Wratten number

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, but is arbitrary and does not encode any information (the 80A–80D are blue, the next filters in numerical order, 81A–81EF, are orange); letters increase with increasing strength.

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.[1]

Wratten filters are much 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 14 or 2 in (32 or 51 mm) filter threads. For imaging interference filters are used. Wratten filters are also used in photomicrography.[2]

Filters made by various manufacturers may be identified by Wratten numbers but 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 their manufacturing techniques. 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.

Some filters are listed in tables of Wratten filters with codes which do not follow the letter-number scheme, e.g. K2, G, X0, FL-W;[3] CC-50Y.[4]

In digital photography, where the color temperature can be adjusted and color corrections can be easily accomplished in firmware (in the camera) or in software, the need for color filters has all but disappeared. Thus, it has become difficult to find Wratten filters in photography stores.

Reference table

The commonly available numbers and some of their uses include:

Wratten
number
Visible colorFilter factor
or alternate designation
F-Stops correctionUses and characteristics
1A Called a skylight filter, this absorbs ultraviolet radiation, which reduces haze in outdoor landscape photography.
2A pale yellowAbsorbs ultraviolet radiation. longpass filter blocking wavelengths below 405 nm
2B pale yellowAbsorbs ultraviolet radiation, slightly less than #2A. longpass filter blocking wavelengths below 395 nm
2C Absorbs ultraviolet radiation. longpass filter blocking wavelengths below 390 nm
2E pale yellowAbsorbs ultraviolet radiation, slightly more than #2A. longpass filter blocking wavelengths below 415 nm
3 light yellowAbsorbs excessive sky blue, making sky look slightly darker in black and white images. longpass filter blocking wavelengths below 440 nm
4 yellowlongpass filter blocking visible wavelengths below 455 nm
6 light yellowK1not a longpass filter
8 yellowK21Absorbs more blue than #3. longpass filter blocking visible wavelengths below 465 nm
9 deep yellowK3Absorbs more blue than #8. longpass filter blocking visible wavelengths below 470 nm
11 yellowish-greenX02Color Correction. not a longpass filter
12 deep yellowMinus blue1.3Minus blue filter; complements #32 minus-green and #44A minus-red. Used with Ektachrome or Aerochrome Infrared films to obtain false-color results. Used in ophthalmology and optometry in conjunction with a slit-lamp and a cobalt blue light to improve contrast when assessing the health of the cornea and the fit of contact lenses. longpass filter blocking visible wavelengths below 500 nm
13 Green2Color Correction. not a longpass filter
15 deep yellowG1.6Darkens the sky in black and white outdoor photography. longpass filter blocking visible wavelengths below 510 nm
16 yellow-orange1.6Performs like #15, but more so; longpass filter blocking visible wavelengths below about 520 nm
18A visually opaqueBased on Wood's glass, transmits small bands of ultraviolet radiation and infrared radiation.
18B very deep violetSimilar to 18A but with wider bands of transmittance in both the ultraviolet and infrared, a less 'pure' filter.
21 orange2Contrast filter for blue and blue-green absorption. longpass filter blocking visible wavelengths below 530 nm
22 deep orange2.3Contrast filter, greater effect than #21. longpass filter blocking visible wavelengths below 550 nm
23A light redlongpass filter blocking visible wavelengths below 550 nm
24 redUsed for color separation of Kodachrome transparency film, complements #47B and #61. longpass filter blocking visible wavelengths below 575nm. Red for 'Two Color Photography' (daylight or tungsten). White flame arc tri-color projection.[5]
25 red tricolorA3Used for color separation and infrared photography longpass filter blocking below 580 nm.
26 redlongpass filter blocking below 585 nm
29 deep redF4Used for color separation, complements #47 and #61. In black and white outdoor photography makes blue skies look very dark, almost black. In infrared photography, blocks much visible light, increasing the effect of the infrared frequencies on the picture. longpass filter blocking below 600 nm.
32 magentaMinus-green. Complements #12 minus-blue and #44A minus-red.
33 magentaContrast filter for strongest green absorption. For photomechanical color masking.
34A violetUsed for minus-green and plus-blue separation.
38A blueAbsorbs red, some UV and some green light.
40 light greenGreen, for two color photography (tungsten).
44 light blue-greenminus-red filter with much UV absorption.
44A light blue-greenminus-red, complements #12 is minus-blue and #32 minus-green.
47 blue tricolorC5Used for color separation. Complements #29 and #61.
47A light blueBy removing lots of light that is not blue, blue and purple objects show a broader range of colors. Used for medical applications that involve making dyes fluoresce.
47B deep blue tricolorUsed for color separation. It is also commonly used to calibrate video monitors while using SMPTE color bars.[6]
50 deep blue
56 light green
57 greenGreen for 'Two Color Photography' (daylight).
58 green tricolorBColor separation.
60 greenGreen for two color photography' (tungsten).
61 deep green tricolorNColor separation, complements #29 and #47.
70 redUsed for color separation and infrared photography longpass filter blocking below 650 nm.
74 dark green monochromatTransmits 10 percent of green radiation and virtually no yellow radiation from mercury-vapor illumination.[7]
80A blue42Color Conversion. Raises the color temperature, causing a 3200 K tungsten-lit scene to appear to be daylight lit, approximately 5500 K. This allows use of a daylight balanced film with tungsten lighting.
80B blue31+2/3Similar to 80A; 3400 K to 5500 K.
80C blue21Similar to 80A; 3800 K to 5500 K. Typically used so that old-style flashbulbs can be used on a daylight film.
80D blue1.51/3Similar to 80A; 4200 K to 5500 K.
81A pale orange1.41/3Warming filter to decrease the color temperature slightly; this can also be used when shooting tungsten type B film (3200 K) with 3400 K photoflood lights. The opposite of 82A.
81B pale orange1.41/3Warming filter, slightly stronger than 81A. The opposite of 82B.
81C pale orange1.51/3Warming filter, slightly stronger than 81B, opposite of 82C.
81D pale orangeWarming filter, slightly stronger than 81C.
81EF pale orange1/3Warming filter, stronger than 81D.
82A pale blue1.31/3Cooling filter to increase the color temperature slightly. The opposite of 81A.
82B pale blue1.42/3Cooling filter, slightly stronger than 82A and opposite of 81B. Can also be used when shooting tungsten type B film (3200 K) with household 100 W electric bulbs (2900 K).
82C pale blue1.52/3Cooling filter, slightly stronger than 82B and opposite of 81C.
85 amber1.52/3Color conversion, the opposite of the 80A; this is a warming filter that takes an outdoor scene lit by sunlight (which has a color temperature around 5500 kelvins) and makes it appear to be lit by tungsten incandescent bulbs around 3400 K. This allows an indoor balanced film to be used to photograph outdoors. These filters were used in Super 8 movie cameras that were designed to use Tungsten film.
85B amber1.52/3Similar to 85; converts 5500 K to 3200 K.
85C amber1.5Similar to 85; converts 5500 K to 3800 K.
85N3 amberNeutral density of 1 stop + color conversion, the opposite of the 80A; this is a warming filter that takes an outdoor scene lit by sunlight (which has a color temperature around 5500 kelvins) and makes it appear to be lit by tungsten incandescent bulbs around 3400 K. This allows an indoor balanced film to be used to photograph outdoors.
85N6 amberNeutral density of 2 stops + color conversion, the opposite of the 80A; this is a warming filter that takes an outdoor scene lit by sunlight (which has a color temperature around 5500 kelvins) and makes it appear to be lit by tungsten incandescent bulbs around 3400 K. This allows an indoor balanced film to be used to photograph outdoors.
85N9 amberNeutral density of 3 stops + color conversion, the opposite of the 80A; this is a warming filter that takes an outdoor scene lit by sunlight (which has a color temperature around 5500 kelvins) and makes it appear to be lit by tungsten incandescent bulbs around 3400 K. This allows an indoor balanced film to be used to photograph outdoors.
87 opaquePasses infrared but not visible frequencies. blocks wavelengths below 740 nm
87A opaquePasses infrared but not visible frequencies. blocks wavelengths below 880 nm
87B opaquePasses infrared, blocks visible frequencies. blocks wavelengths below 820 nm
87C opaquePasses infrared, blocks visible frequencies. blocks wavelengths below 790 nm
88 opaquePasses infrared, blocks visible wavelengths below 700 nm.
88A opaquePasses infrared, blocks visible frequencies. below 720 nm.
89B near-opaque redR72Passes infrared, longpass filter blocking visible wavelengths below 690 nm (very deep red). Aerial photography is one use.
90 dark grayish amberUsed for viewing scenes without color before photographing them, in order to assess the brightness values. Not used for actual photography.
92 redcolor densitometry. longpass filter blocking visible wavelengths below 625 nm
93 greencolor densitometry.
94 bluecolor densitometry.
96 grayvariesneutral density filter. Blocks all frequencies of visible light approximately evenly, making scene darker overall. Available in many different values, distinguished by optical density or by filter factor.
98 blueLike a #47B plus a #2B filter.
99 greenLike a #61 plus a #16 filter.
102 yellow-greenColor conversion; makes a barrier-level type photocell respond as a human eye would.
106 amberColor conversions; makes an S-4 type photocell respond as a human eye would.

See also

References

  1. Tiffen website, offering Kodak Wratten filters
  2. Florida State University website, Kodak Wratten Filters for Black & White Photomicrography
  3. Wratten Filter Codes and Uses
  4. Transmission of Wratten filters, detailed numerical information compiled by Allie C Peed Jr for the Eastman Kodak Company
  5. Kodak Wratten Filters, Fourth Edition 1969, Kodak Limited London.
  6. Filming The Fantastic – Mark Sawicki – Focal Press – June 1, 2007 – ISMB 0240809157
  7. Handbook of Kodak Photographic Filters, page 127 Pub B-3, Cat 1528108, ISBN 0-87985-658-0 Transmission of Wratten Filters, Allie C. Peed www.karmalimbo.com/aro/pics/filters/transmision%20of%20wratten%20filters.pdf
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