Wratten numbers are a labeling system for optical filters, usually for photographic use comprising a number sometimes followed by a letter. The number denotes the color of the filter and its spectral characteristics, and these numbers can be grouped into broad categories, but the numbering system is arbitrary within a group and does not encode any information. For example, within the photometric group of color correction filters, the 80A–80D are blue, while the next filters in numerical order, 81A–81EF, are orange. Letters almost always increase with increasing strength (the exception being 2B, 2A, 2C, 2E).

History and use

They are named for the founder of the first photography company, British inventor Frederick Wratten. Wratten and partner C.E.K. Mees sold their company to Eastman Kodak in 1912, and Kodak started manufacturing Wratten filters. They remain in production, and are sold under license through the Tiffen corporation. The numbering scheme can be divided into broad categories by use including color, monochromats, photometrics, and miscellaneous. Wratten filters are often used in observational astronomy by amateur astronomers. Color filters for visual observing made by GSO, Baader, Lumicon, or other companies are actually Wratten filters mounted in standard 1+1/4 in or 2 in (nominal, 48 mm actual) filter threads. For imaging interference filters are used. Wratten filters are also used in photomicrography. Other manufacturers of photographic filters may use Wratten numbers for identification, but these may not precisely match the spectral definition for that number. This is especially true for filters used for aesthetic (as opposed to technical) reasons. For example, an 81B warming filter is a filter used to slightly "warm" the colors in a color photo, making the scene a bit less blue and more red. Many manufacturers make filters labeled as 81B with transmission curves which are similar, but not identical, to the Kodak Wratten 81B. This is according to that manufacturer's idea of how best to warm a scene, and depending on the dyes and layering techniques used in manufacturing. Some manufacturers use their own designations to avoid this confusion, for example Singh-Ray has a warming filter which they designate A‑13, which is not a Wratten number. Filters used where precisely specified and repeatable characteristics are required, e.g. for printing press color separation and scientific work, use more standardized and rigorous coding systems. In digital photography, where the color temperature can be adjusted and color corrections can be easily accomplished in the camera (by firmware) or in software, the need for color filters has all but disappeared. Thus, it has become difficult to find Wratten filters in photography stores.

Subsets

Some filters are listed in tables of Wratten filters with codes which do not follow the original number-letter scheme, e.g. K2, G, X0, FL‑W; CC‑50Y. In some cases, these are alternate designations for the filters, and used by Kodak to recommend sets of filters for use with specific purposes. For instance, some sets were for commercial photographers to adjust contrast in black-and-white photography with orthochromatic and panchromatic films.

Color correction filters

The Wratten filters numbered with 80, 81, 82, and 85x are color conversion filters used to avoid unnatural colour casts when photographing scenes where the color temperature of the light source does not match the rated color temperature of the film, which is available in Tungsten and Daylight types. While the Wratten 80 and 82 series are cooling filters with blue tones, the 81 and 85 series are warming filters. The 80/85 series are regarded as "color conversion" filters, while the corresponding 82/81 series are "light balancing filters" which generally have a weaker effect than the 80/85 series. To avoid confusion, some filter manufacturers use the mired shift to name their filters, which quantifies the effect of a color conversion filter; the mired shift is distinct from the Wratten number. The mired value associated with a given color temperature is computed as the reciprocal of the color temperature, in Kelvin, multiplied by 10^6: The shift is the difference in the mired values of the film and light source. Sometimes the decamired is used, where 10 mired = 1 decamired, as the smallest perceptible color temperature change is from a 10 mired shift. From the equation, when the film has a higher color temperature than the light source, a negative mired shift is required, which calls for a "cooling" filter; these have a perceptible blue color, and the more saturated the color, the stronger the cooling effect. Likewise, when the film has a lower color temperature than the light source, a positive mired shift is required, which calls for an amber "warming" filter. Stacking color conversion filters creates an additive mired shift: for example, stacking a Wratten 80A (-130 mired) with a Wratten 82C (-60 mired) results in a total mired shift of -190. A typical set of color conversion filters has a geometric sequence, e.g. ±15, ±30, ±60, and ±120 mired, which approximates the sequence established by the Wratten filters, and allows intermediate values to be obtained by stacking.

Reference table

The commonly available numbers and some of their uses include: