Camera module lens selection——(What is CRA)
January 20, 2024
Preface: When selecting a lens, an important indicator we must pay attention to is whether the CRA of the sensor and lens match. When the CRA of the sensor and lens do not match, it will lead to the appearance of luma shading or color shading, which will affect the later imaging. The effect has a great influence. This article mainly introduces the concept of CRA.
1. What is CRA?
The performance of the sensor is related to the sensitivity of the sensor itself and the angle of light incident on the sensor. The angle at which light enters the sensor pixel is determined by the Lens' CRA and the sensor's Micro Lens opening layout (sensor's CRA).
CRA is the abbreviation of Chief Ray Angle, which means key light angle. On the sensor side of the lens, the maximum angle at which light can be focused onto a pixel is defined as a parameter called the dominant angle (CRA). The pixel response at this angle is reduced to 80% of the pixel response at zero degrees (where the pixel is perpendicular to the light).
We will have a CRA parameter when selecting a lens, and there will also be a CRA parameter when selecting a sensor (there is a microlens on the sensor pixel, which is why there is a sensor CRA). When selecting the lens and sensor, the CRA The parameters should be as close as possible (generally no more than 3 degrees). Otherwise, when the lens CRA is larger than the Sensor CRA, obvious color shading will appear. When the lens CAR is smaller than the Sensor CRA, luma shading will appear. It is recommended to choose the configuration where the lens CRA is smaller than the sensor CRA, because luma shading is easier to handle than color shading and the backend ISP. .
2. Why does CRA lead to lu ma shading?
When lens CRA is smaller than sensor CRA, it is understood from the definition that the maximum light angle of light incident on the pixel from the lens is smaller than the maximum light angle of light incident on the pixel through the sensor micro lens, but the imaging light path is the lens- --> sensor micro lens --->pixel, so when the light incident to the lens CRA is satisfied (that is, the range of the light incident angle is 0-CRA, which is the maximum incident light angle), when passing through the sensor micro lens, the micro The CRA of the lens is larger, which causes the pixels at the edge of the sensor to lose light, resulting in vignetting.
3. Why does CRA cause color shading?
The above picture is the internal structure of the sensor. The function of the micro lens on the sensor is to focus the light and introduce the incident light to the correct pixel point (you can see that when there is no micro lens, the light is vertically incident, and the light at the edge cannot illuminate the pixel. ), when the CRA angle of the incident light exceeds the CRA of the sensor, it will cause the light passing through the R-filter to shine on the G pixel, causing crosstalk between the pixels and color shading;
As shown in the figure above, when light is vertically incident (that is, when CRA is 0), light can be correctly incident on the pixel, but the CRA of the Lens is difficult to be 0. The larger the field of view of the lens, the greater the CRA value. , the micro lens on the sensor needs to match the lens. In fact, by moving the micro lens, the incident light at a large angle is guided to the correct pixel, so as to ensure a normal image (as shown in Figure b). Therefore, the maximum light path angle that the sensor micro lens can correct is the sensor CRA. However, when the lens CRA is larger than the sensor CRA, the sensor cannot correct all large-angle incident light rays to the correct pixels by displacing the micro lens, which will lead to color shading. Appear.