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.

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.

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; CC-50Y.

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.
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.
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

• Wratten Light Filters, 4th ed, by Eastman Kodak Company, 1920. (Google ebook edition.) Gives graphical spectral power distribution of all Wratten filters available as of the publication date.
• Transmission of Wratten filters, detailed numerical information on spectral power distribution of Wratten filters, compiled by Allie C Peed Jr for the Eastman Kodak Company
• FAQs for DPs: filters
• Filters table