Why a camera needs an infrared filter
A typical digital camera sensor responds to more than the colors people can see. Its silicon pixels can also register near-infrared light, just beyond red in the spectrum. The sensor does not know that this light is invisible to the eye; it simply records the energy that reaches it.
That extra sensitivity would interfere with ordinary color photography. A camera estimates color by placing tiny red, green, and blue filters over its pixels. Infrared can pass through those color filters and add unwanted signal, making colors less accurate and sometimes softening the image. Manufacturers place an infrared-blocking filter—often called a hot mirror—in front of the sensor to keep most of that light out.
What the remote-control test reveals
A remote control sends commands as rapid pulses of infrared from a small light-emitting diode. You cannot see those pulses directly, but a camera without a perfect infrared barrier may register them. Pointing the remote at a camera and pressing a button can therefore make its front light appear to flash on the screen.
The filter does not make every camera equally sensitive to infrared. Some block it so effectively that the test is faint or invisible; others let enough through for the flash to show. The screen is displaying what the sensor detects, not revealing a beam visible to human eyes.
When the filter is removed
Some cameras intended for night vision or infrared photography have a weakened or removed infrared filter. That lets more infrared reach the sensor, but ordinary colors may no longer look reliable. In a standard camera, the filter is a quiet compromise: it sacrifices sensitivity to invisible light so that the colors in a photograph better match the scene people actually see.
