JP2670405B2 - Weak light measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a weak light measuring device such as a streak camera system, a weak image pickup device or the like.

[0002]

[Prior Art and Problems to be Solved by the Invention] FIG.
Shows the configuration of a conventional photon counting streak camera system. The conventional streak camera system includes a streak tube 101, a driving circuit 102 for operating the streak tube 101, and a fluorescent screen 10 of the streak tube 101.
The streak image formed on the 1a is displayed on the relay lens 104.
A high-sensitivity television camera 105 that captures images via the
The image processing apparatus 106 is configured to store and process. The streak tube 101 is formed on the inner surface of the light receiving face plate on which the measured light shown in the figure is incident, and emits photoelectrons when the measured light is incident (not shown), and an acceleration electrode 101b for accelerating the photoelectrons. , A deflection electrode 10 for sweeping accelerated photoelectrons in a direction orthogonal to the traveling direction thereof
1c and a pair of microchannel plates (MC that emits multiplied electrons by multiplying swept photoelectrons by secondary electrons).
PDP) and the MCP device 101d.

The temporal change of the light to be measured is converted by the streak tube 101 into a streak image in which the fluorescent intensity (luminance) spatially changes on the fluorescent screen 101. The streak image is captured by the high-sensitivity camera 105 and converted into a video signal. The video signal is converted by the image processing device 106 into A
The image is integrated as image data in order to improve the S / N ratio of the signal. The image data obtained by the image processing device 106 includes coordinate values and intensity values. The coordinate value is the position on the time axis corresponding to the time change of the measured light,
This corresponds to a position on a spatial axis in the longitudinal direction of a slit (not shown) provided on the photoelectric surface side of the streak tube 101. The intensity value corresponds to the light intensity of the measured light. The image processing device 106 can extract the time waveform of the measured light intensity from the obtained image data, and further display / analyze it.

Regarding the conventional streak camera system as described above, reference is made to "SPIEol.693".
p99, High Speed Photograph
y, Videography, and Photoni
cs IV (1986) ", JP-A-59-58745 and the like.

However, in the conventional streak camera system, since the MCP device 101d provided in the streak tube 101 is operated in a saturated state with respect to the incidence of a single photoelectron, an extremely weak light phenomenon can be easily and accurately measured. It was difficult to measure well. More specifically, in the case described above it was multiplied until saturated single photoelectron about 10 ⁶ times by superimposing two pieces of MCP. Therefore, the detection accuracy of the incident position of a single photoelectron on the MCP has been reduced due to the spatial spread of the multiplied electrons. Although such detection accuracy, that is, the decrease in resolution, can be solved to some extent by obtaining the center of gravity of the bright spot spread on the phosphor screen, complicated calculation processing is required to detect the center of gravity of the bright spot, and a considerable processing time is required. There was a problem that needed. In addition, putting two MCPs in the streak tube lowers the yield rate of the streak tube and increases the cost. The streak tube failure rate with one MCP is 3
If it is 0%, the defective rate becomes 50% by inserting two sheets, which is a problem in providing an inexpensive system. Furthermore, the high-sensitivity TV camera 106 is used for imaging the streak tube.
However, there is a problem that it becomes a large-scale system.

FIG. 9 shows a conventional example of a weak image pickup device using an image intensifier (light image intensifying type image conversion tube) and a CCD camera. The image to be measured is formed on the photocathode 202a of the image conversion tube 202 by the objective lens 201. Photoelectrons proportional to the intensity of the image are emitted from the photocathode 202a, and are incident on the MCP 202b having a large number of fine holes having secondary electron emission surfaces formed on the inner surface. MC
The electrons incident on the P202b are amplified about 10 ⁴ times by the MCP 202b, collide with the fluorescent surface 202c, and are converted into an optical image (fluorescent image) again. The output image of the phosphor screen 202c is guided to the fiber plate 203, and the CCD 2
04 and is converted into an electric signal. The output signal of the CCD 204 is amplified by the video amplifier 205, A / D converted, and input to the adder shown in FIG. The adder reads the contents of the image stored in the image memory up to the previous time and adds the input signals. The result is stored again in the image memory as new data.

A technique for improving the S / N of a detected image by repeatedly performing such an adding operation is well known and is widely used because it is simple. In this case, the signal level is increased by the addition, but since the dark current of the CCD element and the noise generated by the video amplifier are also the objects of this addition, the noise level is also increased and the detection of a weak image in the single photoelectron counting area is detected. Had the problem of being difficult.

A photon counting image measurement method is known as a method for acquiring an image of a weak light phenomenon in a single photon counting region which cannot be detected by a weak image pickup device as shown in FIG. Photon counting image measurement method is a multi-stage M
In electron multiplier device comprising a CP, a single photoelectron corresponding to the optical phenomenon (event) to amplify more than 10 ⁶ times, and the noise and CCD readout noise generated inside the image intensifier, a weak optical phenomena This is a method of clearly separating the signal due to the single photoelectron of (3) and detecting only the signal component with high sensitivity.

FIG. 10 shows a weak image pickup device using this photon counting image measuring method. In the illustrated device, in order to amplify a single photoelectron more than 10 ⁶ times, two-stage M
MCP with CP202d and 202e connected in cascade
The device is used. Since the MCPs are connected at regular intervals as shown in the figure, the MCP 202 of the first stage is connected.
The output electron of channel 1 of d is the MCP20 of the second and subsequent stages.
It is input to a plurality of channels 2e (usually 30 or more) (see FIG. 11A). For this reason, the output electrons in the second stage for a single photoelectron are a combination of electrons from a plurality of channels, and therefore the output electrons spread spatially (see FIG. 11A), so that they are on the phosphor screen 202c. The bright spot expands. Further, at this time, since the second-stage MCP 202e is used in a region where the number of output electrons is saturated with respect to the number of input electrons, the bright spots on the phosphor screen are flattened as shown by the solid line in FIG. 11 (b). It becomes a luminance distribution. As a result, the resolution (positional resolution) is significantly deteriorated. FIG. 11 is a diagram showing bright spots generated on the phosphor screen by multiplication of single photoelectrons. FIG.
1 (a) is two MCs connected at regular intervals.
Shows the incidence of the electron group multiplied by P on the phosphor screen,
FIG. 11B shows the distribution of the emission intensity on the phosphor screen by the multiplied electrons.

In order to improve such resolution degradation,
In the weak image pickup device of FIG. 10, a barycentric position detection function is provided in the preceding stage of the image memory to calculate the barycentric position of the bright spot, and the calculation result is sent to an adder and stored in the image memory. In the process of detecting the center of gravity, the readout noise (C
The noise generated by the CD and the image amplification circuit) is completely removed, and does not increase even in a long-time calculation.

As described above, the weak image pickup device of FIG. 10 is an excellent method for detecting a weak image of a single photon region, but has the following problems.

(1) In order to saturate the number of output electrons with respect to the number of electrons input to the electron multiplying means, it is necessary to use two or more MCPs. The non-defective rate of the image conversion tube decreases, and the cost of the image conversion tube increases. The MCP is, for example, a collection of 3 million or more glass tubes and is apt to cause clogging and discharge defects. If a multi-stage MCP is used, the defective rate becomes worse by multiplying the number of MCPs, which causes a cost increase.

(2) In order to improve the resolution degradation,
The center of gravity position detection function is required. Usually, the reading rate of a TV camera is 30 frames per second, and the barycentric position detection function requires a considerably high speed computing ability. Such a function is extremely expensive.

(3) Since the output fluorescence image for a single photoelectron incident on the MCP is spatially large, the detection accuracy of the incident position of the single photoelectron is poor, and sufficient position resolution cannot be obtained. That is, when two light spots on the photocathode gradually approach, the two points cannot be separated from the place where the output fluorescent images start to overlap, and the light spot is detected as one point. For this reason, the measurement accuracy is significantly deteriorated when the amount of light is large to some extent or when there is a partially bright place. FIG. 12 is a diagram specifically illustrating a decrease in resolution. As shown in FIG. 12A, if the points P ₁ and P ₂ are sufficiently separated, the single photoelectron images do not overlap and are measured independently. On the other hand, when the points P ₁ and P ₂ approach each other as shown in FIG. 12B, the fluorescent images resulting from the single photoelectrons generated at the points P ₁ and P ₂ partially overlap on the fluorescent screen. Therefore, the calculation result of the center of gravity becomes the center point of the points P ₁ and P ₂ , and the two cannot be separated. That is, even if the function of detecting the position of the center of gravity is provided, the deterioration of resolution cannot be completely overcome.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has as its object to improve the yield of image conversion tubes, eliminate the need for a center-of-gravity position detection function, and have a high level of weak optical phenomena. It is to provide a small and lightweight weak light measuring device capable of performing precise measurement with position resolution.

[0016]

In order to solve the above-mentioned problems, a weak light measuring apparatus according to the present invention comprises: (a) a photoelectric conversion means for generating photoelectrons in response to the incidence of light to be measured; An electron multiplying means for multiplying photoelectrons from the means so as not to be saturated by the incidence of one electron, and emitting the multiplied electrons from a position corresponding to the incident position of the photoelectrons; and an electron multiplied by the electron multiplying means. (B) an imaging unit having a light emitting unit for converting the light image into an optical image; and (b) an output imaging unit for converting a light image from the imaging unit into a video signal output having a level corresponding to the light intensity, and for each pixel, (C) A photoelectron detection signal corresponding to a signal portion exceeding a predetermined level determined based on the level of noise generated from the video signal output from the image pickup means without being caused by the incidence of one electron to the electron multiplier. , And extracted for each pixel of the image pickup means,
(D) photoelectric conversion by accumulating the output of the signal extracting means for each pixel for a predetermined time based on the timing of the video signal output from the imaging means; Signal processing means for measuring photons incident on the means.

[0017]

According to the above weak light measuring device, the electron multiplying means constituting the imaging means corresponds to one event.
It is not saturated by the incidence of electrons. Therefore, the area of the region where the multiplied electrons enter the light emitting means is not so large,
The detection accuracy of the incident position of one photoelectron corresponding to one event can be improved. Moreover, since a sufficient luminance distribution is generated in the light image obtained by the light emitting means, the center of gravity of the obtained light image is detected by extracting the signal portion from which the noise portion has been removed from the electric output by the signal extracting means. Without this, the incident position of one photoelectron can be easily specified.

The position of one photoelectron incident on the two-dimensional electron multiplying means is specified by extracting a signal component above the predetermined level in the electric output, binarizing the signal component, This is performed by detecting the peak of the signal component. When the event under the same condition is repeated, photon counting is performed by integrating these signal portions for each pixel for a predetermined time.

(1) The incident position of the photoelectrons is specified by determining the signal portion extracted by the signal extraction means, the peak of the signal portion detected by the signal peak detection means, and the binary signal detected by the binary signal detection means. This can be done by storing for each region corresponding to each pixel of the image pickup means, based on the timing of electric output from the image pickup means.

If the electron multiplying means is composed of a single microchannel plate, the yield rate of the device can be increased and the resolution can be increased more than ever before. In this case, by fiber-coupling the imaging means and the imaging means, it is possible to effectively eliminate the problem of insufficient gain caused by performing photon counting using a single microchannel plate. As a result, the optical coupling efficiency is 0.0
It is improved from 2 to 0.3 by one digit or more. Therefore, MCP
Despite the fact that there is only one, the signal level of a single photoelectron can be made sufficiently higher than the noise level of the image pickup means, and highly accurate photon counting detection is possible.

When the imaging means is a streak tube, the signal processing means can give a time change of the measured light based on the signal portion extracted by the signal extracting means.

When the imaging means is an image intensifier tube, the signal processing means can give a distribution image of the measured light based on the signal portion extracted by the signal extracting means.

A fiber plate may be used as a means for guiding the light image from the light emitting means provided in the imaging means to the imaging means.

When a CCD, a photodiode array, or other solid-state image pickup device is used as the image pickup means, the weak light measuring device as a whole can be downsized and an inexpensive system can be constructed.

[0025]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a block diagram of a streak camera system according to an embodiment. As shown in the figure, this system is formed on a streak tube 11 that forms a streak image from incident measured light, a drive circuit 13 for operating the streak tube 11, and a fluorescent screen 11 a of the streak tube 11. The solid-state imaging device 15 is configured to capture a streak image via the fiber plate 14 and a signal processing device 16 that processes an output video signal of the solid-state imaging device 15.

The streak tube 11 is a single MCP (micro channel plate) 1 for performing unsaturated electron multiplication.
1b. MCP has many small holes (channels)
Is a plate-shaped two-dimensional electron multiplying means provided with, and carries out secondary electron multiplication of the electrons incident from one surface while maintaining the positional information, and emits from the other surface. Photoelectrons are emitted from the photocathode 11c of the streak tube 11 in response to the incident light to be measured. The emitted photoelectrons are stored in the accelerating electrode 11d.
After being accelerated by the anode electrode 1 through the focusing electrode 11e.
It is incident on the opening at 1f. The photoelectrons that have passed through the opening of the anode electrode 11f are deflected at appropriate timing when passing between the deflection electrodes 11g, and enter the MCP 11b.
The photoelectrons incident on the MCP 11b are multiplied by the secondary electron emission surface formed on the inner surface of the fine aperture and then incident on the fluorescent surface 11a, and a bright spot is generated on the fluorescent surface 11a. In this case, since the MCP 11b operates so as not to be saturated with respect to the incidence of a single photoelectron, the spread of the bright spot corresponding to the incidence of a single photon, that is, a single photoelectron becomes extremely narrow. I have. Instead of the streak tube 11 described above, a combination of a streak tube without an MCP and an image intensifier with a non-saturated MCP may be used as the imaging means.

The fiber plate 14 is formed by bundling a number of thin optical fibers into a rectangular plate or a column. By adhering and arranging the fluorescent screen 11a and the solid-state image sensor 15 with the fiber plate 14 interposed therebetween, a streak image formed on the fluorescent screen 11a is optically efficiently (brightly) formed on the solid-state image sensor 15. Can be a statue. Although the fiber plate 14 is constituted by one sheet,
A plurality of these may be used. Further, a magnification can be applied to the fiber plate 14, and the area on the fluorescent screen 11a for imaging by the solid-state imaging device 15 can be changed by the magnification of the fiber plate 14.

As the solid-state image pickup device 15, for example, a CCD which is a two-dimensional solid-state image pickup device or a one-dimensional photodiode array can be used.

The signal processing device 16 has a built-in storage element and the like, and outputs a signal obtained by removing noise from an output video signal (a noise such as a video noise generated without being caused by a single photoelectron incident on the MCP). Signal exceeding the level corresponding to the solid state imaging device 15 corresponding to the phosphor screen 11a.
The optical time waveform can be formed by integrating each pixel.

Next, the operation of this embodiment will be described.
The incident measured light 10 is the photocathode 1 of the streak tube 11.
The photoelectrons projected and emitted to 1c are multiplied by the MCP 11b, and the multiplied electrons are projected to the phosphor screen 11a. At this time, the photoelectrons are deflected at appropriate timing by the deflection electrode 11g in the process of reaching the MCP 11b from the photocathode 11c, so that the measured light having a temporal intensity change has a streak image with a spatial brightness change. Is converted to Since the phosphor screen 11a and the solid-state imaging device 15 are optically coupled with the fiber plate 14 interposed therebetween, the converted streak image is passed through the fiber plate 14
5 is projected. The solid-state imaging device 15 converts the projected streak image into a video signal and supplies the video signal to the signal processing device 16. Then, the signal processing device 16 removes video noise from the video signal, and integrates only the signal portion of the measured light to form an optical time waveform.

Next, a method of removing video noise from a video signal, which is a feature of this embodiment, will be described with reference to FIGS.
This will be described with reference to FIG. When the measured light is weak, the video signal is a train of pulse signals as shown in FIG. In the figure, each pulsed signal is transmitted to a streak tube 11.
It corresponds to each photon of the measured light in each event detected by. Video noise generated in the solid-state image sensor 15 and the signal processing device 16 is also superimposed on the video signal, but the video noise has a smaller signal level (lower level) than the pulse-shaped signal described above. Therefore, a threshold is provided corresponding to the level of noise not caused by the single photoelectron incident on the MCP 11b, such as video noise, and only the video signal exceeding this threshold level is integrated to remove the video noise. It is possible to extract only the signal part.

A specific method of integration will be described with reference to FIGS. In the first method, as shown in FIG. 3, only the signal component (see FIG. 3B) exceeding the threshold level in the video signal (see FIG. 3A) is directly passed to the signal processing device 16. This is a method of integrating each pixel on a built-in storage element. FIG. 4 is a conceptual diagram showing integration into the storage element of the signal processing device 16. As shown in the figure, the noise-removed signal portion of the video signal supplied to the signal processing device 16 is added to the previously stored data in the storage element and stored again. Then the second
As shown in FIG. 5, the method of FIG.
(See FIG. 5B) is a binary signal obtained by binarizing (see FIG. 5) with a threshold level.
This is a method of sequentially accumulating pixel by pixel on the storage elements built in the signal processing device 16. In the third method, as shown in FIG. 6, a pulse signal (FIG. 6B) exceeding a threshold level in a video signal (see FIG. 6A) is used.
In this method, the number of peak detections is counted for each pixel by detecting the peak of the corresponding pixel and adding 1 to the value of the storage element corresponding to the position of the peak.

Regarding the integration or counting method, when the solid-state image pickup device 15 is, for example, a two-dimensional CCD, it is performed on one horizontal scanning line of the video signal or two-dimensionally. When the solid-state image sensor 15 is one-dimensional, the one-dimensional operation is naturally performed.

FIG. 7 is a diagram showing the structure of a weak image pickup device according to the embodiment. The image to be measured is formed on the photoelectric surface 52a of the image conversion tube 52 by the objective lens 51. Photoelectrons corresponding to photons forming an image are emitted from the photocathode 52a. After being accelerated by the acceleration electrode 52b, the photoelectrons enter the aperture of the anode electrode 52d via the focusing electrode 52c. The photoelectrons that have passed through the aperture of the anode electrode 52d enter a single MCP 52e that performs unsaturated electron multiplication. The photoelectrons that have entered the MCP 52e are multiplied here, and then enter the phosphor screen 52f while maintaining the positional information to generate bright spots. In this case, MCP52f
Since it operates so as not to be saturated with respect to the incidence of a single photoelectron, the spread of the bright spot corresponding to the incidence of a single photon is extremely narrow.

The output image from the fluorescent screen 52f is guided to the fiber plate 53 and converted into an electric signal by the solid-state image sensor 54. The video signal output from the solid-state image sensor 54 is processed by the signal processing device 55 and displayed on the TV monitor 56. The signal processing device 55 includes a noise removal unit 55a including an amplifier, a threshold processing circuit, an A / D converter and the like. This noise removal unit 5
Reference numeral 5a is a means for extracting a signal portion from the video signal output from the solid-state image sensor 54. An adder 55b and a pixel memory 55c are connected to the noise removal unit 55a. The adder 55b and the pixel memory 55c
Is a signal processing means for integrating the signal portion extracted by the noise removal unit 55a for each pixel of the solid-state image pickup device 54. The output of the noise removal unit 55a is
It is added by the adder 55b and stored in the image memory 55c. As a result, it is possible to obtain an image in which noise is removed and only signals from the measured light are integrated. The video noise removal processing in the signal processing device 55 is the same as that shown in FIGS. 3 to 6, and therefore detailed description will be omitted.

[0037]

As described above, according to the weak light measuring device of the present invention, the electron multiplying means constituting the imaging means is not saturated by the incidence of one electron corresponding to one event. Therefore, the area of the region where the multiplied electrons enter the light emitting means is not so large, and the detection accuracy of the incident position of one photoelectron corresponding to one event on the electron multiplying means can be improved. Moreover, since a sufficient luminance distribution is generated in the light image obtained by the light emitting means, the center of gravity of the obtained light image can be detected by extracting the signal portion from which the noise portion is removed from the electric output by the signal processing means. It is possible to easily specify the incident position of one photoelectron on the electron multiplying means without doing so.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a streak camera system according to an embodiment.

FIG. 2 is a diagram showing a video signal.

FIG. 3 illustrates a first method of removing video noise from a video signal.

FIG. 4 is a diagram showing processing in a signal processing means.

FIG. 5 is a diagram showing a second method of removing video noise from a video signal.

FIG. 6 is a diagram showing a third method of removing video noise from a video signal.

FIG. 7 is a configuration diagram showing a weak image pickup device according to an embodiment.

FIG. 8 is a configuration diagram showing a conventional streak camera system.

FIG. 9 is a configuration diagram showing a conventional image intensifier system.

FIG. 10 is a configuration diagram showing a conventional weak image pickup device.

FIG. 11 is a diagram illustrating a problem of the weak image pickup device of FIG. 10;

FIG. 12 is a view for explaining a problem of the weak image pickup device of FIG. 10;

[Explanation of symbols]

11, 52 ... Photon counting means, 11c, 52a ... Photoelectric conversion means, 11b, 52e ... Electron multiplication means, 11a, 52f
... Light-emitting means, 15, 54 ... Imaging means, 16, 55 ... Signal processing device.

Claims (11)

(57) [Claims] A photoelectric conversion means for generating photoelectrons in response to the incident light to be measured; a photoelectron from the photoelectric conversion means being multiplied so as not to be saturated by the incidence of one electron; An imaging means having an electron multiplying means for emitting electrons from a position corresponding to the incident position, a light emitting means for converting the electrons multiplied by the electron multiplying means into a light image, and An image pickup means for converting an optical image from the means into a video signal output of a level corresponding to the light intensity and outputting the video signal for each pixel; and one electron from the video signal output from the image pickup means to the electron multiplying means. A photoelectron detection signal corresponding to a signal portion exceeding a predetermined level determined based on the level of noise generated without being caused by the incidence of the light is extracted for each pixel of the image pickup means, and the video signal is output. Signal output means based on the timing of, and based on the timing of the video signal output from the image pickup means, by integrating the output of the signal extraction means for each pixel of the image pickup means for a predetermined time, A weak light measurement device comprising: a signal processing unit that measures an incident photon to the photoelectric conversion unit. 2. The weak light measurement device according to claim 1, wherein the photoelectron detection signal is a signal obtained by digitizing a level of a signal portion exceeding the predetermined level for each pixel of the image pickup means. 3. The photoelectron detection signal is a signal in which one pixel in each pixel of the image pickup means having a peak level in a signal portion exceeding the predetermined level has a value of 1.
The weak light measurement device described. 4. The weak light measurement device according to claim 1, wherein the photoelectron detection signal is a signal in which all the pixels of the imaging unit included in the signal portion have a value of 1. 5. The weak measurement device according to claim 1, wherein the signal processing unit further includes a storage unit that stores the integration result integrated for the predetermined time for each pixel of the imaging unit. 6. The weak light measuring device according to claim 1, wherein the electron multiplying unit includes a microchannel plate that multiplies the incident photoelectrons and emits the multiplied electrons. 7. The imaging means further comprises sweeping means for sweeping photoelectrons emitted from the photoelectric conversion means and traveling toward the electron multiplying means in a direction orthogonal to the traveling direction. The weak light measurement device according to claim 1. 8. The weak light measuring device according to claim 1, wherein the photoelectric conversion means is a photoelectric surface that emits photoelectrons from a position corresponding to an incident position of the light to be measured. 9. The weak light measuring device according to claim 1, further comprising a fiber plate for guiding an optical image from the light emitting means to the image pickup means. 10. The weak light measuring device according to claim 1, wherein the image pickup means is a CCD in which the plurality of pixels are two-dimensionally arranged. 11. The weak light measuring device according to claim 1, wherein the image pickup means is a photodiode array in which a plurality of pixels are arranged in one dimension.