Hot Mirror vs. Cold Mirror: Which Light Should Pass, and Which Should Reflect?

The names can be misleading. A hot mirror is not a mirror that must run hot, and a cold mirror is not a cooling device. They describe two different ways to separate visible and infrared light.

Draw where you want each part of the spectrum to go. Once the transmitted and reflected paths are clear, the choice is much easier.

Two different optical paths

These are family-level descriptions. The precise wavelength limits, angle and efficiency depend on the design. Some products combine additional ultraviolet control; others do not.

Choose a hot mirror when the useful visible beam continues through

Imagine a white-light source illuminating a visible-light imaging system. If unwanted infrared should be diverted while the visible beam continues forward, a suitable hot mirror may serve that function.

Do not choose solely by the appearance of the glass. Check visible transmission across the required band and infrared rejection across the part of the source spectrum that matters. Also identify where the reflected infrared will travel.

Choose a cold mirror when the useful visible beam is reflected

In a folded illumination path, the visible beam may need to turn toward the target while infrared continues through the optic. A suitable cold mirror can provide that separation.

The transmitted infrared still carries energy. Provide an appropriate path and thermal design for it. Separating that light from the useful beam does not make the energy disappear.

Read transmission and reflection separately

A transmission curve shows what passes through. It does not, by itself, prove that everything else is reflected: some energy may be absorbed or scattered. When the reflected path is important, ask for the relevant reflection information as well.

This matters when comparing KUPO designs. A cold-mirror product's transmitted infrared region describes one path; the performance of its useful reflected visible path needs its own stated requirements.

Angle and heat belong in the specification

A design intended for one angle is not automatically interchangeable with another. Tell us the intended incidence angle and the range of ray angles. Consider both spectral performance and how the reflected beam fits inside the housing.

Share the source type, power conditions, available cooling, glass size and mount. Optical filtering can support heat management, but it does not replace a thermal assessment or establish a safe operating temperature on its own.

What if I only need infrared blocking for a camera?

Start with the camera's desired response. You may need an infrared-cut function without using a reflected beam at all. A hot mirror is one way to achieve infrared rejection; the exact coating and substrate still need to suit the camera and light source.

Explore KUPO hot mirror and UV/IR-cut designs and cold mirror designs. Send us a sketch showing the light entering, passing through and reflecting. Even a simple drawing can prevent choosing the right spectrum for the wrong path.