Introduction of Sensor : The structure and principle of the camera module CCM

We often say that the Sensor, that is, the image sensor is only a part of the entire Camera Module, mainly responsible for converting the received optical signal into an electrical signal, in fact, the entire camera module (Camera Compact Module, CCM) is composed of many parts. Other modules other than the Sensor will be responsible for focusing, filtering, shock absorption, data transmission and other functions to better capture images.

SONY and Samsung are the two camera module suppliers favored by many mobile phone manufacturers, of course, there are also many good domestic suppliers.

The camera module mainly consists of the following parts:

Lens: Focus light onto the image sensor.

Voice coil motor (VCM) : Autofocus the camera.

Infrared Filter (IR-cut Filter) : filter out the non-visible light of the human eye.

Image Sensor: Converts optical signals into electrical signals.

Flexible printed circuit board (FPCB) : Connects the camera module to the main processor and transmits data.

The following picture shows the structure of several core components of a common mobile phone camera module.

Step 1: Lens

The main role of Lens is to gather the light collected by the outside world to the image sensor, the principle is small hole imaging, with different lenses, so that the light is focused on the sensor chip at the bottom of the lens (that is, the sensor), to form an image.

Modern lenses are generally composed of several lenses together to achieve more complex imaging effects. The lens is divided into Glass material and Plastic material, such as 2G2P means that this set of lenses is composed of two glass lenses and two plastic lenses, and the general glass lens has a better imaging effect and is more expensive.

2. Voice coil motor

Voice Coil Motor (Voice Coil Motor), also known as the voice coil motor, its principle is similar to a loudspeaker, in a permanent magnetic field placed in a coil, the coil will be energized to create a magnetic field (ampere rule), which will create a gravitational or repulsive force with the external permanent magnet, causing the coil to move. If the lens is placed on the coil, the coil can drive the lens forward and backward movement by changing the current size, so as to control the position of the lens and complete the focusing function.

Ampere's rule: Also called the right hand spiral rule, is a rule that indicates the relationship between the current and the direction of the magnetic inductance line of the current excited magnetic field. Hold the solenoid with your right hand and let the four fingers point in the direction of the current, so that the end pointed by the thumb is the N pole of the solenoid. Figure below left.

The voice coil motor is called this name because it works on the same principle as a loudspeaker, which is the movement of an electrified coil in a permanent magnetic field. See below to the right.

VCM is usually used together with VCM Driver IC(VCM driver chip) and AF(Auto Focus) algorithm. First, the image collected by the Sensor is input to the ISP, and the AF algorithm in the ISP calculates the number of focus steps of the current image, and transmits it to the Driver IC through I2C. The Driver IC calculates the current required to move the coil to the position accordingly, so as to accurately control the behavior of the lens and achieve the autofocus effect. Of course, the method can also achieve the effect of Optical Image Stabilizer (OIS), the principle is the same.

3. Infrared filter

The function of the infrared Filter (IR-cut Filter) is to filter infrared light. The wavelength range of light that the human eye and the electronic sensor can feel is different, and the wavelength range of light that the electronic sensor can feel is much larger than that of humans, therefore, the light that the human eye cannot feel should be filtered out to prevent them from causing color bias and ripple phenomenon, and improve the color reduction degree. This will have a great impact on the subsequent ISP tuning work.

Ir-cut filter is divided into ordinary IR and blue glass IR two categories, the former will reflect infrared and ultraviolet light, it does not enter the Sensor, the latter will absorb infrared and ultraviolet light, so that it does not enter the Sensor. The former is easy to cause multiple reflections of infrared light, resulting in Shading, resulting in a larger color difference in the final imaging effect. Although the latter does not have this problem, the effect is better, but the cost is also ten times that of the former.

4. Image sensor

Image Sensor (Image Sensor) is the core component of the entire module, is also the focus of this introduction, it is a semiconductor chip, its surface by hundreds of thousands to millions of photodiodes, these photodiodes will generate electric charges when exposed to light, thereby converting the light signal collected by the lens into an electrical signal. Therefore, if the role of Lens is equivalent to the lens of the human eye, then the role of the Sensor is similar to the retina, and the light-sensitive diode on the Sensor is the visual cells on the retina.

The Sensor is mainly used to convert the collected external optical signals into electrical signals for subsequent electronic systems to process and store. The Sensor's photosensitive units, that is, each pixel unit, are arranged into an array of pixels, which are distributed with photosensitive diodes. These diodes will be stimulated by external light, according to the intensity of light will produce different strength of electrical signals, and then through the internal A/D conversion discrete into digital signals.

The photodiode can be roughly regarded as a capacitor, the stronger the care, the more charge accumulated on the capacitor at the same time, the greater the voltage generated. If the voltage at both ends of the capacitor is converted into A digital signal through A/D, a digitized value can be obtained, and all the pixels can be combined to obtain a gray scale map. The higher the illumination, the greater the value, the closer the pixel is to white; The weaker the light, the smaller this number, the closer the pixel is to black. This is also consistent with our usual cognition.

According to the different types of photosensitive diodes, the sensors widely used on the market can be divided into CCD sensors and CMOS sensors. CCD is a Charge Coupled Device, that is, a charge-coupled device. CMOS is Complementary Metal Oxide Semicondutor, that is, complementary metal oxide semiconductor. The specific photographic principles of the two semiconductors are not the focus of this paper and will not be detailed. At present, CMOS Sensor has basically replaced the status of the past CCD Sensor due to the advantages of smaller power, faster processing speed and lower manufacturing cost, especially in the field of mobile phones and mobile devices. However, the advantage of the CCD Sensor is that it performs better in low-light conditions, will not be affected by digital noise like CMOS, and will not rely so much on the noise removal work of the ISP.

5. Flexible printed circuit board

The main role of the Flexible Printed Circuit Board (FPCB) is to connect the component with the main processor, and complete the original data transmission function between the Sensor and the main processor.