Dichromatism
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Apart from the meaning described in this article, the term dichromatism is sometimes also used as a synonym for Dichromacy (colour blindness).
Dichromatism (or polychromatism) is a phenomenon where the hue of the colour in materials or solutions are dependent on both the concentration of the absorbing substance and the depth or thickness of the medium traversed. [1]
In most substances which are not dichromatic, only the brightness and saturation of the colour depend on their concentration and layer thickness. Examples of dichromatic substances are pumpkin seed oil, bromophenol blue and resazurin. When the layer of pumpkin seed oil is less than 0.7 mm thick, the oil appears bright green, and in layer thicker than this, it appears bright red. The physicochemical–physiological basis of this phenomenon was recently explicated [2]. Dichromatic properties can be explained by Beer-Lambert law and by the excitation characteristics of the three types of cone photoreceptors in the human retina. Dichromatism is observed in any substance, that has the absorption spectrum with one wide but shallow and one narrow but deep local minimum. The deep local minimum on the absorption spectrum may also be limited by the end of the visible range of human eye, and in this case, it may not necessarily be narrow. The substance changes the hue from that defined by the position of the shallow local minimum (in thin layer) to the hue of deep local minimum (in thick layer). The absorbance spectrum of the pumpkin seed oil has a one wide but shallow minimum in green region and deep local minimum in red region of spectrum. In thin layer the absorbance in green is not as low as it is in red, but the overall appearance is green. This is due to the broad local minimum or depression at the green portion of the spectrum, where green cone photoreceptors are most sensitive. On the other hand, the reception of the red colour is limited by the upper limit of long-wavelength cone photoreceptor sensitivity. According to Beer-Lambert law, the absorbance is linearly related to concentration of an absorbing species and the length of the optical pathway. As the thickness of the oil layer increases, the absorbance (A) increases proportionally at all wavelengths. The transmittance (T, proportion of transmitted light), on the other hand, does not decrease proportionally (T=10−A). The intensity of transmitted light with wavelengths shorter than 630 nm is greatly reduced, whereas the intensity of red light with wavelengths longer than 630 remains high. Due to the increased ratio between red and green transmitted light, the apparent hue of the colour is turned from green to red.
[edit] References
- ^ Kennard IG, Howell DH (1941) Types of coloring in minerals. Am Mineral 26:405–421
- ^ Kreft S and Kreft M (2007) Physicochemical and physiological basis of dichromatic colour, Naturwissenschaften 94, 935-939. On-line PDF
