Chief Ray Angle(CRA) is known as Chief Ray Angle, Image Sensor (Image Sensor) CRA exists because Image Sensor surface of the Mirco Lens FOV (Field of view) (Field of view), and the size of the CRA value depends on the Image Sensor Micro Lens and silicon photodiode position there is a horizontal error value. The size of the CRA value depends on the position of the Image Sensor’s Micro Lens and the silicon photodiode, which is made to match the lens better.

Choosing the CRA of the matching Lens & Image Sensor ensures that more accurate photons are captured into the silicon photodiode, thus reducing optical crosstalk.

For Image Sensor with small pixels, the main ray angle has become an important parameter. This is because light has to pass through the depth of the pixel to reach the silicon photodiode at the bottom of the pixel, which helps to maximise the amount of light that correctly enters the optical silicon photodiode and reduces the amount of light that enters the silicon photodiode of the neighbouring pixel (creating optical crosstalk).

So Image sensor can ask Image Sensor manufacturer & lens manufacturer for CRA curve diagram for matching when choosing lens; it is generally recommended to control the CRA angle difference between Image Sensor and lens within +/-3 degrees, of course the smaller the Pixel the higher the requirement.

Impact of mismatch between lens CRA and sensor CRA

1.  The mismatch between the CRA of the lens and the CRA of the sensor will lead to the crosstalk of the colour imbalance of the whole image, which will lead to the reduction of the Signal to Noise Ratio (SNR); because the CCM needs to increase the digital gain to compensate for the loss of signals in the photodiode.
2. CRA does not correspond to the image will cause unclear, foggy, low contrast, colour fading, depth of field and other problems.
3. Lens CRA is smaller than Image Sensor CRA will produce colour shading.
4. If the Image Sensor is smaller than the CRA of the lens, it will produce Lens shading.

Therefore, we should first ensure that there is no Color shading, because Lens shading is easier to solve through debugging than Color shading.

CRA not only affects the clarity and distortion of the image, but is also directly related to the uniformity of illumination of the imaging system. If the CRA is not reasonably designed, it may cause the image centre to be bright and the edges to be dark, a phenomenon that is especially not expected in the field of professional photography. Reasonable CRA design can ensure that the light is evenly distributed on the sensor, making the imaging more balanced.

In the actual imaging system design, the selection of CRA needs to fully consider the characteristics of the sensor and the optical design of the lens. For example, some large-size sensors may require a smaller CRA to ensure that the light can evenly cover the entire sensor surface to prevent dark corners. Conversely, small-size sensors may be more suitable for using a larger CRA due to their own physical limitations.

In order to optimise CRA, it is necessary to take into account the characteristics of the lens design and the image sensor. First, in the lens design stage, advanced optical elements should be used as much as possible to reduce optical aberrations and improve light transmission efficiency. This not only helps to improve image quality, but also reduces the need for strict CRA matching.

Next, when selecting a chip, factors such as the spectral response, pixel size and layout of the chip should be taken into account, all of which will affect the chip’s adaptability to CRA. Specifically, during the assembly of the imaging system, the CRA can be fine-tuned to achieve the best imaging effect by adjusting the physical distance between the lens and the chip.

Considering the different requirements for imaging quality in different application scenarios, the selection and adjustment of CRA is particularly important. For example, in the field of security monitoring, in order to ensure that clear images can also be obtained at night, it is usually necessary to select a low CRA lens matched with a high sensitivity chip to increase the amount of light intake.

On the other hand, in the field of medical imaging, the precise adjustment of CRA is especially critical due to the need to obtain extremely high image details. By accurately controlling CRA, not only can the edge sharpness of the image be improved, but also the distortion can be reduced to ensure the accuracy of medical diagnosis.

It is clear from the above analysis that the selection and adjustment of CRA is a key factor affecting the performance of the imaging system. Reasonable CRA matching is not only related to image quality, but also directly affects the application effect and field of the imaging system. Therefore, whether in the lens design, chip selection, or the final tuning process of the imaging system, the importance of CRA is an important part of improving the imaging performance and meeting the needs of different applications.