of wavelengths modulation point ideally the modulator is transparent to other wavelengths control circuits such as a voltage source u are used to apply a control signal to modulator 1 to move its modulation point as required a luminous intensity detector 2 is used in conjunction with modulator 1 it receives the light which has passed through the modulator and outputs a signal which is a function of the luminous intensity received this signal is displayed on a detector device det modulator 1 is illuminated by the beam e to be analyzed if the modulator 1 modulation point does not correspond to any wavelength in beam e all the light is transmitted to detector 1 which therefore detects the maximum intensity the control signal then shifts the modulator modulation point when this point corresponds to a wavelength in beam e the modulator absorbs all or part of this wavelength and the intensity of the beam transmitted to detector 2 is reduced by an amount corresponding to the intensity absorbed we shall now describe in reference to fig5 a and 5b an example of a preferred embodiment of the invention in which modulator 1 and detector 2 are quantum well structures stacked on the same substrate s the device therefore consists of substrate s a stack of doped lgs sgs lgs layers forming a multi quantum well radiation detector mqwrd 2 trapped between two thick and highly doped layers of sgs which provide electrical contact and above one of these.