Filters by the Wratten Numbers

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 2008, 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.
Cokin Filter Guide
The commonly available numbers and some of their uses include:

Wratten
number		 Color     Filter
factor		 F-Stops correction Uses and characteristics Check Prices,  Availability
1A Skylight     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     Absorbs excess blue, slightly darkening sky, emphasizing clouds  
8 yellow   Absorbs more blue than #3. f/stop 2  
8-11 Yellow -green        
9 deep yellow     Absorbs more blue than #8.  
11 yellowish-green   Color Correction.  
12 yellow   Cuts haze in aerial photos, for use with aerial Ektachrome Infrared  
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.  
13 Green     dramatic dark skys, marine scenes, aerial photos, contrast in copies  
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       Dark Yellow Filter 023 (15) [SCHOTT OG 530] The even greater blue suppression and the attenuation into the blue-green range leads to a further increase in the effects described above. Interesting for snow scenes under a blue sky, because the darkened blue shadows in the snow make the shapes of the landscape look more dimensional. Freckles and skin blemishes are diminished strongly, but lips are rendered more pale (to compensate for you may use a dark lipstick!). f/stop 3  
23A red   contrast effects, darkens sky and water, architectural photos, pan film only  
24 red          
25 red tricolor   Used for color separation and infrared photography.  
25A red     8 Strong contrast red filter used for color separation and infrared photography in black and white film.  f stops +3  
26 red     8 ideal for strong contrast improvement since it absorbs green and blue completely and enhances red. Clouds are dramatically improved almost to the mood of a thunderstorm. It is more effective in cutting through haze, and moonlight effects are easily accomplished. It is also suited for infrared photography. Filter Factor +3  
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. This Dark blue filter lightens blue objects for detail. Although originally intended for use with black and white film, these filters can also be used as a creative tool in color imaging.  
50 deep blue          
56 light green   darkens sky, good flesh tones, pan films only  
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.  
80A blue 4 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. f/stop 2  
80B blue 3 Similar to 80A; 3400 K to 5500 K. f/stop 1+2/3  
80C blue 2 Similar to 80A; 3800 K to 5500 K. Typically used so that old-style flashbulbs can be used on a daylight film. f/stop 1  
80D blue 1.5 Similar to 80A; 4200 K to 5500 K. f/stop 1/3  
81A pale orange 1.4 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. f/stop 1/3  
81B pale orange 1.4 Warming filter, slightly stronger than 81A. The opposite of 82B. f/stop 1/3  
81C pale orange 1.5 Warming filter, slightly stronger than 81B, opposite of 82C. f/stop 1/3  
81D pale orange     Warming filter, slightly stronger than 81C.  
81EF pale orange   Warming filter, stronger than 81D. f/stop 1/3  
82       100 deg K increase  
82A pale blue 1.3 Cooling filter to decrease the color temperature slightly. The opposite of 81A. f/stop 1/3  
82B pale blue 1.4 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). f/stop 2/3  
82C pale blue 1.5 Cooling filter, slightly stronger than 82B and opposite of 81C. f/stop 2/3  
85 amber 1.5 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. f/stop 2/3  
85B amber 1.5 Similar to 85; converts 5500 K to 3200 K. f/stop 2/3  
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.  
85       for infrared film only, no visual transmission  
87C IR opaque       Passes infrared but not visible frequencies.  
89B IR 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.  
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 f/stop.  
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.  
  Circular Polarizer     Polarizers provide color and contrast enhancement. Reflected light often shows up as whitish glare that washes out color in an image. A Polarizer corrects this problem producing deep, dramatically blue skies. It also removes glare from non-metallic surfaces   
  Linear Polarizer     Not for digital cameras  
  Warm Polarizer     Combines the benefits of the Linear Polarizer with the warming effect of the 812 Color Warming filter