Wratten number

As a color filter as pre-filters for cameras are referred to, which can happen only a certain color ( radiation of a certain wavelength ) or filter out a particular color ( much rarer ). They are mostly made ​​of colored glass, plastic or gelatin films.

When lighting can also filter sheets of plastic or corresponding reflectors use.

The term is also used for image editing software that change the color spectrum and increase contrast, or eliminate color errors or alienate the image artistically.

• 3.1 Comparison " Classic " Photography and Digital Photography

• 6.1 strip of filter
• 6.2 mosaic filter

Color Photography

In color photography, in particular the correction of unwanted color reproduction is very important.

Correction filter

Correction filters are optical filters (glasses, plastic sheets, films) that are incorporated into the optical path of the lens system of an optical device in order to avoid unwanted pictures ( correct ). As far as they are used to adjust the color temperature of the lighting in the footage, it is called conversion filter.

Such filters are particularly useful in color photography to films of importance. In the Slide photography the final colors are already established at the time of the recordings. In preparing the paper image though color correction is possible in a wide range, the filtering during the recording, however, usually leads to a more balanced playback.

In photography, glass filters are screwed or plugged in front of the lens, or put plastic sheets or films (gelatin or plastic) in a holder in the following colors:

Subtractive filter:

• Y: Yellow / Yellow;
• M: magenta / purple ( blue-red );
• C: Cyan / Blue Green.

Additives filters:

• B: blue;
• G: Green;
• R: Red

With digital cameras ( still and video cameras ), an electronic color matching to be made by an appropriate calculation of the image data, the so-called white balance. There is also the possibility of color change by recalculation of the already stored image. In this case, stored in a raw format images are more flexible than compressed jpeg files. In principle, however, is to be expected that especially extreme conversions, such as to provide exceptional color casts to avoid, also lead to some loss in quality (eg, due to noisy sensor or compression effects due ).

Conversion filter

Conversion filters are used to adjust the color reproduction of the film to the surrounding light. This allows daylight color film to adapt to different types of artificial light, artificial light on daylight film material. It can be specifically influenced own Grautonpalette into each film type and the implementation of the color scale on black and white movies.

The conversion filters also include the delicate reddish colored skylight filter, which is often used to avoid a blue cast when shooting with the sun is high, especially in a high proportion of blue sky or in the snow.

Conversion filter, there are basically in two forms: bluish-colored types increase the color temperature to photograph, for example, with daylight film under artificial lighting (incandescent, halogen spotlights). Conversely serve reddish colored filter to compensate for light with high color temperature to, for example, tungsten film to be used with flash units, but also to motifs located in the shade, to play back in daylight. There are also special conversion filters that are tuned to the use of discontinuous spectrum lighting (fluorescent lamps).

For the description of the conversion filter, two completely different and incompatible systems have been established. Widely used is the identification the Kodak Wratten system in which the individual types are assigned arbitrary number combinations. The second system uses numerical values ​​that indicates the shift of the color temperature in step Dekamired in blue filters B with a in red supplemented with R.

Determination of correction filtering

With a simple color temperature meter can the necessary strength and type of the conversion filter determine: For example, it is in artificial light of 3200 Kelvin with a daylight film a conversion filter KB15 (or B15, see above) need.

It is more difficult in the light, which has no continuous spectrum as daylight or tungsten light, eg fluorescent lamps, vapor pressure or gas discharge lamps, etc.: Here you need a three-color color temperature meter, which red, green and blue measures. It then shows, depending on the type of film daylight film or tungsten film, the correction required to filtering. This consists of a KB ( blue) or KR ( red-orange ) conversion filter and an M ( Magenta), or G (green) correction filter.

Alternatively, one can resort to tables of the film maker, which can be found in the data sheets for the professional films where read is what filter combination (usually of two (or three ) of the additive colors of R (red ), G (green ), B ( blue ) and / or the subtractive Y ( Yellow / Yellow ), M ( magenta ), C ( cyan) ) apply to certain types of lamps.

However, one can not rely entirely on the measurement itself such an expensive device, nor as to the above tables. To get to a really good color reproduction, test series with different filtering must be performed at a given illumination.

Black and White Photography

While the various color filters to produce the correct color reproduction in color film, this has in black and white material is very different. Here the use of the conversion filter results in a shift in the Grautonumsetzung. The color of the filter is increased, that is, they will appear brighter, while the complementary color is suppressed, that is, is darker. This effect allows the image design with black and white footage change very interesting.

This description refers to the " classic photograph " with chemically sensitized film materials that react to the filter element.

The effects of the filter for the black and white photograph may therefore be summarized as follows:

• Yellow: Darkens a blue sky and enhances the contrast of the clouds, as well as atmospheric haze is slightly weakened.
• Yellow - green: Conveniently located at outdoor portraits, suppressed slight blemishes.
• Green: differentiation present in the natural shades of green, red tones ( lips in the human portrait ) are darkened, but also blemishes.
• Orange: In the action between yellow and red filters.
• Red: A landscape in the sunshine impacted by the strong blocking effect of red glass as illuminated by the full moon, blue sky is almost black. Eventually existing extractor is strongly suppressed, skin tones are more waxy, red lips almost white.

The use of blue filter glasses in black and white photography has little meaning, however, can be achieved by the use of a blue conversion filter when illuminated with incandescent or candle light may a more natural reproduction of skin tones as well as mist and fog reinforce something.

A special case are infrared filters that block all visible light and almost black to the human eye or deep dark act. With suitable film or infrared-sensitive sensors of some digital cameras can be with infrared photography (not to be confused with thermal imaging cameras) achieve impressive effects. A metering through the lens is not possible with these filters, but also in orange and red filters it comes with TTL exposure metering often considerable deviations. Orange and red filters can not be used with orthochromatic sensitized black and white film.

Image processing

In the photographic work in the laboratory filter to achieve the desired color effect can be used on paper image. Likewise, color filters are used for contrast control for black and white documents with variable gradation. Also in the darkroom light or when modeling for black and white material coming Photo Filter or their effect used.

Compare " Classic " Photography and Digital Photography

Both the photography with conventional film and digital photography in the filters can be used. In many cases, however, allows the digital white balance or adjusting the color temperature of the waiver of the filtering in digital cameras.

However, the camera-side white balance with extremely different from the standard daylight lighting conditions can lead to increased noise or false color filtering. In particular, the illumination with ( mostly ) monochromatic artificial light leads to a disturbing irritation of the camera electronics and the built-in logic, so that the use of filters to achieve low-noise image data is useful.

In addition, the digital photography allows for filtering in the digital post-processing. This applies in particular when the raw digital data from the sensor are stored in high quality. By creating color histograms and generation of arbitrary color channels focused and sometimes easier editing is possible, as it allows the analog image processing.

If, in a digital camera, a color filter placed in front of the lens, the auto white balance could recognize the color shift and compensate (partially). In this case, should be performed and the automatic system can be disabled with filter without filter, a manual white balance.

Influence on the exposure

Filters have their complementary colors, a light blocking effect reduction result, which is usually indicated on the filter. This light loss must be taken into account when photographic use. This is particularly difficult in conversion filters in Rottonbereich, since the light reduction is not static but can vary quite considerably depending on the subject.

This is especially true for the exposure to external devices (manual exposure metering ) or due to exposure tables. The metering through the lens basically takes into account the reduced amount of light measuring cell in the camera can also be deceived by the modified spectral composition of light.

This also applies to digital photography. If the histograms of the exposure appears only as the sum of the channels red, green and blue, can already significant overexposure occur in a channel without realizing it at the histogram. This also applies to the overexposure warning of some cameras in the control reproduction of the image.

Application in the lighting

For the lighting, color filters are used to achieve particular effects: In theater lighting and photographic recordings. This often foil filters are to use.

In particular, in the English speaking abbreviations for the names of the individual colors established, such as " CTO " for " Color Temperature Orange" (literally " color temperature orange "), " CTB " for " Color Temperature Blue" ( " Color Temperature Blue " ), and so further. This refers hereby orange (blue, ... ) filters that color the light accordingly, and so, for example, as a warmer ( orange - not to be confused with the actual color temperature) or colder (blue ) ensure perceived light. When reproducing ( enlarger ) they are used for subsequent correction. By the use of a color mixing head to thereby achieve a stepless change of light color.

To evaluate the image section a red filter can be swiveled in front of the lens in many enlargers. Thus, the loaded photo paper is not exposed, the image can also be assessed anyway.

Also use Safelights usually color filter, to avoid exposure of the photographic material during processing.

Video technology and digital cameras

Strip of filter

Filter strips are used to split the color spectrum before an image converter. They consist of vertical strips of the primary colors red, green and blue. By the row-wise scanning of the image sensor so the color intensities are transmitted sequentially in time. Accordingly, these filters are also used for image reproduction, in which a white light beam, a color image is formed. Through the stripe arrangement moire interference is unavoidable.

Mosaic filter

Mosaic filters are a further strip of filter. The staggered arrangement of the color surfaces the moiré effects can be greatly mitigated. See the use of digital photography in order to waive a beamsplitter. This is necessary when the production costs should be kept low (video camera for the consumer sector ) or the average length of the lens can not accommodate the beam splitter can ( Compact design, wide-angle lens of higher light intensity ).

• Description, see Bayer sensor.