Luminescence detecting devices

In a beam index cathode ray tube including a photosensitive element comprising a PIN diode which has a peak response at a relatively high wavelength relative to wavelengths of visible light, said photosensitive element further including a luminescence detecting device comprising:

• a luminescence sensitive member composed of a transparent material containing at least two types of organic phosphors dispersed therein, the first type of organic phosphor being excitable by incident luminescence of a wavelength substantially lower than said relatively high wavelength entering therein and operative to absorb fluorescence in a first wavelength range which overlaps with the range of wavelengths in said incident luminescence and emit fluorescence in a second wavelength range overlapping with the longer wavelength portion of said incident luminescence, the second type of organic phosphor being excitable by flourescence in a third wavelength range overlapping with at least a part of said second wavelength range and emit fluorescence in a fourth wavelength range overlapping with the longer wavelength portion of said third wavelength range, and
• luminescence sensing means comprising said photosensitive element disposed in proximity to said luminescence sensitive member and capable of immediately detecting at least the flourescence in a fourth wavelength range emitted by said luminescence sensitive member to produce an output signal approximating said relatively high wavelength peak response of said photosenstivie element in response to the flourescence detected thereby.