Precautions for selecting facial recognition filters
How to choose the light source for facial recognition filter
In facial recognition image acquisition devices, high-power infrared diodes with wavelengths of 850 nm and 940 nm are commonly used as light sources. Although LED products purchased on the market have nominal values of 850 nm or 940 nm, there are still many deviations when measuring the center wavelength of specific LED products. Taking 850nm light-emitting diodes as an example, the actual center wavelengths are 835nm and 865nm, and the bandwidth of a single 850nm LED is about 50nm. If the center wavelengths are different, the spectral bandwidth of multiple LEDs stacked together will be very wide. This is very unfavorable for the selection of bandwidth, energy utilization, and signal-to-noise ratio of subsequent narrowband filters, so when choosing LED light sources, their center wavelengths should be consistent. In addition, as the working temperature increases, the center wavelength of the LED light source shifts towards longer wavelengths. For every 10 ℃ increase, the center wavelength of the light-emitting diode shifts towards longer wavelengths by 1nm. As the operating temperature increases, the luminous efficiency of the LED rapidly decreases. When the temperature rises to around 85 ℃, the output efficiency of the LED decreases to around 50%. In addition, when choosing the divergence angle of LED, the smaller the divergence angle, the better, which can improve the energy utilization efficiency of the light source.
How to choose the thickness of the facial recognition filter?
Considering cost factors, CCD receivers and corresponding lens groups used in facial recognition systems are basically ready-made, universal, and widely used in network cameras or mobile phone cameras. The focusing range of this universal camera is relatively small. If a filter is placed in front of the CCD, it will introduce additional optical path difference to blur the image. When the optical path difference is relatively small, adjusting the focal length can make the image clear. However, when the filter thickness is large, the introduced optical path difference is also large, and cannot be adjusted by adjusting the focal length, resulting in blurry images. Therefore, many people place the filter in front of the CCD camera lens instead of the CCD sensor, as it acts as a filter that does not interfere with the imaging path. However, the filter placed in front of the lens is large in size, inconvenient to install, and not aesthetically pleasing. Therefore, many people hope to reduce the area of the filter and place it in front of the CCD sensor, which can be built into the camera. This not only saves the cost of the filter, but also does not affect the appearance. To place the filter inside the camera, it is required that the thickness of the filter be very thin. After practice, it is suitable to use 0.55mm or 0.7mm thick filters. Ruiyan Optics has successfully achieved the technology of coating narrowband filters on glass substrates above 0.2mm using all dielectric hard film coating technology.
Precautions to be taken when capturing large angle views
If you need to capture a large angle of view, the narrowband filter needs to be placed closest to the CCD or CMOS, that is, built-in to the camera. If the filter is placed directly in front of the camera lens, the shooting angle is generally around 20 degrees Celsius; Within.
Precautions to be taken when there is interference light with a large angle present
Even if there is no need to capture a large angle of view, if there is interference light at a large angle, especially when the signal wavelength is slightly shorter than the signal wavelength, in order to improve anti-interference ability, it is recommended to place the filter in the camera, close to the front of the CCD or CMOS, and behind the lens, which is conducive to reducing interference light in the large angle field of view.