Definitions

skylight filter

Wratten number

Wratten numbers are a labeling system for optical filters, usually for photographic use.

They are named for the man who founded the first company, Frederick Wratten, a British inventor. Wratten and partner C. E. K. Mees sold their company to Eastman Kodak in 1912, and Kodak continued to produce "Wratten Filters" for decades. Even now, as of 2006, Wratten filters are still produced by Kodak, and sold under license through the Tiffen corporation.

Wratten filters are very much an active part of observational astronomy.

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 which do similar but not exactly the same filtering of light, 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 for printing press color separation or scientific photography tend to have less variation.

Reference Table

The commonly available numbers and some of their uses include:

Wratten
number
Color Filter
factor
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.
2B pale yellow Absorbs ultraviolet radiation, slightly less than #2A.
2C Absorbs ultraviolet radiation.
2E pale yellow Absorbs ultraviolet radiation, slightly more than #2A.
3 light yellow Absorbs excessive sky blue, making sky look slightly darker in black and white images.
4 yellow
6 light yellow
8 yellow 2 Absorbs more blue than #3.
9 deep yellow Absorbs more blue than #8.
11 yellowish-green Color Correction.
12 deep yellow 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.
15 deep yellow Darkens the sky in black and white outdoor photography.
16 yellow-orange Like #15, but more so.
18A Based on Wood's glass.
21 orange Contrast filter for blue and blue-green absorption.
22 deep orange Contrast filter, greater effect than #21.
23 light red
24 red Used for color separation of Kodachrome tranparency film, complements #47B and #61.
25 red tricolor Used for color separation and infrared photography.
26 red
29 deep red 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.
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.
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 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 Color separation.
50 deep blue
56 light green
58 green tricolor Color separation. By removing light that isn't green, green objects like foliage show a broader range of colors.
61 deep green tricolor Color separation, complements #29 and #47.
70 red Used for color separation and infrared photography.
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 increase 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 100W 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.
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 kelvin) 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 kelvin) 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 kelvin) 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
87C opaque Passes infrared but not visible frequencies.
89B near-opaque Passes infrared, blocks visible wavelengths below 720 nm (very dark red). Aerial photography is one use.
90 dark grayish amber Used for viewing scenes without color before photographing them.
92 red color densitometry.
93 green color densitometry.
94 blue color densitometry.
96 grayish varies neutral density filter. Blocks all frequencies of light 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.

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