Why a Clear Window Can Hide a Warm Body
A thermal camera does not measure temperature directly. It detects infrared radiation—energy emitted by objects—and converts the signal into an image that represents apparent surface temperature. The camera’s sensor is tuned to particular infrared wavelengths, often ones that ordinary window glass does not transmit well.
Visible light and thermal infrared behave differently in glass. A pane can pass the wavelengths your eyes use to see a person while absorbing or reflecting much of the longer-wavelength infrared a thermal camera relies on. So the camera cannot simply look through the window as your eyes do.
What the Camera Sees Instead
The glass itself emits infrared according to its surface temperature. It can also reflect infrared from objects on the camera’s side of the pane, much as a mirror reflects visible light. The resulting image may therefore show patterns from the window’s temperature and surroundings, not a clear outline of what is behind it.
That distinction matters when interpreting a thermal image. A bright or dark patch on a window is not reliable evidence of a warm object inside; it may be the pane, a reflection, or both. Thermal cameras are useful for finding temperature differences on surfaces they can observe, but a window can interrupt that view even when it looks perfectly transparent.
