Television photography of the test object with pulsed exposure of the photodetector by a flash pack..
Smelkov Vyacheslav Mikhailovich, Candidate of Technical Sciences, Associate Professor
TELEVISION PHOTOGRAPHY OF THE CONTROL OBJECT WITH PULSE EXPOSURE OF THE PHOTO DETECTOR WITH A PACKAGE OF FLASHES
The practical implementation of the single-shot video signal generation mode (TV photography) in modern TV cameras is due to the advent of solid-state “light-signal” converters based on matrix CCDs. Over the past twenty years since the publication of this mode in the work [1], called MONOSHOT by foreign inventors, technological advances in the production of CCD matrices and the chips surrounding them have made the possibility of performing TV photography using hardware embedded in a mobile phone a common occurrence.
In the article [2], methods for automatic selection of exposure time when using a video signal as a sensitivity control signal were analyzed for the MONOSHOT mode. It should be noted that all of them relate to continuous exposure of the photoconverter.
Let us evaluate the implementation of television photography in radiation introscopy. In this area of technology, the continuous radiation mode created by an X-ray apparatus during shadow scanning of the test object is not energetically favorable for the apparatus itself. For portable devices intended for express analysis of relatively small metal-intensive objects, the pulsed mode of the X-ray emitter is most suitable. For it, from the standpoint of radiation output, in order to reduce the radiation dose received by the service personnel during radiation inspection, it is advisable to implement radiation in the form of individual pulses or a sequence of pulses. Then the X-ray pulses penetrating the object will cause flashes of visible spectrum illumination on the single crystal (radiation detector), and the TV camera will be assigned the task of television photography from the screen of the single crystal of this specific image.
Below is a technical solution for a television camera that implements a method of television photography with pulsed exposure of a CCD photodetector, when its target is illuminated by individual flashes following with a certain period, and the required exposure is created by a sequence (packet) of these flashes. It is assumed that the task of automatically selecting the duration of the accumulation time of the photodetector remains. Therefore, the operating mode of the television camera in this article is called MONOSHOT-AUTO-PULSE.
The structural diagram of the television camera, the device of which is recognized as an invention [3], is shown in Fig. 1. The camera contains a lens (1), a CCD matrix (2) consisting of a series-connected charge-coupled accumulation section (2-1), a storage section (2-2) and an output register (2-3), as well as a synchronizing generator (3), four level converters (LC) in positions (4), (5), (6) and (7), a video amplifier (8), a trigger unit (9), an amplification and formation unit (AFU) in position (10), a charge measurement unit (CMU) in position (11), a peak detector (12), an “OR” element (13), a comparator (14) and a monostable multivibrator (15).
The pulsed light source (PLS) in position (16) is not included in the television camera and is considered here as an equivalent feature for a set of two devices: an X-ray machine and a single-crystal screen [4].
Note that the duration of the radiation pulses and their repetition period are specified by the PLS (16) as setting parameters from the television camera side.
The BUF (10) contains two series-connected ring counters (not shown here). The first counter counts with a period of 128 cycles of input counting pulses, and the second counter — with a period of 312 output pulses of the first counter. Let us designate the period of pulses generated by the first counter as a «conventional line», and the period of pulses of the second counter as a «conventional frame». Then the «conventional line» contains 128 cycles, and the «conventional frame» 312 «conventional lines». The selected indicators of the ring counters determine the rate of forced pulse exposure performed by the IMS (16), as well as the time processes of measuring the charge relief on the phototarget of the CCD matrix after each flash of illumination.
The CCD matrix (2) has a frame transfer organization with a three-phase charge transfer. Its feature is the introduction of a drain region into the accumulation section (2-1), which is under the potential DA and is equipped with a gate GA. The latter serves as an electronic gate of the photodetector. If there is a low (relative to the substrate) potential on the gate GA, it is closed, and the potential wells under the phase electrodes of section (2-1) are isolated from the drain region due to this barrier bias. Then the process of accumulation of charge photoelectrons is initiated on the phototarget itself.
When a high potential is applied to the GA gate, the potential barrier is removed, and the process of photoelectron accumulation in section (2-1) is excluded. This is explained by the fact that the carriers, without lingering in the potential wells under the phase electrodes, rush into deeper wells created by the DA potential in the drain region, and then recombine into the photodetector substrate. An example of such a CCD matrix is the domestic device FPPZ-134M with the number of elements 520-580 and an n-type channel.
Let's assume that a trigger pulse is received at the «Start» input of the TV camera at the moment to. Then the trigger unit (9) goes into a new state, in which the logical «1» level is set at its direct output, and the logical «0» level is set at its inverse output.
The appearance of a low logic level at the control input of the BUF (10) ensures the installation of a low signal level at the enabling input of its ring counters. Therefore, starting from the moment to, the first counter counts the clocks at the input, forming pulse sequences with a period of the “conditional line”. The second counter counts the “conditional” lines at the input and generates pulses with a period of the “conditional frame”.
As a result, a pulse with a period of the “conditional frame” is formed at the output of the BUF (10), in which a low logical level is transmitted during the interval Tn, including the first two cycles of the first “conditional line”. A low signal level at the control input of the IMS (14) ensures the formation of the first flash of light in the interval Tn, which acts through the lens (1) on the photo target (2-1) of the CCD matrix.
On the other hand, the appearance of a low level in the interval Tn at the control input of the synchronizing generator (3) ensures the installation of a low signal level at its first, third and fourth outputs and a high level at its second output. The designation of these synchronizing generator outputs is shown in Fig. 1 (see numbered references). At the outputs of the PU (4…7), the input logical signals are repeated. As a result, during the interval Tn, the electronic shutter of the section (2-1) of the CCD is closed, and the accumulation of photocharges from the first flash in the potential wells formed under the second phase buses is carried out on the phototarget itself.
Fig. 1. Structural diagram of the television camera.
At the moment t1, accumulation from the first flash ends, and during the interval (t1…t2) the accumulated photocharges are transferred from section (2-1) to section (2-2).
Then, in the following “conditional frame”, these photocharges are stored under the second phase buses of section (2-2). During the first 104 cycles of the last “conditional line” of this conditional frame”, according to the signal from the output of the BUF (10), a high level is additionally applied to the first phase electrodes of section (2-2). Due to this, the photocharges in section (2-2) are evenly distributed in the potential wells located under the first and second buses.
During the last 24 cycles of the last “conditional line” of this “conditional frame”, the high level in the control signal of the first phase of section (2-2) decreases linearly, therefore, in each element of section (2-2), the process of charge transfer from the potential wells under the first phase buses to the potential wells located under the second buses begins. The potential wells under the first phase electrodes are monotonically destroyed, and in the circuit of the second electrode of section (2-2), a current arises, which is maximum at the first moment, and then monotonically decreases. The value of the current in this external circuit is the sum of the currents in each element of section (2-2) of the CCD matrix. Thus, the external current contains information about the charge distribution over the entire area of section (2-2).
BIZ (11) performs the current-voltage conversion, and the necessary information conversion is carried out during the measurement interval. For this, the necessary permission signal is fed to the control input of the unit (11) from the output of the unit (10).
The peak detector (12), previously reset to zero by the signal from the output of the BUF (10), remembers the maximum value of the voltage from the output of the unit (11). Therefore, the output voltage of the unit (12) is proportional to the current level of the potential relief in section (2-2) of the CCD matrix. Depending on the intensity of the light flash, the process of accumulation of the charge relief in section (2-1) and its non-destructive measurement in section (2-2) can occur during a single interval of the “conditional frame”, or an integer number of times longer with a corresponding increase in the number of light flashes.
Let us assume that after a certain number of flashes the value of the voltage from the output of the peak detector (12) is less than the value of the threshold voltage Uп of the comparator (14). In this case the comparator retains the previous state, and in section (2-1) of the CCD matrix the accumulation process continues for at least one more interval Тн, and then the potential relief in section (2-2), increased due to the last accumulation of charges, is again measured by the block (12).
Let the voltage from the peak detector output (12) exceed the voltage Up of the comparator (14) as a result of the last measurement at the moment t3. Then the comparator flips over. At the control input of the BUF (10), a high signal level is set by the signal from the inverse output of the trigger unit (9), which, arriving at the enabling input of the ring counters, stops the pulse counting in them. Stopping the counters eliminates the possibility of new light flashes from the IMS (16), and section (2-1) of the CCD matrix returns to the non-accumulation state, since the high level on the shutter of its drain region causes the injection of all charges from the phototarget into the substrate.
At moment t3, the negative voltage drop from the direct output of the launch unit (9) goes to the input of the monostable multivibrator (15), which forms an “image ready” signal at the output of the TV camera, the low level of which informs the consumer about the possibility of receiving information.
Let us assume that the consumer sends an “image request” signal to the input of the TV camera at the moment t4. When the low level in the “image request” signal coincides with the end of the nearest frame blanking pulse, the camera generates an “image confirmation” signal at the output of the synchronization generator (3), simultaneously reading the information frame from the storage section (2-2) through the output register (2-3) of the photodetector in the interval (t5…t6). The “charge – voltage” conversion performed in the CCD matrix by the output device with noise dispersion s2 ensures the formation of an electrical signal of a single image at the output of the “video TV camera.
Conclusion
The proposed solution for a television camera implements the principle of braking noise information in a photodetector, which consists in the fact that the useful signal is accumulated over time in charge form in the CCD matrix itself many times after each flash of illumination, and noise with dispersion s2 is introduced only once in the process of picking up an electric video signal.
Literature
1. French application No. 2589301 dated 10/28/85. IPC HO4N 3/15, 5/238. Electronic shuttering device. Applicant – I2S (France).
2. V.M. Smelkov Selecting the exposure time for a security camera in the single-shot video signal generation mode/Special equipment, 2003, No. 2, pp. 25 – 28.
3. Patent 2146080 RF. MKI7 HO4N 3/14, 5/335. Device for single-shot image signal generation/V. M. Smelkov, V. N. Mikhailov, V. Ya. Maklashevsky //B. I., 2000, No. 6.
4. Klyuev V. V., Leonov B. I., Sosnin F. R., Gusev E. A., Krongauz A. N. Industrial radiation introscopy. – Moscow: “Energoatomizdat”, 1985.