Because the human eye and CMOS sensor respond differently to different wavelengths, the human eye cannot see infrared light but the sensor can detect it. Therefore, an IR cut filter is needed to block infrared light.
IR cut filters come in two types: reflective (ordinary IR) and absorptive (blue glass). The difference between the two can be seen in the image below under different lighting 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 advantages, which are reflected in two aspects:
1. Ordinary IR will produce a more severe halo phenomenon:
Below is the effect of normal IR:
Ordinary IR filters use reflection, exhibiting high transmittance and low reflectance in the visible light region, but the opposite is true in the infrared region, with high reflectance. Therefore, when a camera takes a picture at an angle, infrared light will be significantly reflected on the IR film. After multiple reflections, a halo effect will appear on the photograph.
Below is the effect of blue glass:
Blue glass is an absorptive glass that absorbs infrared light and has minimal reflection. The copper ions in the glass also absorb infrared radiation. When a camera takes a picture at an angle, the light passes through the main absorption of the blue glass, preventing halo effects in the photograph.
Second: Ordinary IR spectrum shifts with angle, which can cause shading problems.
Ordinary IR has a center wavelength offset of about 30nm when the incident angle is 0°~30°, which will cause uneven color. Blue glass IR has a center wavelength offset of about 2nm when the incident angle is 0°~30°, and will not have black corners or red spots.
Summarize:
As mobile phone cameras become increasingly thinner, the optical design of the lens and the CRA of the sensor are further enlarged, making the aforementioned defects of reflective IR more prominent. Blue glass significantly improves color cast, stray light, and ghosting issues, resulting in softer and more natural colors in photos. Therefore, blue glass is the must-have for cameras that pursue ultimate performance.
