Random phase plate for holography and process for manufacturing same

A process for producing a random phase holographic plate coated with polyvinyl butyral resin in which the thickness H of the resin varies in a continuous and random manner over the said plate relative to light of a wavelength .lambda. such that

• exp -4.pi..sup.2 (n-n.sub.o).sup.2 .sigma.h.sup.2 /.lambda..sup.2 .ltoreq.0.10
• where
• h.sub.m is a mean value of H, h=H-h.sub.m ;
• .sigma.h is a standard deviation of h;
• n is a refractive index of the solidified coating material for light of the wavelength .lambda.;
• n.sub.o is a refractive index of air for light of the wavelength .lambda.;
• and
• exp indicates exponentiation, comprising the steps of:
• (a) applying a 17.5% by weight solution of polyvinyl butyral resin in benzene to the upper surface of a transparent substrate, and
• (b) solidifying the solution by rotating the substrate about a perpendicular rotary axis at a rate of from 200 to 1500 rpm to evaporate the solvent.