used for this purpose if they are substantially constant in thickness in a lens system in which the same optical path lengths on and out of the optical axis cannot be obtained by only a single lens the on axis and out of axis optical path lengths can be balanced with respect to the total optical path length by using a plurality of single lenses into which the light absorbing material is incorporated in accordance with this invention photoreceptors other than the cds type photoreceptor for example selenium type photoreceptor having high sensitivity in the short wavelength range can be also compensated in spectral sensitivity a spectral transmission factor obtained in this case is shown in fig6 wherein for the near ultraviolet range a transparency at 50 selectivity efficiency is seen to occur for wavelengths of substantially 450 m u03bc m u03bc nm for example by incorporating a light absorbing material capable of absorbing light in the short wavelength range into a lanthanum glass material of sio 2 b 2 o 3 la 2 o 3 pbo al 2 o 3 type a lens for absorbing light in a particular short wavelength range can be obtained without any change of the optical constant and also the basic design for the optical element as in the previously mentioned light absorption in the longer wavelength range for a photoreceptor.