September 1, 2021
1. Camera imaging principle
The scene is projected onto the surface of the image sensor (Sensor) through the optical image generated by the lens (LENS), and then converted into an analog electrical signal, which is converted into a digital image signal after A/D (analog-to-digital conversion) conversion, and then sent to digital signal processing The chip (DSP) is processed in the chip (DSP), and then transmitted to the CPU for processing through the I/O interface, and the image can be seen through the LCD.
2.1. FF & AF
FF module (Fix Focus): fixed focus module. The focal length is fixed, auto focus cannot be achieved
AF module (Auto Focus): auto focus module. Auto-focus is achieved by adding VCM and Driver IC matching algorithms.
VCM: Voice coil motor. The full name is Voice Coil Montor, the voice coil motor in electronics, which is a kind of motor. Because the principle is similar to that of a speaker, it is called a voice coil motor, which has the characteristics of high frequency response and high precision. The main principle is to control the stretched position of the spring leaf by changing the DC current of the coil in the motor in a permanent magnetic field, thereby driving up and down movement. Mobile phone cameras widely use VCM to achieve auto-focusing. Through VCM, the position of the lens can be adjusted to present clear images.
Generally speaking, the higher the pixels, the more lenses the lens needs. 3P lenses will be used in products below 5M, and products at 5M and above will use 4P～6P lenses.
Field Of View. The field of view, the size of the camera's angle of view, is inversely proportional to the focal length.
F-Number. Aperture ratio = F/D (focal length/entrance pupil size), which determines the brightness of the lens.
TV distortion. The phenomenon that a straight line becomes curved after being photographed by the lens is called distortion.
As shown in the figure below: (a) Ideal situation; (b) Pincushion distortion; (c) Barrel distortion.
3.4 RI (relative illumination)
Relative illuminance. The brightness of the lens depends on the F value (the aperture value), which represents the brightness of the image on the optical axis of the lens, which is the center of the screen. The amount of peripheral light is affected by lens vignetting and the law of fourth power of cosine, so the brightness is inevitably reduced compared to the center. The communication industry standard YDT 1607-2007 stipulates a test method for considerable illuminance.
Resolution. Reflects the ability of the lens to distinguish the details of the object.
3.6 Flare & Ghost
Stray light and ghost images have a lot to do with the coating of the lens.
Effective Focal Length
Back Focal Length
The principal angle of incidence (CRA). The light that passes through the center of the aperture and finally reaches the edge of the imaging plane is called the chief ray, and the angle between the chief ray and the horizontal direction is called the main incident angle.
The CRA of the lens and sensor must be matched, otherwise serious problems such as vignetting and color cast will occur.
IR=infrared=infrared; CUT=filter, subtract.
The function of IR-CUT is to filter out infrared light and reduce the stray light irradiated on the sensor surface.
Black level, when the amount of light is zero, the RGB value of the data obtained by the sensor is defined as Black level
Reason for existence: In theory, Black level=0, in fact, Black level>0
Function: Make the Black level as close as possible to 0
Implementation method: Part of the pixel is covered and not photosensitive, so as to obtain an offset
Lens Shading Correction
Reason for existence: the lens attenuates the transmitted light from the center to the surroundings
Function: make the human eye feel that the lens is attenuating the light
Realization method: compensate the attenuation by additionally compensating the RGB values of the pixels around the image
Denoising/edge enhancement. Basic principle: reduce/increase the difference between the current pixel and the surrounding pixels
Auto Exposure Control
Function: The brightness of the final image is as suitable as possible for the human eye
Realization: By controlling the exposure time (line) and gain value, the brightness of the image reaches the target
Banding/Flicker: Fluorescent lamps have an energy cycle, and sensors use line exposure mode
Elimination Flicker method: line exposure time is an integer multiple of the light source energy period
5. Output data format and interface
5.1. Data format
It is the original record of the level when the sensor converts the optical signal into the electrical signal, and it is obtained by simply digitalizing the image data without any processing, that is, the electrical signal directly obtained by the imaging element.
It is a color coding method adopted by the European television system. Among them, "Y" represents brightness (Luminance or Luma), which is the grayscale value; and "U" and "V" represent chrominance (Chrominance or Chroma), which describe the color and saturation of the image, and are used to specify the color of the pixel.
Serial Peripheral Interface: Serial interface, used for ultra-low end.
Digital Video Port: Parallel port, commonly used.
Mobile Industry Processor Interface: Camera uses differential signals, high speed, low power consumption, and is applied to high-pixel products.
6. the software
3A technology is auto focus (AF), auto exposure (AE) and auto white balance (AWB).
3A digital imaging technology uses AF autofocus algorithm, AE auto exposure algorithm and AWB auto white balance algorithm to achieve the maximum image contrast, improve the overexposure or underexposure of the subject, and compensate the chromatic aberration of the picture under different light. So as to present higher quality image information. The camera with 3A digital imaging technology can guarantee the accurate color reproduction of the image and present the perfect day and night monitoring effect.
ZSL (zero shutter lag): Zero second delay. In daily life, there are often some delays in the experience when using a mobile phone camera to take pictures. ZSL was developed to eliminate this delay and provide a "shoot and see" experience.
Analog circuit power supply
Digital circuit power supply
8. testing & debugging items
Take a picture of the ISO12233 chart with a machine, analyze the center and surrounding h, v directions with tools, and read the line pairs that can be distinguished in millimeters (in LP/mm).