Basic camera module literacy
August 5, 2021
Basic camera module literacy
The commonly used structure of a mobile phone camera is shown in Figure 37.1, which mainly includes the lens, base, sensor and PCB part.
CCM (compact camera module) type
1.FF (fixed focus) fixed focus camera
Currently, the most used cameras are mainly used in mobile phones with 300,000 and 1.3 million pixels.
2. MF (micro focus) two-speed zoom camera
Mainly used for 1.3 million and 2 million pixel mobile phone products, mainly used for distant and close-range shooting, scenery shooting in the long-range shooting, business cards in close-up shooting and other objects with magnetic barcodes.
3.AF (auto focus) automatic zoom camera
Mainly used in high-pixel mobile phones, with MF function, used in 2 million and 3 million pixel mobile phone products
4.ZOOM automatic digital zoom camera
Mainly used for higher pixel requirements, pixel quality above 3 million.
For the lens, its function is to filter out the invisible light, let the visible light enter, and project it on the sensor, so the lens is equivalent to a band pass filter.
CMOS Sensor part
For now, sensors are mainly divided into two categories, one is CMOS, the other is CCD, and now CMOS is a trend.
For the lens, a lens can only be applied to one sensor, and the size of the general lens should be the same as that of the sensor.
For the sensor, the Bayer array is still being used, as shown in Figure 37.2 below. Figure 37.3 shows the working process, illumination→charge→weak current→RGB signal→YUV signal.
Figure 37.4 shows the working principle of the sensor, which is exactly the same as OV7670 and OV7725.
So for the pixel part, we often hear 300,000 pixels, 1.2 million pixels, etc. What do these mean? Figure 37.5 explains these terms.
From the above introduction, it can be concluded that we take 300,000 pixels as an example, 300,000 pixels ~= 640 * 480 = 30_7200; it can be seen that the so-called number of pixels is the number of pixels in a frame of image, we can associate image processing Related knowledge, the value of the number of pixels here is the gray value we often say. The higher the number of pixels, the better the quality of the displayed image and the clearer the image, but the corresponding storage requirements are also put forward. In image processing, we will also hear a concept called resolution. In fact, this concept It should be concretely called the spatial resolution of the image, such as 72ppi, which means 72 pixels per inch. A better camera can reach 490ppi.
Common sensor vendors
At present, most of the market is occupied by OV Howe Technology's supply, and micron also has a certain market share.
Currently, there are two main types of packaging for sensors. DICE. The process corresponding to CSP is SMT, and the process corresponding to DICE is COB. The related concepts are explained as follows:
CSP: chip scale package, this package format is mainly used by OV.
COB packaging is chip on board, which is to stick the bare chip on the interconnection substrate with conductive or non-conductive glue, and then perform wire bonding to realize its electrical connection, which is mainly used by Samsung and micron.
Then the two packaging forms are shown in the figure below.
For AF and AZ cameras, the way to achieve auto focus is basically to use voice coil motor, stepping motor, piezoelectric motor, here is the main Introduce how the voice coil motor works.
In the above description, we mentioned MTF, the modulus transfer function.
The largest can focus on the sensor, and can cover the entire pixel plane. It is better to match the CRA of the lens with the CRA of the sensor. If there is a deviation, it is best not to deviate by 2°.
Lens: CRA is smaller than sensorCRA, the surroundings will be dark, and the light will not reach the edge of the pixel;
The lensCRA is larger than the sensorCRA, and the light is refracted to the adjacent pixels, causing crosstalk between pixels and the color cast of the image, which becomes more obvious around the image, because the CRA rises in a curve from the center of the image to the periphery and gradually becomes larger.