Infrared radiation like all electromagnetic radiation travels in straight lines in space. This is clearly evident with visible light since shadows are formed where the light path is interrupted by an object.

Radiations are emitted from their source in all directions so that if the source is relatively small the radiation will spread out equally in all directions and will feel weaker -- less focused. Likewise, the energy passing through a unit area per unit time - the intensity, will decrease with distance.

The relationship between the distance from the source and intensity of radiation is expressed in the inverse square law which states that the intensity of the radiation from a point source is inversely proportional to the square of the distance from the source:

I = 1/d²

Where: I = Intensity, d = Distance between the source and the point of calculation

So, to double the distance from an infrared source decreases the intensity to (1/2)² = 1/4 of its original value.

This is, of course, only strictly true if there is no scattering or absorption of the radiation but for much radiation it is effectively true in air and of great practical importance.

The consequence is that small changes of distance will cause large changes of intensity. Doubling the distance between the source and the irradiated surface will reduce the intensity to one-quarter; tripling the distance would reduce the intensity to one-ninth and so on. Similarly, halving the distance will quadruple the intensity.