Spectrophotometry determines the spectral transmittance and the spectral reflectance of objects … to compare at each wavelength the radiant flux leaving the object with that incident upon it. A spectrophotometric match thus occurs only when the two objects being compared are identical in their colour structure. Such a match will be maintained regardless of viewing conditions. Spectrophotometric matches are seldom encountered and rarely necessary; in practice, the usual objective is to achieve a metameric match. Metameric matches, however, appear identical only under one set of specified viewing conditions.