Light Intensity Effect

According to the tri-receptor theory of vision, the human eye detector evaluates the intensity of an image by summing the stimuli from the three receptors, while the chromatic attributes, hue and saturation, are determined by the ratios of the stimuli. Thus, light sources having widely different spectral distribution may give exactly the same visual color sensation as long as the amount and ratios of the total simulation are equal. To investigate the effect of the strength of illumination on the hue-temperature calibration curve, Wang (1996) performed the LCT calibration test at two different intensity levels of illumination. The experiment was repeated under similar test conditions but with 50 percent of the fluorescent tube surface covered. The author observed that the change in illumination intensity does not alter calibration except at the high temperature end of the curve.

The clarity of the picture seen by the camera depends on the background intensity. If it is low, then the picture will be dark and the camera may not be able to capture the color changes. If the background intensity remains high, the actual color may be lost in the white-light effect. Therefore, it becomes necessary to establish the background intensity, provide uniform lighting and ensure correct threshold setting to record meaningful color information. An optimum intensity for the background is typically between 80 and 100, on an eight-bit scale of 0-255 (Ekkad, 2000).

A sufficiently bright and stable white light source without the infrared (IR) and ultra-violet (UV) radiation in the output spectrum is ideally needed for liquid crystal thermography. Any IR energy present in the incident light will cause unwanted radiant heating of the test surface. Exposure to UV radiation can cause rapid deterioration of the liquid crystal surface, leading to an unreliable performance in terms of the color-temperature response. Consistent light source setting and viewing arrangement between calibration and the experiment is essential to minimize color-temperature interpretation errors. In the experience of the authors, the LC test surface is best illuminated with a collimated white light source, which utilizes four 150 W ANSI code halogen bulbs covered by a sheet of heat absorbing glass.