Intense laboratory light sources always generate massive amounts of unwanted infrared heat. A traditional mirror made with a shiny metal coating reflects both the visible light and the burning heat. A cold mirror solves this problem using a clever physics trick called thin-film interference.
Engineers coat a piece of glass with many microscopic layers of different chemical compounds. They often alternate extremely thin layers of titanium dioxide and silicon dioxide. These layers are meticulously measured to be exactly a quarter of a wavelength thick.
When visible light hits these stacked layers, the light waves bounce back and amplify each other. This constructive interference reflects all colors we can see, spanning from 400 to 700 nanometers. Meanwhile, the much longer infrared wavelengths do not bounce back at all.
They pass harmlessly through the chemical coating and out the back of the glass substrate. Because the mirror transmits the heat instead of absorbing it, the mirror stays surprisingly cool. Traditional absorptive heat filters often crack or melt because they trap the thermal energy inside the glass.
Students often wrongly assume that a cold mirror actually cools the air around it. It merely redirects the existing heat away from the main working path of the light beam.