lipid layer and undergo constructive and destructive interference in the lipid layer an image of this surface is the backdrop over which the interference image is seen and it should be as spatially uniform as possible the illumination means 100 illuminates a large area of the face which creates a 2 5 mm high by 5 mm long viewable area centered beneath the pupil 310 see fig1 which is adequate for lipid layer thickness determination and correlation to dry eye by u201c viewable area u201d it is meant the active area that satisfies the criteria for viewing interferences fringes i e approximately 2 5 mm u00d7 7 mm for the half eye illuminator full eye illumination excluding the pupil area reveals additional information about the whole eye pattern of lipid distribution the geometry of the illuminator 100 can be most easily understood by starting from the camera lens and proceeding forward to the eye and then to the illuminator the fundamental equation for tracing ray lines is snell 39 s law where n is the refractive index of the medium containing the ray and u03b8 the angle of the ray relative to the normal from the surface for a reflected ray that doesn 39 t enter the lipid layer n 1 n 2 and under these conditions snell 39 s law reduces to the classical u201c angle of incidence is equal to the angle of reflectance u201d statement according to the present invention it is necessary to determine only the extreme rays the ones at the outermost boundary of the desired viewing area to define.