Technology

Night vision and IR range: what the data sheet figure really says

11 September 20262 min readcctvfinder

Pixel density does not change at night. The distance stays the same, so does the lens, and the standard does not lower its requirement when the sun goes down. What changes is the amount of light reaching the sensor. And it drops faster than most people expect.

The range figure measures something else

When a data sheet says "IR range 50 m", it means the camera still throws measurable infrared light that far. It does not mean a face is recognisable there. The measurement is taken against a highly reflective surface, not against a person in dark workwear.

The two diverge because clothing reflects very differently. A light jacket gives a usable image, a dark one swallows the light. The data sheet figure is an upper bound under the most favourable conditions, not a promise for the ordinary case.

IR range tells you how far the light travels. It says nothing about what is still recognisable at that distance.

Why the light falls away so quickly

Infrared behaves like any other illumination: its intensity falls with the square of the distance. Twice the distance means a quarter of the illuminance, not half.

Distance	Illuminance at the target
10 m	100 %
20 m	25 %
30 m	11 %
40 m	6 %

The values are relative to 10 metres. At 40 metres a sixteenth of the illumination arrives. That is why a night image does not degrade gradually but becomes unusable across a few metres.

The lens has a say

The infrared diodes sit in the camera housing and light a fixed cone. A wide lens sees beyond that cone: the edges stay dark while the centre is bright. A narrow lens sees only the lit part, and there the light is spread across less area.

That aligns with the daytime calculation. A narrow field of view puts more pixels on the metre and at the same time more light on the surface. Choose the optics tightly for the daytime job and you win twice at night.

What is left of the resolution at night

A sensor short of light has two ways out, and both cost image quality. It amplifies the signal, and then the image is noisy and fine structure disappears in the grain. Or it exposes longer, and then anything moving draws a trail.

The calculated pixel density is untouched by this. The pixels are there, they simply carry less information. A setup that reaches exactly the required 125 pixels per metre in daylight has no reserve left at night.

Plan the daytime job with room above the threshold. What is barely enough by day is not enough by night.

How to check it

Set the spot up in cctvfinder and look at the daytime figure first. If it sits only just above the required level, the setup is planned too tightly for the dark. A narrower lens or a camera closer to the target raises both figures at once.