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| CONVENTIONAL AND FLUORESCENT COLOURS |
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Colours we see and ascribe to substances are related to the selective absorption of light incident upon their surfaces. If spectral components of the visible spectrum are absorbed, visible colour will result. The perceived colour then depends upon the wavelengths of the reflected or transmitted light. In case of conventional colours, the absorbed light excites the substance to higher energetic states |
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resulting in increased molecular movements, collisions and vibrations, and if energy levels achieved are high enough, chemical reaction and even decomposition may follow. Some particular substances under specific conditions however are capable of converting the absorbed energy by re-emission of radiation. Such processes are generally termed 'Fluorescence'. |
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Fluorescent substances thus are able to de-energize absorbed waves by re-emission of light. If wavelengths of absorbed and re-emitted light are equal, the term 'resonance fluorescence' is used. Fluorescent systems however also lose energy by non-fluorescent modes of decay, and the emitted light will be of lower energy or longer wavelength. Practically this means that absorbed rays, for instance ultra-violet rays, can be converted into visible light and that fluorescence consequently is manifested as colour. The latter type is known as 'Stokes fluorescence'.
The duration of the fluorescent after-glow is of the order of 10-8 seconds; radiation only persists as long as the exposure is subjected to an exciting lightsource.
Phosphorescent colours, comparable to fluorescent ones as far as the re-emission of light is concerned, differ however in their time of after-glow. Retention between absorption and re-emission is much higher and production of light can be perceived after seconds, minutes and even after
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hours. The most important fluorescent substances are the ones giving visible radiations. Some of these have the property of exhibiting fluorescence by responding only to ultra-violet radiation, producing thereby vivid colours in the visible spectrum. The fluorescent effect however is negligible when the excitation is switched to the visible region in the form of artificial or daylight illumination. These substances are said to exhibit 'Ultra-violet Fluorescence' .
Other substances will exhibit a strong fluorescent effect either under ultra-violet or daylight exposure, and will appear intensely coloured, with very bright and pure shades. The perceived colour here is a composite of the normal colour, due to reflection of visible light, plus the colour of the fluorescent re-emission. These substances absorb UV-light or the short waves of the visual spectrum, or both, and convert them to a visible radiation, reinforcing the normal colour. Such substances are said to exhibit 'Daylight Fluorescence' .
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