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Rubies seen through 'crossed' filters

A set of synthetic rubies seen through a deep-red filter (RG715) and illuminated with blue light (6500K LED + 5mm/BG38). The camera only sees light arising from the chromium fluorescence from the rubies. This is what is known as the 'crossed-filter' technique which is a very sensitive way of detecting fluorescence (especially red).

The Sony camera in 'night-shot' mode has very high sensitivity at the wavelength of the ruby fluorescence (694nm).

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