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. 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. 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: M = \frac{{10}^{6}}{T} 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. \Delta M = M_{film} - M_{light} = \frac{{10}^{6}}{T_{film}} - \frac{{10}^{6}}{T_{light}} 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 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==