Optical filters are a great tool for a wide variety of lighting applications. One common use of optical filters includes changing the correlated color temperature of a light source. For example, changing the appearance of light from a tungsten lamp so that it looks more like daylight; or, changing light from a flash lamp to look more like light from a tungsten lamp. The class of optical filters used to make these types of color changes are called Color Temperature filters.
The term color temperature comes from the natural phenomenon of colored light emitted by warm objects. Very warm objects, such as a candle flame, emit deep red and orange light. The temperature of a candle flame is roughly 1500K. If you increase that temperature the light emitted begins to look more blue. An example that comes to mind is a piece of hot iron worked by a blacksmith. A hotter fire heating the iron will cause the iron to glow increasingly with a blue hue. The blue appearance of the iron indicates that the temperature of the metal is up above 6500K.
Of course, optical filters don’t really change the temperature of the object emitting the light. We are able to achieve our magic by using color temperature filters to remove some of the light of wavelengths of our choosing. So for example, we can use a filter to absorb or reflect away some of the orange and red light emitted by a tungsten lamp. This makes the remaining light look more blue and results in a higher correlated color temperature. Conversely, we can use a filter to remove some of the blue light emitted by a flash lamp making the remaining light look more orange and having a lower correlated color temperature.
Fortunately these color temperature filters have been standardized. There are two types typically supplied by Abrisa Technologies. These two types are Color Temperature Orange (CTO) and Color Temperature Blue (CTB). Each type is also divided into a few different standard values indicating the amount or degree of color temperature shift they induce. A Full CTO is specified to have a color temperature shift that changes a 5500K color temperature to approximately 3200K. The other standard CTO filters supplied by Abrisa Technologies are 1/8, 1/4, and 1/2 CTO. Respectively, these shift a 5500K light source down to approximately 4900K, 4500K, and 3800K.
A full CTB filter does just the opposite and shifts 3200K up to 5500K. Other standard values of CTB include 1/4 and 1/2. The 1/4 CTB filter shifts 3200K up to approximately 3500K. And the 1/2 CTB filters shifts 3200K up to approximately 4100K.
Standard Abrisa Technologies color temperature filters are supplied on Borofloat® glass substrates in sizes as large as 24″ in diameter. Standard and custom sizes are available. Additionally, we are happy to provide custom filters for your color application.