the object to be identified is affixed with an identification medium comprising a reflective layer consisting of a reflective hologram or diffraction grating . the high polymer cholesteric liquid crystal provides an inexpensive identification medium , and the use of the second light receiving unit for providing a reference signal provides a high s / n output signal which is highly distinct and resistant to the contamination of the identification medium . also , the elimination of the need for expensive optical elements such as beam splitters also contributes to the reduction of cost .