Referencing our recent post on hot mirrors can help greatly in understanding how a cold mirror functions as they are both heat control coatings.
Cold mirrors are much like hot mirrors in that they are used to separate IR from the non-IR. The major difference between the two is that cold mirrors transmit IR bands and reflect one or more non-IR bands. As can be seen in our UV Cold Mirror graph, the coating represented is specially designed to reflect more than 95% of UV rays from 350 to 450 nm while transmitting more than 90% between 550 and 1200 nm at 45 degrees angle of incidence. This means that you can use this coating to split a beam and have the longer bandwidths (550 to 1200nm) transmitted while UV rays (350 to 450 nm) are reflected and sent another route. This can help isolate bands that are needed for a particular application while unneeded or possibly harmful bands can be isolated and removed from the application.
One particularly useful application for this kind of dichroic filter we’ve discussed before is in reference to optical fiber. Optical fiber can be damaged by Ultra-Violet and Infra-Red radiation. When a cold mirror is used in conjunction with a hot mirror you can isolate application specific visible light from harmful UV and IR bandwidths such that the visible is transmitted to the optical fiber to be used and the UV/IR are reflected away from the fiber where they will not damage or harm the application.
Cold mirrors are often used in lighting applications where excess heat is not desired and IR radiation is not helpful. In these applications the visible light is reflected to the application and the IR is transmitted away from the application.