Some spectrophotometers have a filter for removing or partially removing ultraviolet (UV) radiation from the light source. If an object has fluorescent agents, UV radiation can change the apparent reflectance of an object ("apparent" to the spectrophotometer).
For instance, to determine if an object is fluorescent, take two readings: one with the UV component included, and one with the UV component excluded. If the object has fluorescent agents, the two readings will be significantly different. The degree of fluorescence depends on the absolute amount of excitation energy present in the light source. For many fluorescent materials, the excitation wavelength is in the ultraviolet region.
Different instruments have different ways of handling UV. Some of them utilize removable or exchangeable UV filters. Readings with the filter in place and with the filter removed can be compared to determine emission wavelength, and degree of fluorescence relative to other fluorescent objects. Be careful when comparing results between two instruments of this type because the percentage reflectance measurement depends on the individual lamp in the instrument. Other instruments have an adjustable UV-filter. UV content of the source may be adjusted to achieve a known quantity of UV, or a known reading on a standard fluorescent material. Absolute degree of fluorescence may then be quantified.
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