heat filter[′hēt ‚fil·tər]
an attachment to or a component of an optical system that is designed to eliminate infrared (heat) rays from the luminous flux passing through the system. The heat rays may be absorbed, as in absorbing heat filters, or removed from the luminous flux, as in cold light interference mirrors.
The simplest heat filter is a glass plate transmitting rays of visible light and absorbing heat rays. Heat filters are used in the light sources of biological microscopes and photomicrographic devices to protect live microorganisms from the harmful effects of heat, as well as in various projection instruments to prevent overheating of the original, whose image is being projected onto a screen.