Patent ID: 8139288

Claim:
A method for controlling the movement of a light beam in a scanning apparatus, as a function of variation in the length of the optical path of said beam, comprising (a) directing said light beam to an angular prism, wherein said angular prism is coupled to a sample at a base interface; characterised in that the base angle θ p of the prism satisfies the equations: ( cos 2 ⁢ θ c - sin 2 ⁢ θ p ) ⁢ ( tan ⁢ ⁢ θ i ⁢ sin ⁢ ⁢ θ p + cos ⁢ ⁢ θ p ) = 1 cos ⁢ ⁢ θ c ⁢ cos ⁡ ( θ p ± θ c ) and n i ⁢ sin ⁢ ⁢ θ i = n p ⁢ sin ⁡ ( θ p - θ c ) wherein θ c is the coupling angle required for light incident at base interface of the prism, θ i is the incident angle of the light beam on the prism, n i is the refractive index of the medium at the interface where the light beam enters the prism and n p is the refractive index of the prism; and (b) moving the prism and sample relative to the light beam, wherein said variation in the length of said optical path of said light beam is controlled by aligning the prism so that said light beam enters said prism normal to the face of entry thereinto, and wherein said light beam and said prism satisfy said equations.