Hot mirrors are heat-reflecting mirrors that are designed to work as a shortpass edge filter by transmitting visible wavelengths @AOI 0°, while reflecting NIR, heat-generating wavelengths.(Reflect NIR heat & transmit VIS light @ 0°. Remove unwanted heat from an optical system.)
    Application: The products are mainly used in precision optical system, such as a rear projection,TV system, digital projection apparatus, digital camera,and CCD imaging detection system, to suppress  infrared heat radiation interference that improve the optical quality.
    Cold mirrors are heat-transmitting mirrors that are designed to work as longpass filters by reflecting visible wavelengths @AOI 45°, while transmitting NIR wavelengths.(Reflect VIS light and transmit NIR heat @45°.Remove unwanted heat from an optical system.) 
    Application: Cold Mirror can be used in such a system which need to split the heat of spectrum and wave band.Visible light is reflected, and the hot part is transmitted when using the cold mirror.