2 shows a model of the brightness distribution on the fluorescent surface 1d caused by one electron generated on the photocathode 1a.

The magnitude of brightness is shown in the direction perpendicular to the fluorescent surface 1d.

The energy of this single brightness is not constant but has a statistical distribution.

Figure 3 shows the energy distribution.

Figure 3 shows that the probability of occurrence of luminance with energy E ₂ is the highest.

This curve is considered to be a combination of a distribution n caused by thermoelectrons and the like and a distribution s caused by photoelectrons excited by photons. More specifically, among the distribution n, those with low energy correspond to thermoelectrons etc. generated by the microchannel plate 1d itself, and these are large in quantity. On the other hand, among the distribution n, those with high energy correspond to thermoelectrons etc. generated at the photocathode 1a, and it is difficult to distinguish them from photoelectrons (distribution s). Therefore, by binarizing with a threshold value corresponding to a predetermined level of this energy, it becomes possible to remove most of the noise components caused by thermoelectrons.

In the present invention, a threshold value corresponding to an intermediate value between energy E ₂ and energy E ₁ is set in the binarization circuit 7 to binarize the video signal. This threshold level can be arbitrarily set while observing the video signal.

The output of the binarization circuit 7 is sent to an arithmetic and storage circuit 9.
It is connected to the.

The calculation and storage circuit 9 includes a first frame memory 9a, a second frame memory 9b, a third frame memory 9c, and a calculation unit 9d. Each frame memory has a capacity of 512 x 512 x 16 BITS, and each address defined by 512 x 512 corresponds to a pixel of television image pickup.

In the background imaging step of the digital imaging mode, the output of the binarization circuit 7 is connected to the first frame memory 9a for a certain period of time.

In the image capturing step, the output of the binarization circuit 7 is connected to the second frame memory 9b for a certain period of time.

The arithmetic unit 9d is a circuit that calculates the contents of the first and second frame memories 9a and 9b in accordance with addresses. The calculation result is stored in the third frame memory 9c. Third frame memory 9c
The contents of are connected to the television monitor 12 via a DA conversion circuit.

The control circuit 10 is a circuit that controls the order of operation of the various parts described above.

Next, an example of use of the device will be explained.

Analog imaging mode is implemented before digital imaging mode.

Objects that are digitally imaged objects cannot normally be observed with the naked eye.

With the output of the video amplification circuit 6 connected to the television monitor 12 by the switching circuit 8, objects to be digitally imaged will be explained using other light sources. The operator can adjust the focus of the imaging lens 21 while observing the screen of the television monitor 12.

In the digital imaging mode, the switching circuit 8 connects the output of the video amplification circuit 6 to the binarization circuit 7. Either the step of imaging the state in which the noise and signal are superimposed or the step of imaging the background may be performed first, but in order to make it easier to understand, the state in which the noise and signal are superimposed will be imaged first. Let's explain step by step. In this step, the control circuit 10 connects the output of the binarization circuit 7 to the second frame memory 9b.

As shown in FIG. 2, the bright spots appearing on the fluorescent surface of the image intensifier 1 correspond to 6 to 8 pixels, assuming that the horizontal scanning line coincides with the highest brightness point. FIG. 2 shows a portion of the frame memory row having 512 addresses corresponding to the scanning line.

Now, if the threshold value of the binarization circuit 7 is at the level indicated by Th in the figure, three 1's will be written to three addresses corresponding to the row of the frame memory 9b corresponding to the scanning line.

FIG. 4 is an explanatory diagram showing the principle of writing.

In the figure, a is the output of the video signal amplification circuit 6 corresponding to a specific scanning line, and b is the output 2 corresponding to the output.
The output c of the value conversion circuit 7 indicates the content written in the row of the second frame memory 9b corresponding to the scanning line.

When a video signal shown as a appears on a specific scanning line in the screen during the first one-screen television scan _F1 , it is binarized by the binarization circuit 7 as shown in b of _F1 .

1 is written to one address in the row of the frame memory 9b corresponding to the first output of the binarization circuit 7,
1 is written to each of the two addresses corresponding to the next output.

In the second television scan _F2 , the scanning line at the same position on the screen as the specific scanning line on the screen is
When the video signal shown in F ₂ (a) appears, it is binarized by the binarization circuit 7 as shown in F ₂ (b).

As a result, in the row of the frame memory 9b,
1, 1 corresponding to the first signal and 1 corresponding to the next signal are written, as shown in F ₂ (c).

In the third television scan F ₃ , the scanning line at the same position on the screen as the specific scanning line on the screen is
When the video signal shown in F ₃ (a) appears, it is binarized by the binarization circuit 7 as shown in F ₃ (b).

As a result, in the row of the frame memory 9b,
1, 1 corresponding to the first signal and 1 corresponding to the next signal are written, as shown in F ₃ (c).

Changes in the contents of the illustrated rows due to the scanning of the three frames are summarized below.

F ₁ 0100000000110 F ₂ 0210001000110 F ₃ 1310001010110 In this way, imaging is repeated a large number of times, and data in which noise and signals are superimposed is stored in the frame memory 9b. For example, if 150 frames are captured in 5 seconds, and there is a region of the photocathode that emits one electron for each frame, the contents of the address corresponding to the photocathode in the frame memory 9b are
It will be 150.

Although the specific row has been described above, similar recording is performed on other rows as well, and data related to the image is recorded based on the arrangement of all rows.

In the background imaging step, the incidence of photons from the object to be imaged on the image intensifier 1 is blocked by a shutter or the like (not shown). At this time, the output of the binarization circuit 7 is connected to the first frame memory 9b by the control circuit 10.

Then, imaging is performed for the same time as the step and imaging time of the image imaging.

As previously explained with reference to FIG. 3, thermoelectrons may be emitted even if no photons are incident on the photocathode of the image intensifier 1.

If the output caused by these thermoelectrons exceeds the threshold of the binarization circuit, it is stored as significant at the corresponding address of the first frame memory 9a in the same way as described above.

The calculation step is performed in the second frame memory 9.
In this step, the content stored in the frame memory 9a and the content stored in the first frame memory 9a are subtracted pixel-by-pixel by the arithmetic unit 9d, and the result is stored in the third frame memory 9c.

A simple example of the contents of corresponding addresses in the first frame memory 9a, second frame memory 9b, and third frame memory 9c will be shown.

9b row n (4436522552110) n+1 row (4436522555110) 9a n row (4433222222110) n+1 row (4433222222110) 9c row n (0003300330000) n+1 row (0003200333000) This The contents of the third frame memory 9 are influenced by the background. This is caused by the removed photons.

The contents of the third frame memory 9c are connected to the television monitor 12 via the DA converter 11 and displayed.

Various modifications can be made to the embodiments described in detail above within the scope of the present invention.

A solid-state image sensor can also be used instead of the vidicon. As described above, the present invention relates to a device for capturing a two-dimensional single photon image, and a single electron emitted from a photocathode 1a by a single photon is captured by a microchannel plate 1c of an image intensifier 1. The light is multiplied and an optical image is generated on the fluorescent surface 1d. Although the electron multiplication factor of the microchannel plate 1c is extremely high, it may itself emit thermal electrons, which are also multiplied and reach the fluorescent surface 1d. However, the brightness level on the fluorescent surface 1d due to thermoelectrons from the microchannel plate 1c itself is generally lower than the brightness level due to electrons incident on the microchannel plate 1c from the photocathode 1a (see FIG. 3); Separation can be performed by the binarization circuit 7. Therefore, since a single electron is generated by a single photon, accurate data can be obtained by the binarization described above.

Furthermore, since the single photon image of the present invention is two-dimensional, the noise image due to thermionic electrons from the microchannel plate 1d is also two-dimensional. For this reason, two-dimensional noise processing is performed by using a frame memory. In particular, this two-dimensional noise is not at a uniform level on a two-dimensional plane and varies depending on the position, so it is particularly important for obtaining accurate images. It should be noted that in this respect, it is completely different from background noise that appears uniformly.

(Description of Effects) As described above, according to the present invention, good imaging can be performed in a weak light region by using a binarization circuit and an image memory without causing a decrease in resolution.

Adjustment of the optical system improves operability by focusing while displaying an analog image and then capturing the image digitally.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an imaging apparatus for capturing weak images according to the present invention. FIG. 2 is a perspective view showing a model of the brightness distribution of one bright spot generated on the fluorescent surface of the image intensifier. FIG. 3 is a graph showing the energy distribution of bright spots. FIG. 4 is a graph showing the relationship between signals extracted by horizontal scanning and storage. DESCRIPTION OF SYMBOLS 1... Image intensifier, 3... Busicon, 4... Image intensifier power supply circuit, 5... Busicon drive circuit, 6... Video signal amplification circuit, 7... Binarization circuit, 8... Switching circuit, 9...Storage and calculation circuit, 9a...First frame memory, 9b...Second frame memory, 9c...Third frame memory, 9d...Subtraction circuit, 10...Control circuit, 11...DA conversion circuit, 12...TV monitor, 21...imaging lens, 22...relay lens.

Claims (1)

[Claims] 1. An apparatus for capturing a two-dimensional single photon image, which includes a photocathode, a microchannel plate,
an image intensifier having a fluorescent surface; a television imaging device for capturing an image formed on the fluorescent screen of the image intensifier; and a television imaging device for capturing an image formed on the fluorescent screen of the image intensifier; A binarization circuit has a threshold value that extracts as significant the central part of the signal waveform caused by a single electron generated by the photon, a frame memory having an address corresponding to the pixel of the television image, and a A significant portion of the output of the binarization circuit is added to the corresponding address of the frame memory for a first period while the photons are incident on the photocathode of the image intensifier, and the image intensifier of the photons from the photon source to be measured is During the second period 2 above, the shifter is blocked from entering the photocathode.
An imaging device that captures a weak image, and includes a control circuit that subtracts a significant portion of a value output from a corresponding address, and an image reproducing device that outputs the calculation result. 2. The imaging device for capturing weak images according to claim 1, wherein the first and second periods are approximately equal in time. 3. An imaging device for capturing weak images according to claim 1, wherein the image intensifier includes a plurality of microchannel plates. 4. An imaging device for capturing a weak image according to claim 1, wherein the imaging device scans the photocathode of the image intensifier by standard television scanning. 5. The imaging device for capturing weak images according to claim 1, wherein the photoelectric conversion section of the imaging device is a low-afterimage type vidicon or a solid-state imaging device. 6 A device that captures a two-dimensional single photon image, comprising an image intensifier having a photocathode, a microchannel plate, and a fluorescent screen, and a television imaging device that captures an image formed on the fluorescent screen of the image intensifier. and 2 having a threshold value for extracting the central part of the signal waveform caused by a single electron generated by the photocathode as significant in the video signal included in the video signal of the television imaging device.
a value conversion circuit; first and second frame memories having addresses corresponding to pixels of television imaging; a computing unit for performing calculations between the respective frame memories; With the input to the photocathode of the shifter blocked, the output of the binarization circuit is switched and connected to the first frame memory, and the significant part of the output of the binarization circuit is sent to the corresponding address for a first period. add it,
The output of the binarization circuit is switched and connected to the second frame memory while photons from the photon source to be measured are incident on the photocathode of the image intensifier, and the output of the binarization circuit is switched for a second period. a control circuit that causes the significant part of the output to be added to a corresponding address and causes the arithmetic unit to calculate a difference for each pixel corresponding address of the first and second frame memories; and an image reproducing device that outputs the arithmetic result. An imaging device that captures weak images. 7 A device for capturing a two-dimensional single photon image, comprising: an image intensifier having a photocathode, a microchannel plate, and a fluorescent screen; and a focus-adjustable optical device that forms a measured image on the photocathode of the image intensifier. a television imaging device for capturing an image formed on the phosphor screen of the image intensifier; a television monitor for reproducing the video imaged by the television imaging device; and an image of the television imaging device. a binarization circuit having a threshold value for extracting the central part of the signal waveform caused by a single electron generated by the photocathode as significant in the video signal contained in the signal; and a second frame memory, a computing unit that performs calculations between the frame memories, and the binarization circuit in a state where photons from the photon source to be measured are blocked from entering the photocathode of the image intensifier. The output is switched and connected to the first frame memory to add a significant part of the output of the binarization circuit to the corresponding address for a first period, and the photoelectric conversion of the photons from the photon source to be measured to the image intensifier is performed. The output of the binarization circuit is switched and connected to the second frame memory while the light is incident on the surface, and the significant part of the output of the binarization circuit is added to the corresponding address for a second period. an object that can normally be imaged at the position of the weak photon source to be measured; Alternatively, an imaging device that captures a weak image is configured to form an image and adjust the focus of the imaging optical system while observing the image on the television monitor.