White light is the sum of all the colors of the rainbow; black is the absence of all these colors. For practical purposes, we can consider white light as composed of equal amounts of three primary light colors-red, green, and blue. For example, if green and red are subtracted, we see blue. We, see many more colors in nature than these three because absorption and reflection of the primaries are rarely complete.

Our perception of a color is influenced by the surrounding colors and brightness level, the surface gloss of an object, and any personal defects in our color vision. Different films also see colors differently due to differences in spectral sensitivity. Filtration used with black-and-white films can control the shades of gray to obtain a technically correct rendition or to exaggerate or suppress the tonal differences for visibility, emphasis, or other effects. Filtration with color films can change the color quality of the light source to produce proper color rendition or to create special effects.

Filters always subtract some of the light reflected from a scene before it reaches the film plane in the camera. A red filter then is not "red" but rather a filter that absorbs blue and green. Similarly, a yellow filter is one that absorbs blue light. A yellow sunflower absorbs blue light and reflects the other parts of white light-red and green, which we see as yellow (lack of blue).

Filters Useful with All Camera Films
Polarizing Filters
Polarizing filters (also called polarizing screens) are used to subdue reflections from surfaces such as glass, water, and polished wood, and for controlling the brightness of the sky. By reducing glare, polarizing filters also increase color saturation. Using a polarizing filter to control the brightness of the sky has several advantages over color filters: (1) The color rendering of foreground objects is not altered. (2) It is easy to determine the effect produced by the polarizing filter by checking the appearance of the image in the viewfinder (for cameras equipped with reflex-type viewfinders), or by looking through the filter when it is held at the same angle as used on the camera. (3) Other filters can be used with a polarizing filter to control the color rendering of objects in the foreground, while the polarizer independently controls the brightness of the sky.

The amount of polarized light from a particular area of the sky varies according to the position of the area with respect to the sun, the maximum occurring at an angle of 90^o from the sun. Panning the camera, therefore, should be avoided with a polarizing because the sky will become darker or lighter as the camera position changes.

The sky may appear lighter than you would expect for these reasons:

• A misty sky does not photograph as dark as a clear blue sky. You can't darken an overcast sky by using a polarizing filter.
• The sky is frequently almost white at the horizon and shades to a more intense blue at the zenith. Therefore, the effect of the filter at the horizon is small, but it becomes greater as you aim the camera upward.
• The sky near the sun is less blue than the surrounding sky and, therefore, is less affected by a filter.

When you begin making exposures with a polarizing filter, be sure to remember that this filter has a minimum filter factor of 2.5 (increase exposure by 1 1/3 stops). This factor applies regardless of how the polarizing screen is rotated. In addition to this exposure increase, you must make any exposure increases required by the nature of the lighting. For example, for the dark-sky effect, the scene must be sidelighted or toplighted, so it will be necessary to add approximately 1/2-stop exposure to the 1 1/3- stop increase required by the polarizing filter factor.

Give an additional 1/2-stop exposure when you use a polarizing filter to eliminate reflections from subjects; reflections often make objects look brighter than they really are. See Figure 50.

Figure 50	A polarizer can eliminate reflections on non- metallic surfaces.

Neutral Density Filters
Neutral density filters, such as the KODAK WRATTEN Neutral Density Filter No. 96, reduce the intensity of light reaching the film without affecting the tonal rendition of colors in the scene. Neutral density filters make it possible to film in bright sunlight using high-speed films without having to use very small lens openings. In black-and-white motion-picture photography, KODAK WRATTEN Gelatin Filters No. 03N5 and 8N5 permit the use of a larger lens opening for depth-of-field reduction. These filters combine a neutral density of 0.5 with the blue and ultraviolet correction capability of WRATTEN Gelatin Filters No. 3 and No. 8, respectively. In color motion picture photography, you can use combination filters, such as KODAK WRATTEN Gelatin Filters No. 85BN3 and 85BN6, to convert the color temperature from 5500 K (daylight) to 3200 K (professional tungsten lighting), and at the same time, obtain neutral densities of 0.3 and 0.6. Since a 0.3ND filter causes a one-stop reduction in exposure, these filters require, respectively, one and two stops of additional exposure.

Filters for Black-and-White Films
KODAK WRATTEN Gelatin Filters are used with a wide range of black-and- white films for many purposes. They can emphasize clouds, reduce the brightness of blue sky and water, penetrate haze in distant landscapes, increase tonal contrast between colored objects, and produce special effects such as simulated night scenes.

The filters used in black-and-white work fall into three main types: (1) Correction filters change the color quality of the exposing light so that the film records all colors at approximately the relative brightness values seen by the eye. (2) Contrast filters change the relative brightness values so that two colors that would otherwise record as nearly the same shade of gray will have decidedly different brightness in the picture. (3) Haze filters reduce the effects of aerial haze.

Correction Filters
Most panchromatic emulsions have a high sensitivity to both ultraviolet and blue radiation. Because this sensitivity is dissimilar to the spectral sensitivity of the eye, blue or violet subjects are often overexposed and rendered too light on the final print. For example in location work, correction filters are often used to overcome an apparent lack of contrast between blue sky and white clouds. At the red end of the spectrum, certain higher speed panchromatic films possess a marked red sensitivity that, unless compensated for, tends to distort the rendering of red subject matter. Deliberate overcorrection is sometimes done to achieve special effects.

Foliage looks slightly darker than we expect when it is photographed on black-and-white film without a filter. By using a yellow or yellow-green filter to absorb some of the unwanted blue and red light, you can record foliage in its proper gray tone.

This may seem to imply a contradiction: If a filter subtracts light, there will be less density on the negative and the print will be darker, so how does the filter make foliage lighter? Actually, the filter darkens the rendering on the print of the color it absorbs, thus making the colors it transmits lighter by comparison.

This becomes apparent when the negative is correctly printed.

Contrast Filters
Used with black-and-white films contrast filters change the relative contrasts between two objects that would normally photograph as nearly the same shade of gray. The following guideline will help you choose contrast filters: A filter transmits its own color, making that color lighter in a black-and- white print. To make a color darker, use a filter that will absorb that color. If you use a No. 25 red filter, which transmits the red of the geranium blossoms and absorbs the green of the grass, the geraniums will be light and the grass dark in your print. Since you probably think of the flowers as being brighter than the grass, this print may look natural to you. But if you use a No. 58 green filter, which absorbs the red of the geraniums and transmits the green of the grass, you'll get the opposite result: dark flowers and light grass. You can also underexpose the film when using a contrast filter to simulate a night effect under daylight conditions; use orange and red filters, such as KODAK WRATTEN Filter Nos. 23A, 25, 29, or 72B.

The color filter circle, Figure 51, will help you decide what filters to use to lighten or darken the gray-tone rendering of most colors. The No. 58 filter is green, for example, which lightens the gray-tone rendering of green, yellow, and blue-green, and darkens the rendering of orange, magenta, and red. The filter factors given are often different for tungsten and for daylight because tungsten light contains relatively more red light while daylight contains more blue.

Haze Filters
The effects of haze can be reduced by filtering out some of the blue and ultraviolet lighy. Yellow filters, commonly used for haze peneration and darkening of the sky, are KODAK WRATTEN Filters No. 3, 8, 12, and 15, in order of increasing absorption. For further darkening of the sky and increased haze penetration, use filters ranging from light orange to deep red, such as filters No. 21, 23A, 25 and 29. These filters absorb varying degrees of blue light and green light.

Figure 51
*For a gray-tone rendering of colors approximating their visual brightness.	Note: If conditions require long time exposures, corrections for reciprocity effect in addition to the corrections for the filter factor may be necessary.

Filters for Color Films
In exposing color films and in making prints and intermediates, there are a number of conditions under which you can obtain good color rendition through the use of correcting filters. Daylight and artificial light differ from one another in spectral quality and are individually subject to considerable variation. When the actual light is different from that specified for a particular film, correction filters can adjust the color quality of the illumination to that for which the film is balanced.

Data sheet tables are usually a reliable guide to the right filters for obtaining optimum color balance and are especially useful as a starting point from which to run tests. However, they cannot cover all such variables as high or low voltage, aging of lamps, or color contribution of diffusers. Color-temperature meters measuring the three primary colors provide an accurate method of determining the spectral-energy distribution of light sources as they relate to the sensitivities of the three layers in color films. Such meters as the Spectral-tricolor meter and the Minolta 3 color meter, while costly, provide the user an excellent means of finding the actual spectral distribution. Two-color meters (much less costly) show the balance between the red and blue light, and are adequate to indicate the spectral distribution of light sources having a continuous energy distribution across the spectrum (such as an incandescent light). They are not satisfactory for sources (such as fluorescent lights) having a skewed or discontinuous distribution.

Some meters give a choice of correcting the balance either wilh color balancing and conversion filters or with color compensating filters. In most instances, making the main correction with color compensating filters requires many filters, whole correcting with light balancing and conversion filters requires two at the most. Because the addition of many filters over a camera lens increase flare and decreases sharpness, color temperature (red- blue) correction is best made with light balancing and conversion filters and green-magenta adjustment is best made with color conipensating filters.

Selecting Filters for Correcting Color Temperature
The color quality of some illuminants can be expressed in terms of color temperature-a measure of the light irradiated by an idea-radiator, that is, a black body heated to incandescence. When the visual color of the illuminant is the same, or nearly the same, as that of the ideal radiator at a given temperature, the illuminant color is described in terms of the corresponding temperature of the ideal radiator, which is expressed in degrees Kelvin (K).

NOTE: Do not confuse sunlight with daylight. Sunlight is the light of the sun only. Daylight is a combination of sunlight plus skylight. The values given are approximate because many factors affect color temperature. Outdoors, the sun angle and the conditions of sky, clouds, haze, or dust particles will raise or lower the color temperature. Indoors, tungsten bulbs are affected by age (and blackening), voltage,type of reflectors and diffusers-all of which can influence the actual color temperature of the light. Usually, a change of 1 volt equals 10K. But this is true only within a limited voltage range and does not always apply to booster voltage operation since certain bulbs will not exceed a certain color temperature regardless of the increase in voltage.

Light Source Conversion with Filters
To evaluate filter requirements for the conversion of light sources, it is helpful to use the reciprocal of the color temperature. The concept of expressing color temperature in reciprocal form is useful because a given sum of reciprocal units corresponds approximately to the same color difference for most visibly emitting sources (in the range from 1000 K to 10,000 K). The reciprocal color temperature is commonly multiplied by 1,000,000 to give numbers of convenient size. The values obtained by this operation have, in the past, been called micro-reciprocal degrees or "mireds."

Recently, the term reciprocal megakelvins (MK^-1) has been used to replace mireds. The reciprocal color temperature expressed in reciprocal megakelvins has the same numerical value as with mireds, but the value is arrived at by first expressing the color temperature in megakelvins (1 MK = 1,000,000 K) and taking the reciprocal. For example, the reciprocal color temperature for a 6000 K source is

Filters such as KODAK Light Balancing Filters and KODAK WRATTEN Photometric Filters modify the effective color temperature, hence the reciprocal color temperature, of any light source by a definite amount. Each filter can be given a visual shift value that is defined by the expression



where T₁ is the color temperature of the light through the filter (both values expressed in megakelvins). Remember that the concept of color temperature relates to the response of the visual system. To match the actual response of films as opposed to the response of the eye, some filters are designed empirically to fit existing photographic requirements. These filters may or may not provide a visual shift that relates to the measured photographic effect. This list give filters that provide the desired photographic result when used for the conversion indicated. The shift value given is a nominal value defined by the equation



and is not a measure of the visual shift that might actually be computed for the filter. A new concept termed photographic color temperature is being developed. If this method proves viable, reporting additional filter data in terms of photographic effect should provide greater assistance in the choice of appropriate filters for photography under a wide range of illuminants.

The light source conversion nomograph shown in Figure 52 is designed to simplify the problem of selecting the proper conversion filter. The original light source, T₁, is listed in the left column and covers the practical range of color temperatures from 2000 to 10,000 K. The right-hand column lists the color temperature of the light through the filter-that is, the converted source, T₂. The center column shows the scale of reciprocal megakelvin (MK^-1) shift values. To find the shift value and consequently the filter required for a particular conversion, it is only necessary to place a straight- edge on the points corresponding to the color temperature of the available source, T₁, and the desired color temperature of the filtered source, T₂, respectively. The straightedge crosses the center column and indicates the reciprocal megakelvin shift value of the required filter. The zero point on this column indicates that no filter is required, values above zero point (+) require yellowish filters, and those below the zero point (-) require bluish filters.

Filters can also be combined, the desired combination being calculated by adding the (MK^-1) shift values of the filters, with due regard to the sign. If you use more than one filter, remember that the illumination loss and flare due to reflection of the multiple surfaces may become considerable.

Light Balancing Filters
Color motion picture films are balanced in manufacture for use either with tungsten light sources (3200 K, type B, or 3400 K, type A) or with illumination of daylight quality (5500 K). KODAK Light Balancing Filters are used over the camera lens to enable the photographer to make minor adjustments to the light reaching the film. If the required color-balance adjustment is small, a single bluish filter of the No.82 series, or a single yellowish filter of the No. 81 series, will be adequate. KODAK Light Balancing Filter No. 82 is intended, in effect, to raise color temperature by 100 K, the 82A by 200 K, the 82B by 300K, and the 82C by 400 K. Those of the No. 81 series (91, 81A, 81B, 81C 81D) are intended to reduce color temperature by 100 K steps. For greater color correction, combine two filters in the same series.

Conversion Filters
If still greater corrections in color are required, you can use light balancing filters and conversion filters. Use conversion filters over the camera lens to make significant changes in the color temperature of illumination (e.g., daylight to artificial light).

Limits to Color Temperature Measurement
Color temperature refers only to the visual appearance of a light source and does not necessarily describe its photographic effect. Although some light sources emit strongly in the ultraviolet region of the spectrum, the color temperature of such a source does not measure this portion of the emission because the eye is not sensitive to radiation below 400 nm. Since a film is usually sensitive to ultraviolet radiation, a scene can record overly blue unless special corrective means are used to filter out the ultraviolet.

Also, color temperature does not take into account the spectral distribution of a light source. Unless the light source has a similar spectral distribution to that of a black body radiator (e.g. various types of tungsten- filament lamps), its effective color temperature alone may not be reliable as a means of selecting a suitable filter for adapting the source for color photography. Fluorescent lamps, for example, do not have the continuous, smooth spectral-distribution curve that is characteristic of a tungsten- filament source.

Although two different light sources may be described as having the same color temperature, the photographic results obtained with each may be quite different.

Ultraviolet-Absorbing and Haze-Cutting Filters
Photographs of distant landscapes, mountain views, snow scenes, scenes over water, and sometimes aerial photographs in open shade made on color films balanced for daylight are frequently rendered with a bluish cast. This is caused by the scattering of ultraviolet radiation to which the film is more sensitive than the human eye. KODAK WRATTEN Filter No. 1A (skylight filter) absorbs ultraviolet light. By placing this filter over the lens, you can reduce the bluish cast and slightly penetrate the haze.

NOMOGRAPH FOR LIGHT SOURCE CONVERSION

ORIGINAL SOURCE IN K	SHIFT VALUE (MK-1)	CONVERTED SOURCE IN K

The nomograph can be used to find the shift value for a particular conversion by placing a
staightedge from an original source (T₁) to a second source (T₂). The shift value can be
read on the center line. Use of the nominal shift values for filters shown on the previous tables
will allow choice of filters that approximate the necessary correction. Shift values are
algebraically additive; filters can be combined to acheive the required shift.

Color Compensating Filters for Color Correction
A color compensating (CC) filter controls light by attenuating principally one or two of the red, blue, or green parts of the spectrum. You can use, them singly or in combination to introduce almost any desired color correction. Use CC filters to make changes in the overall color balance of pictures made with color films, or to compensate for deficiencies in the spectral quality of the light to which color films must sometimes be exposed. Such corrections are often required, for example, in making color prints or in photography with unusual light sources. If the color balance of a test is not satisfactory, the extent of filtering required to correct it can be estimated by viewing the test print through color compensating filters.

KODAK Color Compensating Filters have excellent optical quality and are suitable for image-forming optical systems-over the camera lens, for example. However, because they are gelatin filters, they are very susceptible to scratches and fingerprints, both of which can affect optical quality to a serious degree. Color compensating filters are available in several density values for each of the following colors: cyan, magenta, yellow, red, green, and blue.

The density of each color compensating filter is indicated by the numbers in the filter designation, and the color is indicated by the final letter. In a typical filter designation, CC20Y represents a "Color Compensating Filter with a density of 0.20 that is Yellow."

The densities of color compensating filters are measured at the wavelength of maximum absorbtion (i.e., the density of a yellow filter is given for blue light). That's the reason the term peak density is used in the table. The density values do not include the density of the gelatin on which the filter dye is coated, nor do they include the density of the glass in which a filter may be mounted.

The standardized density spacing of these filter series (5, 10, 20, 30, 40, 50 in each color) helps predict the photographic effects of filter combinations. The red, green, and blue filters each absorb two thirds of the visible spectrum; the cyan, magenta, and yellow filters each absorb one third of the spectrum. In the red, green, and blue series, each filter contains the same dyes in approximately the same amounts as the two corresponding yellow and magenta, yellow and cyan, or magenta and cyan filters.

Combining Color Compensating Filters
The determination of filter combinations can usually be simplified by thinking of all the filters in terms of the subtractive colors:

Red (absorbs blue and green) = yellow (absorbs blue) + magenta (absorbs green)

Green (absorbs blue and red) = yellow (absorbs blue) + cyan (absorbs red)

Blue (absorbs green and red) = magenta (absorbs green) + cyan (absorbs red)

The following method of calculation is recommended:

1. Convert the filters to their equivalents in the subtractive colors-cyan, magenta, and yellow-if they are not already of these colors. For example,
   
   20R = 20M + 20Y.
   
   
2. Add like filters together. For example,
   
   20M + 10M = 30M.
   
   
3. If the resulting filter combination contains all three subtractive colors, cancel out the neutral density by removing an equal amount of each. For example,
   
   10C + 20M + 20Y = 10M + 10Y + 0.10ND (neutral
   density, which can be eliminated).
   
   
4. If the filter combination contains two different filters of equal density, substitute the equivalent single red, green, or blue filter. For example,
   
   10M + 10C = 10B.
   
   

Exposure Allowance for Filters
You must make filters absorb light. You must increase exposure for this loss of light. The published exposure increases for KODAK Color Compensating Filters (see below) provide a rough guide to the exposure adjustments required for a single filter. To determine the exposure increase for two or more filters of different colors run practical tests using initially the sum of the suggested increases for the individual filters.

Filters for Color Printing
Motion picture printers used for printing color films are generally equipped with high-wattage lamps, making it necessary to insert a heat-absorbing glass to protect the mirrors and filters in the printer optical system from damage. Use a dichroic heat-reflecting glass or a heat-absorbing filter. The Heat Absorbing Filter No. 2043 (4 mm) now used in many laboratories is satisfactory. It is available from Kodak. An ultraviolet-absorbing filter may also be required, as specified on the data sheets.

KODAK Color Printing Filters, listed in the table below, are made on an acetate film base and are used singly or in combination for color correction of light sources in subtractive color printing. Color printing (CP) filters are similar to color compensating (CC) filters in that they control principally the red, green, or blue parts of the visible spectrum; unlike CC filters, CP filters cannot be used in the image-forming beam if optimum quality is desired.

See KODAK Publication No. B -3, Handbook of KODAK Photographic Filters, for more technical information concerning the filters discussed in this section.