Infrared Images

In crime dramas and in adventure films the result of an infrared vision device is often reconstructed and shown in a sharp greenish or black-and-white format. We see the infrared faces of people as if they were originally color images seen normally with one’s eyes, but now these color images have been converted to black and white. Is there any physics violation here in depicting the infrared images via this process?

Answer

The actual converted infrared image would be a blurred black-and-white image, not a sharp one. The physics places a limit on the resolution of images we see in the infrared compared to the visible. When we look through binoculars or any lens system in the visible, these systems have a resolution limit that depends on the quality of the optic elements. No matter how much the image is enlarged or enhanced by dithering, etc., the original resolution is not improved even though the image looks cleaner. But the physics is even more restrictive when comparing an infrared image to a visible image because the visible light has been coherently scattered from the object, whereas the infrared light has not, as explained below. Of course, one cannot exceed the Rayleigh criterion for resolution of approximately one wavelength of the light, except by using interference techniques, which we do not consider here.

In the visible part of the electromagnetic spectrum, atoms absorb and emit the photons of light in a two-step process, usually absorbing and emitting in about 10^–16 second. During this time interval, the molecule holding that atom moves very little. Nearby atoms also scattering this impinging beam of numerous photons tend to remain in place during their scatterings. In effect, during the scattering of each photon there always will be a fixed phase relationship among all the atoms scattering light from the object to your light sensors. Hence, photons scattering from different areas on the object’s surface carry detailed phase information with fixed phases to achieve nearly maximum resolution. If the phases actually varied randomly from one location to another, the visible image would become blurred.

In the infrared, the image is blurred because most of the infrared is absorbed and emitted by molecular vibrations and rotations that have random phases over the object’s surface. This scattering process takes much longer about 10^–12 second sufficient time for the molecule to move considerably during the scattering, so there will not be a fixed phase relationship among neighboring molecules on the surface of the object. The same surface that appeared well resolved in the visible will now be quite blurred in the infrared. No magical digital techniques will be able to take an infrared image and make a sharpened black-and white image true to the original object.

In the ultraviolet, the scattering time is very short, but so is the wavelength, so the extent of the coherent scattering area also is very short; thus the image is blurred compared to the visible one. Nature has given us a vision range in the visible that ensures the best resolution of detail.