Diagram of a Thermal Eyetap which can also be used as a night vision system that uses the EyeTap principle). The purpose of this image is to show the EyeTap principle (how an EyeTap works), by way of a particular example, namely the example of the of operation of a thermal EyeTap device.

Rays of eyeward bound heat (heat rays that are collinear with lines passing through the center of projection of the eye) are diverted into a thermal sensor that functions a lot like a thermal video camera, and with the appropriate processing, this measures some quantity (such as temperature) of each ray of heat. Note that the distance "d" (which is called the "eyetap distance" specifies that the camera and the eye must be optically equidistant from the point where the optical axis meets the diverter, which is equidistant for a planar diverter.

Each ray of heat is resynthesized as light, usually by way of a computer controlled laser system called an "aremac". The aremac thus "repaints" or re-synthesizes each ray of heat.

Thermal eyetaps are sometimes used by roofers, so that they can see through the insulation in a roof, and determine where heat loss is. Being able to "see" through the insulation means that the roof leaks can be found because waterlooged insulation conducts heat more, and such areas of the roof show up as white, on black background. Additionally, the eyeglasses help the roofer see the temperature of bitumen or modified bitumen as it's heated in a propane powered kettle, and mopped onto the roof. It is useful to see the mop-down process, and to see when the bitumen or modified bitumen is getting too cold to make a good seal, etc..

Thermal eyetaps can also be used as night vision systems, and they eliminate the undesirable "flashback" effects common with other night vision systems because of adaptation and post adaptation phenomena.