The color of a substantially spherical surface, such as that of a fruit, is measured by rotating the object about a vertical axis and projecting on the surface of the object, downwardly at 45 DEG , an area of light that extends from the zenith to the equator of the object and is generally spherical triangular in shape, but differs from a spherical triangle in that the arc length s of the circles of latitude falling between the boundaries of the illuminated area is related as follows to the arc length s 0 of the 45 DEG circle of latitude: s(x) = s0 (1 + tan x)WHERE X, WITH THE APPROPRIATE SIGN, IS THE DIFFERENCE IN ANGLE BETWEEN 45 DEG AND THE CENTRAL ANGLE OF THE CIRCLE OF LATITUDE PASSING THROUGH THE ILLUMINATED SURFACE. To measure color distribution, the same apparatus is used but only an area is illuminated which is sufficiently small to be regarded as planar and the selected area is manually rotated to underlie the light spot.
MEASUREMENT OF COLOUR OF SPHERICAL OS QUASI-SPHERICAL SURFACES