A brief discussion on the application of filters in machine vision

How to use filters in machine vision

The camera lens assembly consists of a very important component, which is the filter; the IR cut filter, that is, the infrared cut filter, is placed between the LENS and the sensor. Because the human eye and the CMOS sensor respond differently to different wavelengths, the human eye cannot see infrared light but the sensor can sense it, so an IR cut filter is needed to block infrared light.

IR cut filters are divided into two types: reflective (ordinary IR) and absorptive (blue glass). The following figure shows the difference between the two under different light conditions:

The following is a comparison of the characteristics of ordinary IR and blue glass. Blue glass is about 10 times more expensive than ordinary IR, but it has its own advantages, which are reflected in two aspects:

1. Ordinary IR will form a more serious halo phenomenon:

Here are the common IR effects:

Ordinary IR uses reflection, which has high transmittance in the visible light region and low reflectivity, while in the infrared region, it is just the opposite, with high reflection. Therefore, when the camera takes photos at an angle, the infrared light will have a large reflection on the IR film, and after multiple reflections of the light, a halo phenomenon will be formed on the photo.

Here is the effect of blue glass:

Blue glass itself is an absorbing glass that absorbs infrared light without much reflection. The copper ions in the glass have the function of absorbing infrared rays. When the camera takes photos at an angle, the light is mainly absorbed by the blue glass and no halo phenomenon is formed on the photo.

Second: Ordinary IR changes with angle, causing spectrum shift, which will cause shading problems:

The center wavelength offset of ordinary IR is about 30nm at an incident angle of 0°~30°, which will cause color unevenness; the center wavelength offset of blue glass IR is about 2nm at an incident angle of 0°~30°, which will not cause black corners and red spots.

Summarize:

As mobile phone cameras become thinner, the optical design of the lens and the CRA of the sensor are further increased, making the above-mentioned adverse phenomena of reflective IR more prominent. Blue glass can significantly improve the color cast, stray light, and ghosting problems, and the colors of the photos taken are softer and more natural. Therefore, blue glass is a must for cameras that pursue the ultimate effect.