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Color Temperature

The color temperature model is based on the relationship between the temperature of a theoretical standardized material, known as a black body radiator, and the energy distribution of its emitted light as the radiator is brought from absolute zero to increasingly higher temperatures. As the name implies, black body radiators completely absorb all radiation, without any transmission or reflection, and then re-emit all incident energy in the form of a continuous spectrum of light representing all frequencies in the electromagnetic spectrum. Although the black body radiator does not actually exist, many metals behave in a manner very similar to a theoretical radiator.

Basic Principles of Color Temperature - The concept of color temperature is of critical importance in photography and digital imaging, regardless of whether the image capture device is a camera, microscope, or telescope. A lack of proper color temperature balance between the microscope light source and the film emulsion or image sensor is the most common reason for unexpected color shifts in photomicrography and digital imaging. If the color temperature of the light source is too low for the film, photomicrographs will have an overall yellowish or reddish cast and will appear warm. On the other hand, when the color temperature of the light source is too high for the film, photomicrographs will have a blue cast and will appear cool. The degree of mismatch will determine the extent of these color shifts, with large discrepancies leading to extremes in color variations. Perhaps the best example is daylight film used in a microscope equipped with a tungsten-halogen illumination source without the benefit of color balancing filters. In this case, the photomicrographs will have a quite large color shift towards warmer reddish and yellowish hues. As problematic as these color shifts may seem, they are always easily corrected by the proper use of conversion and light balancing filters.

Interactive Java Tutorials

Color Temperature in a Virtual Radiator - Investigate the apparent "color" of a virtual radiator (in this case, a black metal pot) as it is slowly heated through a wide temperature range by external energy. The concept of color temperature is based on the relationship between the temperature and radiation emitted by a theoretical standardized material, termed a black body radiator, cooled down to a state in which all molecular motion has ceased. Hypothetically, at cessation of all molecular motion, the temperature is described as being at absolute zero or 0 Kelvin, which is equal to -273 degrees Celsius.

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 Java nomograph tutorial can be employed to quickly determine the appropriate filter under a variety of illumination scenarios.

White and Black Balance - The overall color of a digital image captured with an optical microscope is dependent not only upon the spectrum of visible light wavelengths transmitted through or reflected by the specimen, but also on the spectral content of the illuminator. In color digital camera systems that employ either charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) image sensors, white and/or black balance (baseline) adjustment is often necessary in order to produce acceptable color quality in digital images.

Incandescent Lamp Filaments - Nearly every source of light depends, at the fundamental level, on the release of energy from atoms that have been excited in some manner. Standard incandescent lamps, derived directly from the early models of the 1800s, now commonly utilize a tungsten filament in an inert gas atmosphere, and produce light through the resistive effect that occurs when the filament temperature increases as electrical current is passed through. This interactive tutorial demonstrates the sub-atomic activity within a conducting incandescent lamp filament that results in resistance to current flow, and ultimately leads to the emission of infrared and visible light photons (with a corresponding rise in color temperature).

Contributing Authors

Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.

Matthew J. Parry-Hill, Thomas J. Fellers, 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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