The basic process of camera and mobile phone imaging:
First, the light of the scene is received from the lens, and the light signal is converted into an electrical signal through the lens and photosensitive element, and then processed by the ISP image, and finally displayed on the screen.
Different components in the lens make the image show different effects. The components of each part play different roles, and the mystery of each part will be introduced in this article.
For more specific content, you can visit- The Secret of Lens Imaging Principles
Table of contents
2.5 photosensitive chip sensor
1. Camera imaging principle
The most basic principle of camera imaging is convex lens imaging. After light passes through the lens, a virtual image is formed, and the virtual image is transmitted to the screen for display.
The lens receives external light and hits the imaging module, and by controlling the expansion and contraction of the lens, a clearer or wider viewing angle image is realized.
The whole camera module can be understood as a combination of a lens and a sensor . The lens is responsible for receiving light from various scenes, and the sensor is responsible for converting the light into an electrical signal that the CPU can process and coloring the display output.
2. Lens module composition
The structure from top to bottom is the lens lens, voice coil motor VCM, filter IR/blue glass BG, bracket Bracket, photosensitive chip sensor, printed circuit board PCB, connectors and SMT mounted components, etc. Each part will be introduced one by one.
2.1 lens lens
Students who play with cameras must be very clear that a good lens is expensive, and different specifications of lenses need to be equipped to take pictures in different scenes. Lens, as the first component in imaging, is very important.
Its overall effect is a convex lens, and its main function is to gather light so that the light from the scene can be gathered on the photosensitive element. The lens determines the lighting rate of the photosensitive element.
Considering the cost and effect of modern lenses, the lens is not just a simple glass lens, but made of glass (Glass) material and plastic (Plastic) material, because glass material is more expensive than plastic material, but the light transmission effect of glass material is better. The more lenses, the higher the cost.
In the picture, the lens barrel mainly supports and protects the lens; the whole is made of light-shielding material to avoid the influence of external light. Generally, the shape of the lens barrel is more obvious in the camera lens;
The spacer is used to support the lens structure and control the lens spacing;
The Press Ring fixes the whole lens, protects and strengthens the lens structure, and also functions as the exit pupil to control the FOV.
Therefore, modern lenses are composed of multiple lenses. For example, the current mainstream lens of mobile phones is 1G3P, which means that the lens is composed of 1 glass lens and 3 plastic lenses to obtain the best imaging effect and the lowest cost.
2.2 Voice Coil Motor VCM
The VCM (Voice Coil Motor) voice coil motor is used in focusing, that is, the AF module, through which the voice coil motor can move the lens to the best clear point.
The principle of VCM is to use the coil current of the motor to change the position of the spring leaf in the permanent magnetic field, causing the lens to zoom and make the image clearest.
2.3 Filter IR/Blue Glass BG
The filter IR Filter (Infrared Ray Cut-off filter), that is, the infrared cut-off filter, is between the lens and the sensor photosensitive element, and is mainly used for light filtering.
Because the wavelength of visible light perceived by the human eye is 380-770nm, and the wavelength range of light recognized by the CMOS sensor is 350-1000nm, the wavelength range of the sensor is wider, and it will receive infrared light, resulting in a difference between the overall transformation of the generated image and what the human eye sees. After filtering, the image is closer to the human eye.
Blue Glass BG (Blue Glass) is an absorbing filter. Unlike ordinary IR Filters, which are reflective, BG has better imaging effects, but the corresponding cost is also higher.
The IR Filter of the ordinary surface is reflective, and has a very low reflectance in the range of visible light, which basically has no effect. In the infrared range, the emissivity is high, and halos will be formed. The blue glass BG is an absorbing glass, and there is no great reflection.
On the other hand, ordinary IR will have a large offset with the incident angle, while blue glass BG has a small offset, and there will be no problems of uneven color, black corners, and red spots.
In the mobile phone market, Apple has used blue glass BG filters since iPhone 4, and many mobile phones also use multi-layer coated blue glass filters to achieve better image quality.
2.4 Bracket
As the name suggests, it is used for support. It is mainly loaded with VCM and lens, and the photosensitive element sensor, driver IC and other components will be placed in the inner cavity.
Among them, the Driver IC is related to the front voice coil motor VCM, and the current is controlled through the Driver IC, thereby controlling the position of the VCM.
2.5 photosensitive chip sensor
The photosensitive chip sensor, also known as the image sensor, is the core component of the entire camera group. Its main function is to convert optical signals into electrical signals, and the quality of sensor components directly affects the imaging effect.
There is a saying in the photography circle that "the bottom class crushes people to death", what does it mean? That is to say, the larger the area of the sensor, the more optical signals it can receive, and the better the image quality it gets. It can be seen that the sensor plays an important role in the camera.
There are two types of photosensitive element sensors widely used: one is CCD (charge coupled) element; the other is CMOS (complementary metal oxide conductor) device.
CCD devices can turn light into charges and store and transfer charges, and can also take out stored charges to change the voltage, while CMOS actually just puts transistors on silicon blocks to output electrical signals. Therefore, the cost of CMOS is lower, and the imaging quality of CCD is better, and it is mostly used in high-end cameras.
Now the camera modules of mainstream mobile phone manufacturers are generally CMOS devices . In a limited hardware environment, the picture quality of the two is very close, and CMOS can produce better results.
This article introduces the main types of CMOS devices, which are divided into front-illuminated and back-illuminated.
It can be seen from the classic structure diagram on the Internet that the difference between front-illuminated and back-illuminated lies in the position of the metal wiring layer and the photodiode. The classic front-illuminated metal wiring layer is above the photodiode, while the back-illuminated metal wiring layer is below the photodiode.
The front-illuminated type with the metal circuit layer above the photodiode results in fewer light signals being blocked by the metal layer, and the light utilization rate of the back-illuminated type is at least 30% higher than that of the front-illuminated type, so that it can take photos with more delicate quality and less noise, and significantly improve the shooting effect under low-light conditions.
However, everyone understands such a simple principle, so why is there a front-illuminated structure? In the final analysis, it is still a problem of production technology. At that time, there was no production level of back-illuminated structure, which made it impossible to enjoy the advantages of back-illuminated structure.
Now, all major mobile phone manufacturers are making a fuss about sensor sensors. From the first few megapixels to the current mainstream 50 million pixel IMX989 one-inch super outsole sensor on mobile phones, mobile phone manufacturers have also done a lot on cameras.
2.6 Circuit board PCB
The PCB circuit board is used to transmit the photoelectric signal of the sensor, connect the camera module with the main processor, and perform data transmission.
Now there are more flexible printed circuit boards (FPCB) on mobile phones, which are light, thin, short and small.
As the functions of the camera become more and more complex, the amount of data to be transmitted is increasing, and the FPCB design is becoming more and more important. It can effectively reduce the noise problem in the signal and improve the image transmission efficiency.
Well, everyone has a basic understanding of the imaging process through the simple principle of splitting.
Whether the image is good or not depends on two parts, one is the lens and the other is the photosensitive element, which is the basis of imaging. Only with good hardware can we display superb photography skills. After all, it is difficult for a smart woman to cook without rice!
There will be an article to introduce the ISP image processing module in the later period, so stay tuned.