I don’t understand some camera technical terms, please answer!!

Charge transfer methods of CCD

CCD*** has three charge transfer methods, namely frame transfer (FT) method, inter-line transfer (IT) method and inter-frame transfer (FIT) way.

1) Frame transfer (FT) mode

CCDs that work in FT mode are the simplest in structure and the easiest to manufacture. The FT CCD was considered a very good CCD structure in the early years. Although its size is twice its photosensitive area, its performance has been improved in many aspects compared with the photoelectric camera tube.

The matrix above is used to receive optical images, so it is where a large amount of charge is concentrated. The pixels in this matrix are designed to be shifted vertically. During the vertical blanking period, all image charges are transferred down as quickly as possible to the underlying matrix, which is covered by a light-shielding material. This charge transfer process clears all charges from the image matrix, allowing a new charge concentration process to begin again.

During the horizontal blanking period of the next vertical scanning process, the non-photosensitive matrix below continues to transfer the charge on a horizontal line downward to the lowest readout register. During a horizontal scan, a readout register clocks the charges representing different pixels horizontally to an output gate. Here, the charge is converted into a video signal.

It is worth mentioning that since the image formed by the optical lens is a real image that is upside down and left and right, the pixels in the lower right corner of the photosensitive matrix actually correspond to the objects in the upper left corner of the scene. So after the vertical blanking ends, the charge generated by the pixel in the lower right corner is read out first.

In addition, the charge accumulation on the pixel is related to the time when the light shines on the matrix. Even during the vertical transmission process during the vertical blanking period, a small part of the charge accumulation on the pixel will occur. , so the vertical tailing phenomenon occurs. The vertical blanking period is approximately equivalent to 6% of the entire charge accumulation process, but the tailing caused by the highlight is greater than 6%. It appears as an upper and lower vertical line passing through the highlight point. This phenomenon is called transmission. Tailing is a serious problem for FT mode CCDs.

The only way to completely prevent transmission tailing is to hold the light during vertical transmission. This can be accomplished by a mechanical shutter on the camera, which was indeed found on early cameras using FT mode CCDs. However, mechanical devices are not suitable for fully electronic cameras. Moreover, the development of CCD in other structures can be used to prevent the occurrence of transmission tail.

2) Inter-line transfer (IT) mode

The photosensitive matrix and the storage matrix are crossed into a single matrix. This CCD structure is called inter-line transfer mode IT. In this structure, each pixel contains two parallel CCD cells, one of which is used to sense light, while the other blocked cell is used to form a vertical shift register. During the vertical blanking period, the charge generated by the photoreceptor cell of each pixel is horizontally transferred to the adjacent vertical shift register cell, and then the vertical shift register transfers these charges to the lowermost horizontal readout cell in clock sequence. out of the register and finally output through the output gate to generate a video signal.

Since the light-shielded vertical shift register is located inside the imaging area, half of the light energy falling on the CCD is lost, resulting in a sensitivity loss of 50% or more. Due to the high efficiency of CCD in photosensitivity, this problem does not appear to be so serious. In addition, this increase in the spacing of the photosensitive parts actually improves the performance of the CCD in terms of MTF value.

The transfer of charge from photoreceptor cells to vertical shift register cells is a very simple process, and it is unlikely to cause tailing. All vertical transmission occurs under light shading, so transmission tailing is effectively eliminated on IT mode CCDs. However, a similar phenomenon still exists in the highlight area, although it is much lower than the highlight level required to produce smearing on an FT mode CCD. This phenomenon is called vertical smearing. It is generally caused by the following reasons: some light leaks from the periphery of the vertical shift register cell; or long-wave light like red light penetrates deeply into the bottom layer to generate charges, and these charges are transferred to the vertical shift in the register. Vertical smearing also produces vertical lines above and below the highlight point, much like the transmission smearing produced on an FT mode CCD, but the highlight level required to produce it is much lower.

3) Frame Interline Transfer (FIT) method

Frame Interline Transfer (FIT) method is a combination of FI and IT concepts. It is the best method for the current CCD structure. It is widely used in high-quality TV cameras.

It works in the same way as IT during charge accumulation, but all pixel charges are moved into the vertical shift register at the beginning of the vertical blanking period. As vertical blanking proceeds, these charges are quickly transferred to the lower half of the light-shielded storage register. This transfer process is so fast that the so-called vertical tailing phenomenon is completely negligible.

During line scanning, the pixel charge of each line is transferred to the readout register in the same way as the FT mode CCD.

Gain is the amplification factor. There is a video amplifier inside the camera that amplifies the signal from the CCD to a standard video amplifier. It has a high sensitivity. However, in a well-lit environment, the amplifier will be overloaded and distort the video signal. When the switch is in AGCH, the lens aperture is fully opened under low brightness conditions and the gain is automatically increased to obtain a clear image. When the switch is AGCL, natural and low-noise images can be obtained at low brightness.

Signal-to-noise ratio: When the camera captures a bright scene, the image displayed on the monitor is usually brighter, and it is difficult for the observer to see the interference noise in the image; while when the camera captures a darker scene, the image displayed by the monitor The picture is relatively dim, and observers can easily see snowflake-like interference noise in the picture. The strength of interference noise is directly related to the signal-to-noise ratio index of the camera. That is, the higher the signal-to-noise ratio, the smaller the impact of interference noise on the picture. The signal-to-noise ratio is the ratio of the signal voltage to the noise voltage, usually represented by the symbol s/n. Since under normal circumstances, the signal voltage is much higher than the noise voltage, the ratio is very large, and the unit of signal-to-noise ratio is expressed in db. Generally, the signal-to-noise ratio value given by the camera is the value when the AGC (automatic gain control) is turned off, because when the AGC is turned on, the small signal will be increased, causing the noise level to increase accordingly. The typical value of the signal-to-noise ratio is 45~55db. If it is 50db, the image has a small amount of noise, but the image quality is good; if it is 60db, the image quality is excellent and no noise appears.

Noise level: Look up.

Sensitivity refers to the minimum target surface illumination required to obtain the specified signal level when the lens aperture is of a certain size. For example: using an F1.2 lens, when the surface illumination of the subject is 0.04Lux, the amplitude of the camera output signal flare is 350mv, which is 50% of the maximum amplitude, then the sensitivity of the camera is said to be 0.04Lux/F1.2. If the illumination on the surface of the subject is low, the monitor screen will be a dark image with difficult to distinguish levels. According to experience, it is generally more appropriate to select a camera whose sensitivity is 1/10 of the illumination of the surface of the subject.