The wavelength range of visible light that the human eye can feel is generally 380~700nm.
There is also near-infrared light in nature that cannot be seen by human eyes. At night, this light still exists. Although it cannot be seen by human eyes, it can be captured by using CMOS sensor.
Taking a CMOS sensor we used in zoom camera module as an example, the sensor response curve is shown below.
It can be seen that the sensor will respond to the spectrum in the range of 400~1000nm.
Although the sensor can receive such a long range of spectrum, the image processing algorithm can only restore the color of visible light. If the sensor receives near-infrared light at the same time, the image will show red.
Therefore, we came up with an idea to add a filter.
The following figure shows the imaging effect of our long range 42X starlight zoom camera module equipped with laser illuminator at night In the daytime, we use visible light filters to filter infrared light. At night, we use full pass filters so that near-infrared light can be received by the sensor, so that the target can be seen under low illumination. But because the image cannot restore color, we set the image to black and white.