How filters work
Filters are a piece of glass that is attached to the front of a lens, which all incoming light has to pass through. Some filters are clear and are designed to protect the front of the lens, whereas others are coated with special chemicals to serve other purposes. Other filters are colored or produce special effects, such as ‘star bursts’ of light. However, the key kind of filter for IR photography is the infrared filter.
There are two types of infrared filters, ones that block IR light while passing visible light and ones that block visible light while passing infrared light. The IR blocking filters are often used in digital video & still cameras that use CCD or CMOS sensors to prevent unwanted IR light from reaching the sensor, which is sensitive to near infrared.
In infrared photography we want the opposite, to block visible light and only passing infrared light.
There are several types of infrared passing filters, also called low-pass filters. The most common used in photography are actually made of ionic or colloid colored glass to absorb the unwanted frequencies. The other kind are made to behave like a mirror but only reflecting certain frequencies while still passing through the others. Cold mirrors are made using a coating that reflects visible light but allows infrared light to pass through. The opposite of a cold mirror is a hot mirror, which reflects infrared light and allows just visible light to pass through.