Color Temperature Nomograph
The color temperature nomograph is a useful tool with which to determine the necessary color balancing and/or correction filter(s) that are necessary to convert a light source from one color temperature to another. To use this type of graph, a straight edge ruler is placed at the color temperature of the original source and is pivoted to connect to the desired color temperature. The region where the ruler intersects the central axis identifies the necessary filter to achieve the color conversion. This interactive nomograph tutorial can be employed to quickly determine the appropriate filter under a variety of illumination scenarios.
The Original and Converted source scales each have movable arrows that are connected by a straight line (blue) running across the graph. To determine a color temperature conversion or balancing filter value, use the mouse to move the arrow on the upper scale (Original Source) to the color temperature of the currently employed light source. Next, move the arrow on the lower scale to the desired (Converted Source) color temperature adjustment. As the arrows are moved, the straight line connecting them will also be translated across the center scale. When the arrows are correctly positioned, the connecting line will cross the center scale at the point where the appropriate filter lies. This filter value (the actual Kodak filter number) will be displayed at the bottom of the tutorial window under the title: Recommended Filter. The actual original and converted source color temperatures in Kelvins are also displayed in the lower section of the tutorial window.
The most common conversion filter (producing large jumps in color temperature) is the Kodak 80A filter (3,200 K to 5,500 K) or the related 80B, 80C, 80D series of filters. For Olympus microscopes, the equivalent daylight-balanced filter is called the LBD filter, and for Nikon microscopes the filter is termed the NCB filter. Kodak 80-series conversion filters display an absorption maximum centered in the 600-650 region of the visible spectrum, which includes most of the yellow and red wavelengths. The 80A filter has the highest extinction coefficient and thus, by absorbing more red and yellow light, will produce the greatest shift in effective color temperature, followed by the 80B, 80C, and 80D.
For small increases in color temperature, the light blue Kodak 82-series filters can be used. Conversely, for small drops in color temperature, the light yellow Kodak 81-series filters are employed. When attempting to convert the color temperature of a daylight-balanced microscope light source such as a xenon lamp or flash tube for use with tungsten-balanced color film, one of the appropriate amber Kodak 85-series filters is inserted. Although it is far more practical to utilize a microscope equipped with a tungsten-halogen light source, especially for critical photomicrography, these filters will serve in cases where it is impossible to use other lamps (such as arc-discharge or flash tubes).
The "fine tuning" filters (Kodak series 81, 82 and similar filters), useful for making smaller adjustments (100 K to 600 K) to color temperature, are called color balancing filters as opposed to color conversion filters that produce large changes (several thousand kelvins) in color temperature. A color temperature nomograph can determine the appropriate filter selection to convert from a known starting color temperature to a desired color temperature. To use this type of graph, a straight edge ruler is placed at the color temperature of the original source and is pivoted to connect to the desired color temperature. The region where the ruler intersects the central axis identifies the necessary filter to achieve the color conversion. A more convenient method of determining the appropriate color temperature conversion filter for color balance adjustment employs a color temperature conversion calculator, which can be found in a variety of reference volumes on the subject.
The Kodak 82-series filters, light blue in color, are useful for making small incremental increases to the color temperature of light sources that produce a color balance less than 3,200 K to 3,400 K. Filters of this type do not actually change the color temperature of the light source, but are useful in simulating a higher color temperature for the purposes of photomicrography. There are four filters in this series: 82, 82A, 82B, and 82C, and each successive filter in the series raises the color temperature by an additional 100 K increment. For example, if a light source has a color temperature of 3,000 K (often found in older microscopes with a tungsten lamp), the microscopist could adjust the apparent color temperature to 3,200 K with an 82A filter for use with type B tungsten-balanced film, or to 3,400 K with an 82C filter for use with type A film. These filters should be an essential part of any photomicrographer's toolkit.
In situations where the color temperature of the light source is too high for a particular film, Kodak offers the 81-series filters. These filters are light yellow in color and produce incremental decreases in color temperature in a similar, but opposite manner from the 82-series filters. Because color temperature in microscope light sources is only rarely too high, these filters are seldom used in photomicrography. An exception to this is the application of 81-series filters to fine tune photomicrographs produced on tungsten-balanced film when using a daylight illumination source, such as a xenon lamp or electronic flash tube.
Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.
Matthew J. Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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