equipment. Defects and relief distortions caused by elastic deformation during copying, excessive ware and mechanical damage of master-matrix lead to degradation of holographic image and loss of its unique security features. Thus the problem of prior master-matrix quality inspection causes a special interest of SH mass manufacturers.
The purpose of this work is to develop a method to carry out an objective assessment about the quality of the master-matrix of security holograms.
Currently, there is no consensus for approaches to assessing the quality of security holograms. Assuming that quality inspection carried out on selected areas of security holograms design, the inspection can be reduced to the measurement of the following parameters of the diffraction grating:
- spatial frequency Ȟ;
- grating depth d;
- number of periods in the grating N; - grating orientation ij.
Due to the period of gratings has a value of about 1 μm, the method based on direct measurement of the diffraction gratings parameters can be used. Such parameters as profile shape, spatial frequency and grating depth can be directly measured with the microscope. However, the disadvantage of this method is labor intensity and duration of the grating parameters measurement.
An alternative to this method is an indirect method of measurement. It is known that:
1) For a given value of wavelength the intensity distribution I over the diffraction orders of the phase grating depends on the grating depth d.
2) For a given value of the incidence wavefront angle, the value of diffraction orders angles depends on the value of the spatial frequency.
3) The orientation of the plane in which there are positive and negative diffraction orders depends on the orientation diffraction grating ij.
4) Angular size of the diffraction orders (or angular selectivity) depends on the number of periods in the grating N.
Therefore, with measuring the value of the intensity distribution, the orientation of the plane and the angular size of the diffraction orders we can uniquely estimate the parameters of the grating.
2. Diffraction in the reflective phase grating
Known mathematical expressions describing within the scalar theory of diffraction process in the reflective phase grating as described by M. Born, E. Wolf (1973) and A.M. Khorokhorov, A.M. Klishyuk et al (2005). Fig. 1 explains the occurrence of phase shift in the diffraction on the sinusoidal reflective grating.
The equation of the phase grating surface can be represented as