The paper carries, at one or more points, materials which, in the excited condition, fluoresce in the visible, ultraviolet, or infra-red spectral range in the characteristic emission spectra. The device includes a light source for exciting fluorescent substances, a condenser lens concentrating the light emitted by the paper, a narrow band interference filter, a focusing lens, and a series of photocells arranged in the focal plane of the focusing lens. The geometric arrangement of the photocells is chosen appropriately to identify an emission-line doublet. The outputs of the photocells are fed to a pre-amplifier and then to a comparator which may effect a subtraction of the amplified photocell output to provide an output signal. In a variation, the quotient of the photocurrents of two photocells is compared to establish the authenticity of the paper.