Application of the hottest CCD image sensor in low

The application of CCD image sensor in low light level TV system

based on the analysis of the characteristics of CCD image sensor, this paper expounds the application of CCD image sensor in low light level TV system, focuses on the coupling mode of CCD and image intensifier, and points out several problems that should be paid attention to in the application and the ways to solve them. Introduction

ccd (charge coupled device), that is, charge coupled device, is a new device with metal oxide semiconductor structure. Its basic structure is a closely arranged MOS capacitor, which can store the optical information charge excited by the incident light in the CCD image sensing unit, and can transfer the stored charge in the form of charge packets under the drive of clock pulses with appropriate phase sequence, so as to realize self scanning, Complete the conversion from optical signal to electrical signal. This kind of electrical signal is usually a video signal that meets the TV standard. It can be restored to the visible image of the object on the TV screen, or the signal can be stored in the tape drive, or input into the computer for image enhancement, recognition, storage and other processing. Therefore, CCD device is an ideal camera device

second, the main characteristics of CCD

compared with vacuum camera tubes, solid-state camera devices have the following characteristics:

(1) small volume, thinner thickness, light weight, less power consumption, fast startup, long service life and high reliability

(2) spectral response range the progress of plastic granulator technology is closely related to the development of the whole national economy. The girth is wide. General CCD devices can work in the wavelength range of 400nm ~ 1100nm. The maximum response is about 900nm. In the ultraviolet region, due to the absorption of the silicon chip itself, the quantum efficiency decreases, but with the CCD thinned by back irradiation, the working wavelength limit can reach 100nm

(3) high sensitivity. CCD has a very high unit quantum yield, and the quantum yield of CCD under front irradiation can reach 20%. If CCD with back irradiation thinning is used, the unit quantum yield can reach more than 90%

(4) wide dynamic response range. The dynamic response range of CCD is more than 4 orders of magnitude, up to 8 orders of magnitude

(5) high resolution. The linear array device has 7000 pixels, and the minimum distinguishable size is 7mm; The area array device has reached 4096 pixels × 4096 pixels, and the resolution of CCD camera has exceeded 1000 lines

(6) it is easy to be cascaded and coupled with low light level image intensifier, and can collect signals under low light conditions

(7) anti overexposure performance. Too strong light will saturate the photosensitive element, but it will not cause chip damage

based on the above characteristics, using CCD in low light level TV system can not only improve the quality of the image displayed at the system terminal, but also enhance, recognize, store and other operations of the image by computer

III. application mode of CCD in low light level TV system

ccd low light level TV system composition structure is shown in Figure 1. Among them, CCD must be coupled with optical image intensifier to play a role. There are three coupling modes between LLL image intensifier and CCD:

(1) fiber optic cone coupling mode

fiber optic cone is also an optical fiber image transmission device, with one end large and the other small. The fluorescent screen of the optical fiber panel of the LLL tube (usually, Φ The enhanced image, which is effectively 18, 25 or 30mm), is coupled to the CCD photosensitive surface (the diagonal size is usually 12.7mm and 16.9mm), so that the purpose of low light level photography can be achieved

the advantage of this coupling method is that the light energy utilization rate of the fluorescent screen is high, but the disadvantage is that: CCD with optical fiber panel input window is required, and the optical fiber coupling of CCD in back lighting mode has the problems of defocusing and MTF reduction; In addition, the optical fiber panel, optical cone and CCD are all discrete imaging elements with several pixel unit arrays. Therefore, the geometric alignment loss between the three arrays and the impact of the defects of the optical fiber element itself on the final imaging quality are issues worthy of serious consideration and strict treatment

(2) coupling mode of relay lens

the output image of the low light level tube can also be coupled to the input surface of the CCD by using the relay lens. Its advantages are easy focusing and clear imaging, which is applicable to CCD with front lighting and back lighting; The disadvantage is that the utilization rate of light energy is low (≤ 10%), the size of the instrument is slightly larger, and the problem of stray light interference in the system needs special consideration and treatment

(3) electron bombardment CCD, that is, EBCCD mode

also needs comprehensive and multi-level energy-saving technical transformation. The common disadvantage of the above two coupling modes is that the overall light quantum detection efficiency and brightness gain loss of low light level camera are large, and the additional noise in the light-emitting process of the fluorescent screen makes the signal-to-noise ratio characteristics of the system less ideal. For this reason, people invented electron bombardment CCD (EBCCD), that is, the CCD is made in a low light level tube to replace the original fluorescent screen. Under the rated working voltage, the (optical) electrons from the photocathode directly bombard the CCD. The experiment shows that the gain is 2857 times under 10kV working voltage. If a reduced magnification electro-optical inverted tube is used (for example, magnification M = 0.33), an additional gain of 10 times can be obtained, that is, the photon charge gain of EBCCD can reach more than 104; Moreover, after careful design, processing and adjustment, the electronic optical system can obtain higher MTF and resolution characteristics than the first two coupling methods, and there is no additional noise on the fluorescent screen

the performance of coupled CCD device is determined by both image intensifier and CCD. The spectral response and signal-to-noise ratio depend on the former, dark current, inertia and resolution depend on the latter, and the sensitivity is related to both

IV. existing problems and solutions

considering the requirements of low light level imaging, the most important thing is to improve the signal/noise ratio of devices. Therefore, the device noise should be reduced (i.e. reduce the number of noise electrons) and the signal processing ability should be improved (i.e. increase the number of signal electrons). Two methods can be used: cooling CCD and electronic bombardment CCD. Its main purpose is to reduce the light flux required for imaging as much as possible when the output signal-to-noise ratio is 1. Please leave your name and basic information (unit or address).

CCD that meets the TV requirements (50FPS ~ 60fps) has obvious dark current at room temperature, which will increase the noise level. In the case of eliminating dark current spikes, the uneven distribution of dark current will also produce a "fixed pattern" of noise when the input light energy is reduced. In addition, when operating at a high frame rate, the utilization of each image unit signal cannot be reduced. Device cooling will significantly improve the dark current in silicon, and the noise will be reduced by half every 8 ℃ cooling. When using ordinary electric refrigeration to -20 ℃ ~ -40 ℃, the dark current will be 100 ~ 1000 times smaller than that at room temperature, but at this time, other noises will become very prominent

combined with refrigeration, the detection effect of CCD is more ideal with the low-noise output of floating gate amplifier (FGA and dfga). Among them, FGA can process 100 noisy CCD image sensor peak signals, and the saturation level of dfga is about 1 of FGA