The spatially periodic amplitude of the first and second component interference patterns 16, 17 in the direction z will be referred to as A1(z) and A2(z), and the resultant intensity pattern I(z) shown in FIG. 12 can be written as the sum of the squares of the intensity patterns, i.e. I ??? ( z ) = K 2 ??? ( A 1 2 + A 2 2 ) ( 1 ) where K is a constant.