With infrared detection, distance is
NEVER measured.
In order to truly measure something, you need absolute control of the medium and method of measurement, which includes units of measurement, such as kilogram or centimeter or hectare. You actually take the ruler and apply it against the object to be measured.
With radar detection, you control the measurement medium -- EM wavelengths that are outside of the visible and infrared spectrum. You control the method -- a specifically designed transmitter. And you control the timing of events -- when to transmit and if you want to receive the reflections.
With infrared detection, you control only if you want to receive any infrared radiation -- or not.
View attachment 192788
The more in control of factors in an environment, the more aware are you of that environment, which includes how long some things happens, to what degree, in which direction, etc. This is why radar detection is still the preferred and premier method of detection for objects that are beyond physical reach.
As far as distance in infrared detection goes, because you are
NOT in control of the detection medium -- EM wavelengths that are in the infrared region -- you do not know the medium's characteristics common in radar such as pulse repetition and duration, you cannot adequately compensate for real time variations of the medium, and because you cannot compensate for variations, you have no choice but to use whatever medium characteristics you can get and work from there.
In infrared detection, the best and always coarse medium characteristic is irradiance.
- the flux of radiant energy per unit area (normal to the direction of flow of radiant energy through a medium).
If there is an increase in irradiance over time, then you can guess, with reasonable certainty, that you are approaching the target, or you are the target and the object is approaching you. The key here is time. The finer the granularity of time measurement, such as millisecond or better picosecond, the better your guess as to how far/near is the target from you. But ultimately, it is still a guess, albeit a calculated one.
Application of this ? Your TV remote control...For example...
http://inside.mines.edu/~whoff/courses/EENG383/reference/appnote2.pdf
Irradiance levels are used by the receiver to cut off responses, meaning that if you hold the TV remote control at a certain distance from the TV, the infrared receiver inside the TV will not respond to any embedded commands. This is to prevent the TV from responding to stray infrared that maybe in the area.
If the radiant intensity of the body increases and you are keeping time with zero as the moment of detection, you can be reasonably certain that the body is approaching you. Conversely, if the irradiance of an object is at a certain level, you can guess its distance from you based upon a preset table.
Again...Infrared distance is never measured, only derived or inferred.