A daylight filter permits that daylight films colour temperature can also be exposed to artificial light at the right colour temperature. The filter consists of a violet foil or a dichroitic, heat-resistant mirror.
Essentially, the daylight filter makes the spectrum of an artificial source of light resemble the sun spectrum. The red wave range that is stronger in artificial light is weakened through the filter, the blue wave range, typically predominant for daylight, remains at the same intensity. By using the filter you simulate a uniform light temperature that also has positive effects in situations with mixed light.