Spectral Response: Monochrome vs Colour Sensors

Spectral Response: Monochrome vs Colour Sensors

Those who have dealt with machine vision sensors may well have noticed that mono cameras have much better response in the near infra-red (NIR) range. The reasons for this are not overwhelmingly obvious at first glance, however are fairly intuitive once they are known. And as often the case, to get to the answers, it helps to ask the right question.

Is a camera an eye?
Let us answer that question in a slightly evasive, but perfectly true, way. The answer is “In some ways yes, in some ways no”.
Is this an important question to ask? Well in order for us to, by no means “dive into”, let us say “to paddle in the shallow waters of” sensor operation, it becomes necessary to speak at a somewhat low level.

Colour cameras are designed to detect, well, colour. Without going into a long philosophical debate about what exactly colour is, let us just say that when we make a colour camera, we want it to emulate what our eyes would observe. To achieve this, colour cameras are equipped with a colour filter array (CFA), also known as a Bayer filter, which is composed of red, green, and blue filters that are placed over each pixel in the ratio of 1: 2: 1.

The Bayer colour filter mosaic - source

The reason for this is our eyes are much more sensitive to green light than the other two colours and, as stated before, we want a colour camera to emulate what our eyes would observe. This ties in nicely with the “in some ways yes” part of our earlier answer to the “Is a camera an eye?” question.
Since these colour filters only let through red, green or blue, not anything in the UV or IR range, this explains nicely why mono cameras have a better response in the NIR range, as no colour filters, red or otherwise, are placed over the sensor. Also, some colour cameras are equipped with an IR cut filter along with the CFA which, as the name suggests, blocks all IR radiation from 650nm and above.

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