A unified approach to the specification of reflectance, in terms of both incident-and reflectedbeam geometry, is presented. Nomenclature to facilitate this approach is proposed. Under specified conditions-including uniform irradiance, a uniform, isotropic, plane surface, and allowance for edge effects due to sub-surface scattering-the geometrical reflecting properties of a reflecting surface are readily characterized or specified in terms of the bidirectional reflectancedistribution function (BRDF). The BRDF is denoted symbolically as fr:

An equation was derived relating t he normal spcctral e mitta nce of a n optically in homogeneous, partiall y trans mitting coating appli ed over an opaque subs trate t o t he t hi ckn cs and optical propert ies of t he coating a nd the reflectance of thc s ubstrate at thc coaLin g-SLI bstrate interface.

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A lase r-so urce bidirectional refl ec to me ter that is fully automated and has angular resolution on th e order of one degree has bee n des igned and built. Th e direc tion of in c ide nce and viewing ca n be inde pe nd e ntly varied over an e ntire he mi s ph e re except for direction s more than 77.5 0 from the normal , and th e two directions mu st be at leas t 2.5 0 apart. Bidirectional re Aectan ces for 15 sa mp les of blac k and whit e coatin gs are presented.

A new e llip soidal mirror re Aec to me te r is d esc rib ed, in whi c h radi a nt Au x from a n infrared mo noc hro lll a t (>I-is fo c use d on a specimen placed a t th e firs t foca l po int of th e ell ip so id a nd a th e rm o pil e d e tec tor is pla cet! at th e seco nd. Errors assoc iat ed with a n gul ar a nd a rea l variation s in se ns iti vity of th e det ector a nd wi th aberra ti o ns in th e opt ics we re largely e limin a te d thro ugh use of a s ma ll ave ragin g s ph e re pl aced ove r th e d e tector. Losses ca used by th e presc nce of th e e nt ra nee ho le in th c el lipsoidal m ir ror a nd frolll mirror abso rption we re e va lu a ted both th eoreti ca ll y a nd ex pe rim e nt a ll y. Co rrecti o ns for th ese losses pe rmitt ed abso lute re Acc ta nce to be o btain ed for bo th d iffu se and parti a ll y diffu se re Aec tin g s pecim e ns. In additi on, th e uniqu e op ti cs of th e e lli pso id a l mirro r provid e more ve rsa tilit y th an is ava il able in Ilrcv iou s re Hec to me te rs. Thi s ve rsa tilit y in c lu d es th e ab ilit y to acc ura te ly meas ure direc ti o na l-he mi s p he ri ca l, s pec ul ar, nons llec ular , and direc ti o na l-a nnul a r co ne re Aec tan ce. An a nal ys is of the acc uracy of th e in s trum e nt indi ca tes th a t an acc uracy of be tt e r th a n one pe r ce nt is poss ibl e fo r a ll e ng in ee rin g materia ls. Th e li se of th e s ulfur ave rag in g s ph e re a lso allowed the cons tru c ti o n of a s impl e acc u ra te s pec ul a r re Aec tome te r for ca li bration of th e mirror re fe re nce stand a rd s use d in th ese meas urement s.Key Words : Avera gin g sp heres, bidirect ional re fl ec t ali ce, diffu se re fl ec tance , e llipso ida l re Aee to me te r, infrared, infra re d det ec tors , re Hec tan ce . re Aee tollle te r, s pat ia l se ns itivit y, s pec tral refle c ta nce, spee ular re Aec tanc e.