1. An optical broad spectral bandwidth low-pass filter for use with an optical imaging system of a solid-state imager, comprising: a transparent substrate and material forming a plurality of transparent spots randomly placed on said substrate for producing a phase difference between a wave front transmitted through said spots and that portion of the substrate other than said spots to define a filter having a spatial sampling cut-off frequency which is approximately independent of the wavelength of light over a specific spectral bandwidth from a lower wavelength .lambda..sub.B to an upper wavelength .lambda..sub.R, said phase difference satisfying the following condition EQU .phi..sub.o =.pi. where .phi..sub.o is the phase difference due to the spots at a certain specific wavelength, .lambda..sub.o, such that EQU .lambda..sub.B .ltoreq..lambda..sub.o .ltoreq..lambda..sub.R and where .phi..sub.o is given by the relationship ##EQU22## so that ##EQU23## where n.sub.o is the index of refraction of the material forming said spots at the specific wavelength .lambda..sub.o ; n.sub.o ' is the index of refraction of the medium surrounding said spots at the specific wavelength .lambda..sub.o ; and t is the physical thickness of said spots and wherein the solid state imager is a monochrome imager characterized by a system spectral sensitivity S.sub.sys (.lambda.) which is common to all pixels in the array comprising said monochrome imager and which has a peak sensitivity at a wavelength .lambda..sub.sys and wherein the specific wavelength .lambda..sub.o is equal to .lambda..sub.sys.