Reach is a square root, not a multiplier
An illuminator emits a fixed amount of light. The beam angle decides what solid angle that light is spread over, and intensity is the light divided by the solid angle. Squeeze the cone and the intensity goes up in proportion. But the distance that intensity carries goes as the square root of it, because illuminance falls off as the square of distance.
The practical version: halving the beam angle roughly doubles the reach, and doubling the number of units only multiplies the reach by about 1.4. Two illuminators side by side is one stop of light and forty percent more distance. That is the trade that makes people think their second unit was faulty.
The doubling rule for the beam angle is close but not exact, and it drifts with the starting angle. Going from 120 degrees to 60 gives 1.93 times the reach; from 20 to 10 it gives 1.998. Halving the beam from every starting angle between a couple of degrees and a full 180 lands somewhere between 1.850 and 2.000, and never outside that. The page computes it from the solid angles rather than using the rule of thumb, so the figure it prints is right for the angles you typed. All of this assumes the total light output is unchanged, which is true when you are genuinely redirecting the same emitters and not true when you are comparing two products.
Matching the beam to the lens
The commonest night-time failure in a camera install has nothing to do with the camera. A 30 degree illuminator behind a 90 degree lens lights 27 percent of the frame width. The automatic exposure meters the bright centre, stops down, and the dark three quarters become black. Somebody then buys a brighter illuminator, which makes the centre brighter and the sides no better at all. The defaults on this page are the milder version of the same mistake: a 60 degree beam through a 90 degree lens covers 58 percent of the frame.
The reverse wastes light rather than ruining the picture. A 120 degree beam behind a 90 degree lens throws about 41 percent of its output onto ground the camera never records — and onto whatever is out there, all night, every night. Neither error shows up on a spec sheet, because the illuminator and the camera are usually specified by different people at different times.
The near object problem
Inverse square is unforgiving at short range. An illuminator on a 10 ft mount lighting a subject at 60 ft is also lighting a porch post at 8 ft, and once the mount height is allowed for in both slant distances the post receives 22.6 times as much — four and a half stops. No camera holds a twenty-two to one range in the same frame, so it does not: the post goes to white, the exposure closes down to control it, and the subject at 60 ft disappears.
Every fix for this is physical. Move the illuminator away from the camera so the near object is lit obliquely rather than head on. Shield the bottom of the beam. Take the near object out of the frame. Turning the gain up, or buying a bigger illuminator, makes it worse in both directions at once.
What the rating is worth
Not much on its own. There is no agreed definition of infrared reach, no standard test scene, and no requirement to state which one was used. Some makers quote the distance at which a person is detectable, some the distance at which their own camera produces a picture they consider acceptable, and some do not say. The beam angle is quoted with the same freedom — at half intensity by one maker and at a tenth by another, which alone changes the number by a large factor.
That is why this page works in ratios. It takes the maker figure as the baseline and tells you what your distance, your scene, your beam change and your losses do to it relative to that baseline. If the baseline is optimistic, every result here is optimistic by the same factor, and the only way to find out is to point it at the scene at night and look.
What the numbers cannot include
Scene return dominates and cannot be tabulated. Dark brick, wet asphalt, evergreen foliage and dark clothing send back a small fraction of what a pale rendered wall does, and the difference between a dry car park and a wet one is larger than most of the changes on this page. Retroreflective surfaces do the opposite and come back far brighter than everything around them, which is why a plate or a hi-vis stripe can be the only white object in an otherwise black frame. The page takes your own estimate of scene return as a percentage and applies it honestly, which is different from being able to predict it.
Questions people ask
Does a narrower IR beam really reach further?
Yes, and by about the square root of the concentration. The same emitters through half the beam angle put roughly four times the intensity into the cone, and four times the intensity carries twice the distance because illuminance falls as the square of it. The page computes the exact figure from the solid angles, which lands between about 1.85 and 2 depending on where you start.
Will two IR illuminators double my night vision range?
No. Two units on the same spot give twice the light, which is one stop, and twice the light reaches about 1.4 times the distance. If you want double the distance from the same output you need to quarter the solid angle, which means roughly halving the beam angle and accepting a much narrower lit patch.
Why is the middle of my night footage bright and the edges black?
Almost always because the illuminator beam is narrower than the camera lens. The lit centre is what the automatic exposure meters, so it stops down, and the unlit edges go from dim to black. A brighter illuminator makes the centre worse. Matching the beam angle to the field of view is the fix, and it usually means a wider illuminator rather than a stronger one.
Why does something close to the camera turn white at night?
Inverse square. A railing at 8 ft from a mount that is lighting a subject at 60 ft receives tens of times more light, and no sensor holds that range in one frame. The camera chooses the bright thing, and the distance goes dark. Moving the illuminator away from the lens, shielding the near part of the beam, or removing the object from frame are the three fixes, and none of them involves a setting.
How far will a 100 ft IR illuminator actually reach?
Unknown, because nobody has defined what the 100 ft means. There is no standard scene, no standard subject and no obligation to say which was used, so two units marked the same can differ by a large factor. This page works in ratios against whatever that figure turns out to be, and the only way to convert it into a real distance is to put it up and look at the recording.