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 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 1,25" 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, bit 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].
The commonly available numbers and some of their uses include:
| Wratten number |
Visible 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. blocks visible wavelengths below 440nm | ||
| 4 | yellow | |||
| 6 | light yellow | |||
| 8 | yellow | K2 | Absorbs more blue than #3. | |
| 9 | deep yellow | Absorbs more blue than #8. blocks visible wavelengths below 465nm | ||
| 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. blocks visible wavelengths below 495nm | ||
| 15 | deep yellow | Darkens the sky in black and white outdoor photography. blocks visible wavelengths below 505nm | ||
| 16 | yellow-orange | Like #15, but more so. blocks visible wavelengths below 520nm | ||
| 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 | G | Contrast filter for blue and blue-green absorption. blocks visible wavelengths below 530nm | |
| 22 | deep orange | Contrast filter, greater effect than #21. blocks visible wavelengths below 550nm | ||
| 23A | light red | blocks below 560nm | ||
| 24 | red | Used for color separation of Kodachrome tranparency film, complements #47B and #61. | ||
| 25 | red tricolor | Used for color separation and infrared photography blocks below 580nm. | ||
| 25A | red | 8 | 3 | Strong contrast red filter used for color separation and infrared photography in black and white film and blocks below 580nm. |
| 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. blocks below 600nm. | ||
| 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 | Used for color separation. It is also commonly used to calibrate video monitors while using SMPTE color bars[5]. | ||
| 50 | deep blue | |||
| 56 | light green | |||
| 58 | green tricolor | Color separation. | ||
| 61 | deep green tricolor | Color separation, complements #29 and #47. | ||
| 70 | red | Used for color separation and infrared photography blocks below 645nm. | ||
| 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 740nm | ||
| 87A | opaque | Passes infrared but not visible frequencies. blocks wavelengths below 880nm | ||
| 87B | opaque | Passes infrared, blocks visible frequencies. blocks wavelengths below 820nm | ||
| 87C | opaque | Passes infrared, blocks visible frequencies. blocks wavelengths below 790nm | ||
| 88 | opaque | Passes infrared, blocks visible wavelengths below 700nm. | ||
| 88A | opaque | Passes infrared, blocks visible frequencies. below 720nm. | ||
| 89B | near-opaque red | R72 | Passes infrared, blocks visible wavelengths below 680nm (very dark 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. | ||
| 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. |