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 man who founded the first photography company, Frederick Wratten, a British inventor. Wratten and partner C. E. K. Mees sold their company to Eastman Kodak in 1912, and Kodak started manufacturing Wratten filters, which 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 1¼″ or 2″ 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, according to that manufacturer's idea of how exactly it is 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. For this reason it has become difficult to find most of the Wratten filters in photography stores anymore.

Reference table

The commonly available numbers and some of their uses include:

Wratten
number 		Visible color Filter factor
or alternate designation 		F-Stops correction Uses and characteristics
1A Called a skylight filter, this absorbs ultraviolet radiation, which reduces haze in outdoor landscape photography.
2A pale yellow Absorbs ultraviolet radiation. longpass filter blocking wavelengths below 405 nm
2B pale yellow Absorbs 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 yellow Absorbs ultraviolet radiation, slightly more than #2A. longpass filter blocking wavelengths below 415 nm
3 light yellow Absorbs excessive sky blue, making sky look slightly darker in black and white images. longpass filter blocking wavelengths below 440 nm
4 yellow longpass filter blocking visible wavelengths below 455 nm
6 light yellow K1 not a longpass filter
8 yellow K2 Absorbs more blue than #3. longpass filter blocking visible wavelengths below 465 nm
9 deep yellow K3 Absorbs more blue than #8. longpass filter blocking visible wavelengths below 470 nm
11 yellowish-green X1 Color Correction. not a longpass filter
12 deep yellow Minus blue Minus 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
15 deep yellow G Darkens the sky in black and white outdoor photography. longpass filter blocking visible wavelengths below 510 nm
16 yellow-orange Performs like #15, but more so; longpass filter blocking visible wavelengths below about 520 nm
18A visually opaque Based on Wood's glass, transmits small bands of ultraviolet radiation and infrared radiation.
18B very deep violet Similar to 18A but with wider bands of transmittance in both the ultraviolet and infrared, a less 'pure' filter.
21 orange Contrast filter for blue and blue-green absorption. longpass filter blocking visible wavelengths below 530 nm
22 deep orange Contrast filter, greater effect than #21. longpass filter blocking visible wavelengths below 550 nm
23A light red longpass filter blocking visible wavelengths below 550 nm
24 red Used 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 tricolor A Used for color separation and infrared photography longpass filter blocking below 580 nm.
26 red longpass filter blocking below 585 nm
29 deep red F Used 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 magenta Minus-green. Complements #12 minus-blue and #44A minus-red.
34A violet Used for minus-green and plus-blue separation.
38A blue Absorbs red, some UV and some green light.
40 light green Green, for two color photography (tungsten).
44 light blue-green minus-red filter with much UV absorption.
44A light blue-green minus-red, complements #12 is minus-blue and #32 minus-green.
47 blue tricolor C5 Used for color separation. Complements #29 and #61.
47A light blue By 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 tricolor Used 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 green Green for 'Two Color Photography' (daylight).
58 green tricolor B Color separation.
60 green Green for two color photography' (tungsten).
61 deep green tricolor N Color separation, complements #29 and #47.
70 red Used for color separation and infrared photography longpass filter blocking below 650 nm.
80A blue 4 2 Color 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 blue 3 1+2/3 Similar to 80A; 3400 K to 5500 K.
80C blue 2 1 Similar to 80A; 3800 K to 5500 K. Typically used so that old-style flashbulbs can be used on a daylight film.
80D blue 1.5 1/3 Similar to 80A; 4200 K to 5500 K.
81A pale orange 1.4 1/3 Warming 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 orange 1.4 1/3 Warming filter, slightly stronger than 81A. The opposite of 82B.
81C pale orange 1.5 1/3 Warming filter, slightly stronger than 81B, opposite of 82C.
81D pale orange Warming filter, slightly stronger than 81C.
81EF pale orange 1/3 Warming filter, stronger than 81D.
82A pale blue 1.3 1/3 Cooling filter to increase the color temperature slightly. The opposite of 81A.
82B pale blue 1.4 2/3 Cooling 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 blue 1.5 2/3 Cooling filter, slightly stronger than 82B and opposite of 81C.
85 amber 1.5 2/3 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. These filters were used in Super 8 movie cameras that were designed to use Tungsten film.
85B amber 1.5 2/3 Similar to 85; converts 5500 K to 3200 K.
85C amber 1.5 Similar to 85; converts 5500 K to 3800 K.
85N3 amber Neutral 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 amber Neutral 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 amber Neutral 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 opaque Passes infrared but not visible frequencies. blocks wavelengths below 740 nm
87A opaque Passes infrared but not visible frequencies. blocks wavelengths below 880 nm
87B opaque Passes infrared, blocks visible frequencies. blocks wavelengths below 820 nm
87C opaque Passes infrared, blocks visible frequencies. blocks wavelengths below 790 nm
88 opaque Passes infrared, blocks visible wavelengths below 700 nm.
88A opaque Passes infrared, blocks visible frequencies. below 720 nm.
89B near-opaque red R72 Passes infrared, longpass filter blocking visible wavelengths below 690 nm (very deep red). Aerial photography is one use.
90 dark grayish amber Used for viewing scenes without color before photographing them, in order to assess the brightness values. Not used for actual photography.
92 red color densitometry. longpass filter blocking visible wavelengths below 625 nm
93 green color densitometry.
94 blue color densitometry.
96 gray varies neutral 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 blue Like a #47B plus a #2B filter.
99 green Like a #61 plus a #16 filter.
102 yellow-green Color conversion; makes a barrier-level type photocell respond as a human eye would.
106 amber Color conversions; makes an S-4 type photocell respond as a human eye would.

External links

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
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