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Dec 29 2017

The application of CCD image sensor in the micro-optic TV system

Warm hints: The word in this article is about 3200 and reading time is about 20 minutes.


CCD is a new type of MOS structure device, its basic structure is a kind of close MOS capacitor, which is capable of storing in CCD as sensitive unit inspired by incident light optical information of electric charge, and the clock pulse in the appropriate phase sequence, driven by the stored charge in the form of charge packet transfer beam transmission, realize self scanning, complete conversion from optical signals to electrical signals. On the basis of analysis on characteristics of the CCD image sensor, this paper expounds the application of CCD image sensor in the low light level television system, CCD and image intensifier is mainly discussed in coupling mode, and points out the application of should pay attention to several problems and solutions.







4.2 Relay lens coupling

II.Main characteristics of CCD

4.3 Electronic bombardment CCD (i.e. EBCCD)

III.The composition of the CCD television system

V.Existing problems and solutions

IV.Image intensifier and the CCD coupling

VI.CMOS/CCD image sensor working principle

4.1 Fiber optic cone coupling







CCD is a new type of MOS structure device, its basic structure is a kind of close MOS capacitor, which is capable of storing in CCD as sensitive unit inspired by incident light optical information of electric charge, and the clock pulse in the appropriate phase sequence, driven by the stored charge in the form of charge packet transfer beam transmission, realize self scanning, complete conversion from optical signals to electrical signals.

CCD image sensor

This electrical signal is usually up to the standard of television video signal, can recover on the TV screen into objects like visible light, signal can also be stored in the tape drive, or input computer, image enhancement, identification, storage, processing, etc. Therefore, CCD device is an ideal camera device.

II.Main characteristics of CCD

A.Compared with the vacuum camera tube, solid camera devices have the following characteristics

  • (1)small volume, light weight, low power consumption, fast start-up, long life and high reliability. 

  • (2)wide spectral response range. Generally, CCD devices can work within the wavelength range of 400nm ~ 1100nm. The maximum response is about 900 nm. In the ultraviolet region, the quantum efficiency drops due to the absorption of the silicon chip itself, but the working wavelength limit can reach 100nm by using the reduced CCD of the back irradiation. 

  • (3)high sensitivity. The CCDS have a high yield of unit light, and the mass production rate of the CCD is up to 20%, which can be as high as 90%. In addition, the dark current of CCD is very small, and the detection noise is very low. Therefore, even in low illumination (10-21x), the CCD can successfully complete the photoelectric conversion and signal output. 

  • (4)wide range of dynamic response. The dynamic response range of CCD can reach up to 8 orders of magnitude in 4 orders of magnitude. 

  • (5)high resolution. The line array device has 7,000 pixels, which can tell the minimum size of 7 mu m. The surface array device has 4096 pixels and CCD camera has more than 1000 lines. 

  • (6)easy to be coupled with the microlight image intensifier, which can collect signals under low light conditions. 

  • (7)anti-over-exposure performance. Excessive light saturates the photosensitive element, but does not lead to chip damage. Based on the above features, the CCD used in low light level television system, not only can improve the quality of the system terminal display image, and can use the computer for image enhancement, identification, storage, etc.


III.The composition of the CCD television system

The composition of the CCD television system


iV.Image intensifier and the CCD coupling

Now, the sensitivity of a single CCD device can work in a low temperature environment, but it is impossible to apply the CCD to a microoptical television system. Therefore, the microlight can be coupled with the CCD to allow the photon to get the gain before it reaches the CCD device. There are three kinds of microoptical image intensifier and CCD coupling. 

4.1 Fiber optic cone coupling

Optic light cone is also a kind of optical fiber image transmission device, it a big head, on the other side is small, by using the theory of fiber optical image transmission, can be light tube fiber panel screen (normally, effective for Φ Φ 18, 25 or 30 mm Φ Φ) coupled to the output by the enhanced image CCD photosensitive surface (diagonal size usually is 12.7 mm and 16.9 mm), which can achieve the goal of LLL video.

Fiber optic cone coupling

The advantage of this coupling mode is that the fluorescence screen has a high utilization rate, and in the ideal case, only the diffuse transmittance of the optical cone is limited to 60%. The disadvantage is that it needs the CCD of the optical fiber panel input window. For the backlighting mode CCD optical fiber coupling, there is a decrease in the focus and MTF; In addition, the optical fiber panel, light cone and cell array CCD are several pixels discrete imaging element, therefore, three array geometry alignment between loss and fiber optic components itself fault on the quality of the final image is worthy of serious consideration and be strict with the problem.

4.2 Relay lens coupling

Using the relay lens, the output image of the microtube can be coupled to the CCD input surface. The advantage is that it is easy to focus and the image is clear, which can be applied to the CCD of the front and back lighting. The disadvantage is that the light energy utilization is low (less than 10%), the size of the instrument is slightly larger, and the problems of the system clutter need special consideration and treatment.

4.3 Electronic bombardment CCD (i.e. EBCCD)

Coupling way more than the first two are the common disadvantages of LLL video overall light quantum detection efficiency and brightness gain loss is bigger, combined with the glow fluorescent screen in the process of additive noise, the SNR performance of system is not very ideal. For this reason, the electronic bombardment CCD(EBCCD) is invented, which is to do the CCD in the microoptical tube, instead of the original fluorescent screen, and under the rated working voltage, the electron from the time pole directly bombards the CCD. The experiment shows that the electron of each 3.5eV can generate an electron - hole pair in the CCD potential well. 10kv working voltage, gain 2857 times. If the reduction of electron optical inverted image tube (e.g., m=0.33) is adopted, the additional gain of 10 times can be obtained. Moreover, the electronic optical system with careful design, processing and installation can obtain higher MTF and resolution features than the previous two coupling modes, without the additional noise of fluorescent screen. Therefore, if the low-noise dfga-ccd is incorporated into m=0.33 reduced ratio of the tube, it is expected to realize the low-light TV camera in the restricted condition of the quantum noise under the condition of the object illumination. 

The core components of a low-light television system are the coupling of the intensifier and CCD devices. The coupling efficiency of relay lens coupling is low and less used. Fiber optic cone coupling is applied to small image CCD. 

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

V.Existing problems and solutions

The main reason for the requirement of microoptical imaging is to improve the signal/noise ratio of the device. This should reduce the noise of the device (i.e. reduce the number of noisy electrons) and improve the signal processing capacity (that is, increase the number of signal electrons). The two methods can be used for cold CCD and electron bombardment CCD. The main purpose is to minimize the amount of light flux required for imaging when the output signal to noise ratio is 1. 

The CCD that meets the TV requirement (50 ~ 60fps) has obvious dark current at room temperature, which will increase the noise level. In the case of eliminating the peak of dark current, the unevenness of dark current distribution will produce a kind of noisy "fixed figure" when the input light can be reduced. In addition, at high frame rates, you don't want to reduce the utilization of each cell signal. The cooling of devices can significantly improve the dark current in silicon. Every 8 ℃ cooling noise will fall by half. Using ordinary electric refrigeration to -- 20 to 40 ℃, the dark current of 100 ~ 1000 times smaller than the room temperature, but then other noise becomes very prominent. Although CCD as sensor at present are known to be the most promising of low light imaging devices, especially in the case of small charge, charge transfer efficiency low light imaging system is not the main restriction, the main restrictions or output and low noise amplifier output detector, therefore, we must know the low noise of L3 imaging detection. With the low noise output (FGA and DFGA) of the floating gate amplifier, the detection effect of CCD is more ideal. 

The FGA can process the CCD image sensor peak signal of 100 noise electrons, and the DFGA saturation level is about 1/10 of the FGA, and it can only handle the peak signal of the image sensor of about 20 noise electrons.

VI.CMOS/CCD image sensor working principle

Both CCD and CMOS, they all adopt the sensor as the basic means to capture images of the, the core of the CCD/CMOS sensor is a photographic diode (photodiode), the diode can produce after accepting light output current, while the strength of the current and the intensity of the light. But on the periphery of the CCD sensor and the CMOS sensor is not the same, the former of the sensor, besides photosensitive diode includes a storage unit is used to control the adjacent charge, photosensitive diode occupies most of the area - to put it another way, effective photosensitive area is larger in CCD sensor, under the same conditions can be received strong light signal, the corresponding output signals are more clear. And the structure of the CMOS sensor is more complicated, except in a core position of photosensitive diode, it also includes the amplifier and modulus conversion circuit, the composition of each image point into a photosensitive diode and three transistors, and the photosensitive diode occupied area is just a small part of the whole element, cause the opening of the CMOS sensor rate is far lower than the CCD (opening rate: effective photosensitive area and the ratio of the area of the sensor). In this case, the light signal that CMOS sensor can capture is obviously smaller than that of the CCD element, and the sensitivity is low. Is reflected on the output, CMOS sensor to capture images more rich content than CCD sensor, lose some image detail and obvious noise, which is early CMOS sensor can only be used for a big reason for the low places.

CCD and CMOS sensor

CMOS low opening rate caused by the other trouble is that it can't be comparable to the CCD pixel density, because with the increase of density, the proportion of the sensor area will be narrowed, and CMOS opening rate is too low, a tiny effective photosensitive area, will lose some image detail is serious. Therefore in the sensor under the premise of the same size, the pixel size of CCD is always higher than the same period of the CMOS sensor, which is a CMOS has long failed to enter the mainstream digital camera market one of the important reasons.

Each sensor has an image point of image sensor, because the sensor can only sense the strength of the light, can't capture the color information, so you must cover color filter over the sensor. In this regard, different sensor manufacturers have different solutions, the most common way is to cover RGB, red, green and blue filters in the composition of 1:2:1 consists of four points as a color pixels (namely red and blue filters cover an image point respectively, the remaining two image points covered green filter), take this proportion is the cause of the human eye is very sensitive to the green. While SONY four color CCD technology will be one of the green filter in Emerald green (Emerald in English, some media called E channel), thus the formation of new R, G, B, E four color scheme. No matter what kind of technology it is, it's important to be upfront and clear about the four pixels that make up a color pixel.


CMOS/CCD image sensor working principle


After receiving the light, the photosensitive element generates the corresponding current, the current size corresponds to the light intensity, so the direct output of the photosensitive element is simulated. In the CCD sensor,

Each sensor without further processing, but will it direct output to the next, the storage unit of the sensor in combination with the element generated analog signals and then output to the third sensor, so on, until combined with the last of the sensor signal to form a unified output. Due to the sensor generated by the electrical signal is too weak, not directly, modulus conversion work, so the output data must be unified in the amplification processing - this task is by CCD sensor, the amplifier for the amplifier after processing, each image point of the amplitude of the signal strength have also increased; But with the CCD itself not to analog signal is converted to a digital signal directly, so also need a specialized analog-to-digital conversion chip processing, the final output in the form of binary digital image matrix to special DSP chip.

For CMOS sensor, the above workflow is completely unapplicable. Each sensor of the CMOS sensors integrated amplifier and modulus conversion logic directly, when photosensitive diode light, produces analog signals, signals are first amplifier amplification of the sensor, and then directly into the corresponding digital signals. , in other words, in the CMOS sensor, each sensor can produce the final digital output, the digital signal after the merger was directly handled by DSP chip - problem is taking place, belong to the simulator of the CMOS sensor amplifier, there is no guarantee that each point of magnification keep strictly, the enlarged image data cannot represent the object's appearance shooting - embodies in the final output results, is in a lot of noise, image quality significantly lower than the CCD sensor.




In the past 30 years, the research of CCD image sensor has made remarkable progress. It has grown from the initial simple 8 pixel shift register to millions to tens of millions of pixels. With the increase of observation distance and requirements are observed under low illumination, the requirement of low light level TV system will be more and more high, so we must develop new high sensitivity, low noise of the camera device, CCD image sensor is high sensitivity and low light imaging quality advantages just cater to the development trend of the low light level television system. As a new generation of microoptical imaging devices, CCD image sensor plays a key role in the microoptical TV system.

Finally, let’s watching an interesting video about ccd image sensors: 

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