Apparatus for measuring absolute reflectance

Claim 1

Apparatus for measuring the absolute reflectance of a sample, comprising: (a) an integrating sphere provided in a spherical wall thereof with four windows having a center of each window lying in a single plane including a center of said sphere, a first one of said four windows having its center coinciding with a diametric line included in said plane while a second and a third one of said four windows are arranged symmetrically with respect to said diametric line, with a fourth one of said windows having a sample set therein so as to face inwardly of said integrating sphere; (b) a light source so arranged as to introduce a beam of light into said integrating sphere; (c) light intercepting means provided in said integrating sphere and arranged in a partial light intercepting relation to said sample so as to prevent diffuse light which is reflected a first time by an inner surface of said sphere from directly impinging on said sample and to allow light reflected by said inner surface said first time to impinge directly on said inner surface so as to be reflected a second time and to directly exit said sphere through one of said second and third windows; (d) light measuring means so arranged as to receive light emerging from inside said integrating sphere; (e) means for rotating said integrating sphere about an axis coinciding with said diametric line so that said integrating sphere selectively takes first and second positions 180° apart from each other thereby to change the operative position of said second and third windows relative to said light source and said light measuring means; and (f) means for processing measured data obtained by said measuring means when said integrating sphere is at said first and second positions, respectively, thereby to obtain the absolute reflectance of said sample.