Application of CCD image sensor in low-light TV system

Application of CCD image sensor in low-light TV system

Abstract: Based on the analysis of the characteristics of CCD image sensors, the application of CCD image sensors in low-light TV systems is explained, focusing on the coupling method of CCD and image intensifiers, and pointing out several issues that should be paid attention to in the application. and solutions.

Keywords: CCD; image enhancement; low-light TV; coupling

1 Introduction

CCD (Charge Coupled Device) is a new device with a metal oxide semiconductor structure. Its basic structure is a densely packed MOS capacitor, which can store optical information charges excited by incident light in the CCD image sensitive unit. , and can directional transfer and transfer the stored charge in the form of charge packets driven by clock pulses with appropriate phase sequence, realize self-scanning, and complete the conversion from optical signals to electrical signals. This electrical signal is usually a video signal that complies with TV standards. It can be restored to a visible light image of the object on the TV screen. The signal can also be stored in a tape drive or input into a computer for image enhancement, recognition, storage and other processing. Therefore, CCD device is an ideal imaging device.

2 Main characteristics of CCD

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

(1) Small size, light weight, low power consumption, fast startup, long life and high reliability.

(2) Wide spectral response range. General CCD devices can work in the wavelength range of 400nm ~ 1100nm. The maximum response is around 900nm. In the ultraviolet region, the quantum efficiency decreases due to the absorption of the silicon wafer itself. However, using a back-irradiated and thinned CCD, the operating wavelength limit can reach 100nm.

(3) High sensitivity. CCD has a very high unit light quantum yield. The quantum yield of a front-illuminated CCD can reach 20%. If a back-illuminated thinned CCD is used, the unit quantum yield can reach over 90%. In addition, the dark current of CCD is very small, and the detection noise is also very low. Therefore, even under low illumination (10-21x), CCD can successfully complete photoelectric conversion and signal output.

(4) Wide dynamic response range. The dynamic response range of CCD is more than 4 orders of magnitude and can reach up to 8 orders of magnitude.

(5) Very high resolution. The line array device has 7000 pixels and can resolve the minimum size of 7 μm; the area array device has reached 4096 pixels, and the CCD camera resolution has exceeded 1000 lines.

(6) It is easy to cascade couple with low-light image intensifiers and can collect signals under low-light conditions.

(7) Anti-overexposure performance. Excessive light will saturate the photosensitive element, but will not cause chip damage.

Based on the above characteristics, using CCD in low-light TV systems can not only improve the quality of images displayed on the system terminal, but also use computers to enhance, identify, and store images.

3 Composition of CCD low-light TV system

4 Coupling of image intensifier and CCD

Now, although the sensitivity of a separate CCD device can work in a low-temperature environment, it is not possible to apply CCD alone to a low-light TV system. Therefore, the low-light image intensifier can be coupled to the CCD, so that the photons gain gain before they reach the CCD device. There are three coupling methods between low-light image intensifier and CCD:

(1) Fiber light cone coupling method

The optical fiber light cone is also a type of optical fiber image transmission device. It has a large end and a small end. Using the principle of fiber optic image transmission, the output of the micro-light tube fiber panel fluorescent screen (usually, the effective Φ is Φ18, Φ25 or Φ30mm) can be enhanced. The image is coupled to the CCD photosensitive surface (the diagonal size is usually 12.7mm and 16.9mm), thereby achieving the purpose of low-light photography.

The advantage of this coupling method is that the utilization rate of light energy of the fluorescent screen is high. Ideally, it is only limited by the diffuse transmittance of the optical fiber light cone (≥60%). The disadvantage is that it requires a CCD with a fiber optic panel input window; for fiber coupling of the backlight mode CCD, there are defocus and MTF drop problems; in addition, the fiber optic panel, light cone and CCD are all discrete imaging elements of several pixel unit arrays. Therefore, the geometric alignment loss between the three arrays and the impact of defects in the optical fiber components themselves on the final imaging quality are issues worthy of serious consideration and strict treatment.

(2) Relay lens coupling method

The output image of the micro-light tube can also be coupled to the CCD input surface by using a relay lens. The advantage is that it is easy to adjust the focus and the image is clear. It is applicable to both front-lighting and back-lighting CCDs; the disadvantage is that the light energy utilization rate is low ( ≤10%), the instrument size is slightly larger, and the problem of stray light interference in the system requires special consideration and processing.

(3) Electron bombardment CCD (ie EBCCD method)

The common shortcomings of the above two coupling methods are that the overall photon detection efficiency and brightness gain of low-light photography are greatly lost. In addition, the additional noise during the luminescence process of the fluorescent screen makes the signal-to-noise ratio characteristics of the system less than ideal. For this reason, people invented the electron bombardment CCD (EBCCD), that is, the CCD is built in a micro-light tube to replace the original fluorescent screen. Under the rated operating voltage, (photo) electrons from the photocathode directly bombard the CCD. Experiments show that every 3.5eV electrons can generate an electron-hole pair in the CCD potential well; at an operating voltage of 10kv, the gain reaches 2857 times. If a reduced magnification electronic optical inverter 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, carefully designed, processed, and assembled electronic The optical system can achieve higher MTF and resolution characteristics than the previous two coupling methods, without additional noise from the fluorescent screen. Therefore, if a DFGA-CCD with lower noise is selected and incorporated into a reduced magnification inverter tube with m=0.33, it is expected to achieve low-light TV photography under the condition of limited scene illumination ≤ 210-7lx photon noise.

The core component of the low-light TV system is the coupling of the image intensifier and the CCD device. The coupling efficiency of the relay lens coupling method is low and is rarely used. The optical fiber light cone coupling method is suitable for CCDs with small imaging areas.

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

5 Existing problems and solutions

Considering the requirements of low-light imaging, the most important thing is to improve the signal/noise ratio of the device. To this end, the device noise should be reduced (ie, the number of noise electrons is reduced) and the signal processing capability is improved (ie, the number of signal electrons is increased). Two methods, refrigeration CCD and electron bombardment CCD, can be used. 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.

CCDs that meet TV requirements (50 to 60 fps) have significant 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 as the input light energy decreases. In addition, when working at high frame rates, it is undesirable to reduce the signal utilization of each image unit. Device cooling will significantly improve the dark current in silicon. The noise will drop by half with every 8°C of cooling. When using ordinary electric cooling to -20 to -40°C, the dark current will be 100 to 1,000 times smaller than at room temperature, but other noises at this time will become very prominent. Although CCD image sensors are currently recognized as the most promising devices for low-brightness imaging, especially in the case of small charges, the charge transfer efficiency of low-brightness imaging systems is not the main limitation. The main limitation is the output amplifier and low-noise output detector. Therefore, we must understand the situation of low-noise detection of L3 imaging.

Coupled with cooling and using the low-noise output of floating gate amplifiers (FGA and DFGA), the CCD detection effect is more ideal. Among them, FGA can handle the CCD image sensor peak signal of 100 noise electrons, while the saturation level of DFGA is about 1/10 of FGA, and it can only handle the image sensor peak signal of about 20 noise electrons.

6 Summary

In the past 30 years, the research on CCD image sensors has made amazing progress. It has developed from a simple 8-pixel shift register to one with millions to tens of millions of pixels. As the observation distance increases and observation is required at lower illumination, the requirements for low-light TV systems will inevitably become higher and higher. Therefore, new high-sensitivity, low-noise camera devices must be developed. CCD image sensors have high sensitivity and The advantage of good low-light imaging quality just caters to the development trend of low-light TV systems. As a new generation of low-light imaging devices, CCD image sensors play a key role in low-light TV systems.