What is the structure and principle of the camera module?

Camera Module, full name CameraCompact Module, abbreviated CCM. CCM contains four major components: lens, sensor, FPC, and image processing chip (DSP). The important components that determine the quality of a camera are: lens, image processing chip (DSP), sensor. The key technologies of CCM are: optical design technology, aspheric mirror making technology, optical coating technology.

Working principle: The object through the lens (lens) gathered light, through the CMOS or CCD integrated circuit, the optical signal into an electrical signal, and then through the internal image processor (ISP) converted into a digital image signal output to the digital signal processor (DSP) processing, converted into a standard GRB, YUV and other formats image signal.

1. Lens

   Lens is a device that can receive optical signals and converge the optical signals on the sensor CMOS/CCD. The Lens determines the light rate of the sensor, and its overall effect is relative to that of a convex lens. It is usually composed of several lenses, from the material point of view, the camera lens can be divided into plastic lenses and glass lenses.

   Usually the camera lens structure is: 1P, 2P, 1G1P, 1G2P, 2G2P, 4G (P stands for Plastic, G stands for Glass) and so on. The more lenses, the higher the cost; Glass lenses are more expensive than plastic. Therefore, a good quality camera should use a glass lens, and the imaging effect will be better than that of a plastic lens. Most camera products on the market now, in order to reduce costs, generally use plastic lenses or semi-plastic and semi-glass lenses (i.e., 1P, 2P, 1G1P, 1G2P, etc.).

   Lens main parameters

   (1) Focal length: the length of the focal length of the lens determines the imaging size of the shooting, the size of the field Angle, the size of the depth of field and the perspective of the picture. Generally speaking, for a single lens, it is the distance from the center of the lens to the focus, and the camera lens is composed of multiple lenses, which is much more complicated. Focal length here refers to the distance between the center point of the lens and the sharp image formed on the photosensitive device (CCD).

   (2) Field of view Angle: We commonly use the horizontal field of view Angle to reflect the shooting range of the picture. The larger the focal length f is, the smaller the field of view Angle is, and the smaller the picture range formed on the sensor is. Conversely, the smaller the focal length f, the larger the field of view Angle, and the larger the picture range formed on the sensor.

   (3) F value (caliber ratio) : F value refers to the brightness of the lens (that is, the amount of light transmittance of the lens). F= lens focal length/aperture diameter. If the F value is the same, the aperture of a long focal length lens is larger than that of a short focal length lens.

   (4) Aperture: aperture is an optically mechanical aperture located inside the lens that can be adjusted to control the amount of light passing through the lens. Variable diaphragm (Iris diaphragm). A mechanism inside the lens that controls the size of the aperture. Or a device used to open or close the aperture of the lens, thereby adjusting the f-stop of the lens.

   (5) Depth of field: When an object is clearly focused, all scenes from a certain distance in front of the object to a certain distance behind it are also quite clear. The distance from front to back where the focus is fairly clear is called depth of field.

2.  Voice Coil Motor (VCM)

   Voice Coil Montor, the voice coil motor in electronics, is a kind of motor. Because the principle is similar to the speaker, it is called the voice coil motor, which has the characteristics of high frequency noise and high precision. The main principle is to control the stretching position of the spring by changing the DC current of the coil in the motor in a permanent magnetic field, so as to drive up and down movement. Mobile phone cameras widely use VCM to achieve autofocus function, through VCM can adjust the position of the lens, present a clear image.

   2.1 VCM Performance Specifications

   The performance of VCM is mainly based on the ratio of current and travel distance. Starting from the starting current, the current rises to be proportional to the travel distance that can be driven, the smaller the required rising current, the higher the accuracy, and also look at the maximum power consumption, maximum power, and size.

   2.2 VCM Classification

   From the structure can be roughly divided into three categories: (1) shrapnel structure; (2) ball type structure; (3) Friction structure.

   In terms of function, it is roughly divided into five categories: (1) Open loop motor; (2) Close loop motor; (3) Alternate central motor; (4) OIS optical anti-shake motor (sub-translation, axis shift, memory metal, etc.); (5) OIS+Close loop six-axis motor.

   2.3 Principles of Auto Focus

   After entering the autofocus mode, the Driver moves the lens from the spot to the maximum position from 0 to the maximum, then the sensor image surface automatically takes pictures and saves them to the DSP, and the DSP calculates the MTF (Modulation transfer function) value of each image through these pictures. Thus, the maximum value is found in this MTF curve, and the current corresponding to this point is obtained through the algorithm, and the Driver is instructed to provide this current to the voice coil again, so that the lens is stable on this imaging surface, so that the automatic zoom is achieved.

   2.4 Zoom and focus

   A: To achieve optical ZOOM using zoom motor (ZOOM)

   By moving the lens inside the lens to change the position of the focus, change the length of the focal length of the lens, and change the Angle of view of the lens, so as to achieve the amplification and reduction of the impact.

   B: Autofocus motor (AF) is used to achieve autofocus

   Move the position of the entire lens (rather than the lens in the lens) by a small distance to control the length of the focal length of the lens, so as to achieve a clear image.

   (Optical focus and optical zoom are different concepts: optical zoom is to change the position of focus by moving the relative position of the lens inside the lens, change the length of the lens focal length, and change the lens Angle of view size, so as to achieve image amplification and reduction; Optical focus is actually adjusting the position of the entire lens (not the lens inside the lens) to control the image distance, so that the image is the clearest)

3. IR-CUT

   There are various wavelengths of light in nature. The wavelength range of light recognized by the naked eye is between 320nm-760nm, and the light exceeding 320nm-760nm cannot be seen by the human eye. The imaging component of the camera, CCD or CMOS, can see most wavelengths of light. Due to the participation of various light, the color restored by the camera is different from that seen by the naked eye. Such as green plants become gray, red pictures become light red, purple and so on. At night, due to the filtering effect of bimodal filter, CCD can not make full use of all the light, no snow noise phenomenon and its low light performance is unsatisfactory. To solve this problem, use IR-CUT double filters.

   IR-CUT double filter refers to the built-in module filter in the camera lens group, when the infrared sensor outside the lens detects the intensity of the light change, the built-in IR-CUT automatic switching filter can automatically switch according to the intensity of the external light, so that the image can achieve the best effect. In other words, in the day or night, the dual filter can automatically switch filters, so whether it is in the day or night, you can get the best imaging results.

   3.1 Composition and principle of IR-CUT

   The IR CUT double filter switcher consists of an infrared cut-off low-pass filter (an infrared cut-off or absorption filter), a full-spectrum optical glass (a full-spectrum filter), a power mechanism (which can be electromagnetic, motor or other power source), and a housing, which is switched and positioned by a circuit control board. When the light is sufficient during the day, the circuit control board drives the switcher to switch and locate the infrared cut-off filter to work, CCD or CMOS to restore the true color; When the visible light is insufficient at night, the infrared cut-off filter automatically moves away, and the full-spectrum optical glass begins to work, at this time, it can sense the infrared light of the infrared lamp, so that CCD or CMOS make full use of all light, which greatly improves the night vision performance of the infrared camera, and the whole picture is clear and natural.

   3.2 IR-CUT Specifications

   a, the infrared cut-off degree of the filter, light transmittance, and light shaping effect.

   b, power drive part

   c, control circuit

   d, filter: generally using the method of IR Coating or blue glass to filter infrared light.

   Holder lens holder

   Lens base if from the material, divided into metal material and plastic material. Lens base from the interface above to distinguish, can be divided into C, CS, M12, M9, M8 and so on a variety of different interface lens, this is according to your needs to choose.

   4.Image Sensor

An Image sensor is a semiconductor chip with millions to tens of millions of photodiodes on its surface. When the photodiodes are exposed to light, they generate an electric charge, converting the light into an electrical signal. Its function is similar to human eyes, so the performance of the sensor will directly affect the performance of the camera.

4.1 Sensor Structure

4.2 Classification

Sensor: CCD, CMOS(PPS and APS)

1. Different processes: front light FSI, back light BSI, stack type

  2.4.3 Indicators

1. 1. Pixels

   There are many light-sensitive units on the sensor, which can convert light into electric charges, thus forming an electronic image corresponding to the scene. In the sensor, each photosensitive unit corresponds to a pixel (Pixels), the more pixels, which means that it can sense more details of the object, so that the image is clearer, the higher the pixel, meaning that the imaging effect is clearer. The product of the camera resolution is the pixel value, for example: 1280×960=1228800

   2. Target surface size

   The size of the sensitive part of an image sensor, usually expressed in inches. As with the TV, this data usually refers to the diagonal length of the image sensor, such as the common 1/3 inch, the larger the target surface, the better the light flow, and the smaller the target surface, the easier to obtain a greater depth of field.

   3. Speed

   That is, the intensity of the incident light is induced by CCD or CMOS and related electronic circuits. The higher the sensitivity, the more sensitive the photosensitive face to light, the higher the shutter speed, which is especially important when shooting moving vehicles and night monitoring.

   4. Electronic shutter

   It is a term coined in reference to the mechanical shutter function of a camera. It controls the sensitive time of the image sensor, because the sensitive value of the image sensor is the accumulation of signal charge, the longer the sensitive value, the longer the accumulation time of signal charge, and the larger the amplitude of the output signal current. The faster the electronic shutter, the lower the sensitivity, suitable for shooting in bright light.

   5. Frame rate

   It refers to the number of pictures recorded or played per unit time. Continuous play of a series of pictures will produce animation effects, according to the human visual system, when the picture playback speed is greater than 15 pictures/second (that is, 15 frames), the human eye can not see the jump of the picture; When it reaches 24 frames /s – 30 frames /s (that is, 24 frames to 30 frames), the flicker phenomenon is basically invisible.

   Frames per second (fps), or frame rate, represents the number of times a graphics sensor can be updated per second while processing the field. High frame rates allow for a smoother, more realistic visual experience.

   6. Signal to noise ratio

   Is the ratio of signal voltage to noise voltage, and the unit of signal-to-noise ratio is expressed in dB. Generally, the signal-to-noise ratio 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 is raised, so that the noise level is also increased accordingly.

   The typical value of SNR is 45-55dB, if 50dB, the image has a little noise, but the image quality is good; If it is 60dB, the image quality is good, there is no noise, and the larger the signal-to-noise ratio, the better the control of noise. This parameter is related to the amount of noise in the image, the higher the signal-to-noise ratio, the cleaner the picture feels, and the less dot noise in the night vision picture.

   ISP

   ISP is short for Image Signal Processor, while DSP is short for Digital Signal Processor. ISP (Image signal processing) mainly completes the processing of digital images, and converts the original data collected by the sensor into the format supported by the algorithm.

   ISP is generally used to process the output data of Image Sensor. Such as AEC (automatic exposure control), AGC (automatic gain control), AWB (automatic White balance), color correction, Lens Shading, Gamma correction, remove bad spots, Auto Black Level, Auto White Level and other functions.

   ISP process:

   1. After the external light passes through the lens, it is filtered by the color filter and irradiated onto the Sensor surface;

   2. The Sensor converts the light transmitted from the lens into an electrical signal, and then converts it into a digital signal through the internal AD.

   DSP

   5 DSP

   DIGITAL SIGNAL PROCESSING (DSP) function: mainly through a series of complex mathematical algorithms to optimize the digital image signal parameters, and the processed signal through USB and other interfaces to the PC and other devices

   Differences between DSP and ISP:

   DSP features are more, it can do some photography and echo (JPEG codec), Video and playback (Video codec), H.264 codec, there are many other aspects of processing. ISP is a special kind of DSP which deals with image signal. At present, ISP can exist independently as a chip, or it can be directly integrated into the Image Sensor. The external output is the YUV Image processed by ISP. If the Image Sensor without ISP is used, the output is the image data in RAW format. It can also be directly integrated in the AP, and directly use the ISP inside the AP for image signal processing.

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