Dichroic mirror reflecting visible light, transmitting heat radiation — essential in front of HMI and tungsten to protect sets and talent. Reduces heat buildup by up to 50 percent.
You need a cold mirror — or you'll get into a conflict with your set designer because the heat destroys the colors and your actors are sweating through their costumes after ten minutes. The device works on a simple optical principle: a dichroic coating reflects visible wavelengths but lets infrared radiation pass through — out of the set, not into it. That's the whole magic.
In practice, you place the cold mirror directly in front of or behind your HMI head or your Tungsten Fresnel. With an 18K HMI, you can achieve a reduction in heat radiation by up to 50 percent — this means your key light sits radiantly white on the face, but the ambient temperature doesn't skyrocket. This is especially important in tight situations — cramped sets, many actors close together, long takes. You notice it immediately: the sweat stains on shirts get smaller, the makeup artist needs fewer touch-ups, and the psychological burden on the performers measurably decreases.
Technically, there are different versions: as a glass insert in reflectors, as a separate frame to be placed in front, or already integrated into modern lamp heads. Important: The cold mirror costs light intensity — plan for half to a full stop less. That's the trade-off. Some DoPs work with a hot filter in combination, some place the mirror directly in front of the lens in the spotlight. The quality of the dichroic coating is crucial — cheap glass shimmers and loses color purity.
Workflow tip: If you're shooting long dialogue scenes and your talent is sensitive to light, position the cold mirror during the lighting test. Let the actor feel for themselves if the heat becomes bearable. The psychological effect of heat radiation is often underestimated — it feels aggressive, even if the temperature isn't objectively extreme. With a cold mirror, your set feels more relaxed, and the performances become more natural. It sounds trivial, but it's not.