Beam splitting prism assembly

Claim 1

A beam splitting prism assembly comprising first, second, third and fourth prism elements arranged consecutively to separate incident light spatially into three different spectral components wherein the first component lies spectrally between the other two components wherein the improvement consists in said first and second prism elements considered in the direction of incident light having adjacent parallel and spectrally substantially non-selective surfaces defining a first air gap, said second and third prism elements being separated by a second air gap, a spectrally selective dichroic coated surface and a spectrally substantially non-selective surface, parallel to said coated surface, bounding said second air gap, one of the adjacent surfaces of the third and fourth prism elements carrying a spectrally selective second dichroic coating, wherein the surfaces defining the first and second air gaps are inclined to the direction of incident light to transmit said incident light and to totally internally reflect the light which has been selectively reflected from the corresponding dichroic surface, and wherein the apex angles of the said first, second and third prism elements lie within the following limits: ##EQU11## where: A is the apex angle of the first prism element; B is the apex angle of the second prism element; C is the apex angle of the third prism element; N is the refractive index of the prism glass at the wavelength appropriate to the colour being selected, and f o is the widest relative aperture of camera objective with which the assembly is optically compatible.