To determine quantum efficiencies of powder phosphors as a function of exciting wavelength, sodium salicylate is usable in the ultraviolet but not in the visible region. As a powder standard in the excitation region, 200–550 nm red luminescent lumogen powder is proposed. The absolute quantum efficiency has been measured. It varies between 40 and 60% in this region and it has a high absorption.

M e th ods of abso lut e a nd re lati ve radi a nl a nd q ua ntum effi c ie ncy m eas ure me nts a re described fo r ultrav iole t , vis ibl e , ca thod e-ray, a nd x·ray exc ita tio ns. Da ta o n so me s ta nd a rd lumin esce nt ma te ri a ls a re gjve n.Key word s: Ca th ode· ray exc itation ; lum inesce/lce; lumin esce nce st a n da rd s; phosp ho rs; qu a ntum e fficie ncies; ra di a nt efficie nc ies; UV exc it a ti on; ,· ray exc it a tio n. . IntroductionMe th ods of absolut e radi a nt a nd qu a ntum e ffi cie ncy meas ure me nts are giv e n togeth e r with meth ods of relati ve e ffi c ie ncy meas ure me nts. Th e me th ods are es pec ia ll y s uit ab le fo r powd e r ma te ri als fo r whi c h th e a ngular di s tributi on of th e e mitted lumin esce nt radi ati on is La mbe rti an.Th e re la ti ve meas ure me nts a re perfo rm ed with th e aid of s ta nd a rd ph os ph ors, whose effi cie ncies have previously bee n de te rmin ed by a bsolute meas ure me nts. Methods are give n fo r excita ti on of th e ph os phors by ultraviole t a nd vis ible radiati on , cathod e rays and x rays.F or samples with non-La mbe rti a n e mi ss io n di stributio ns, a me thod is desc ribed in whi c h a n Ulbri c ht 's s phere or an e lliptical mirror is used. . Ultraviolet ExcitationAll powder phosphors are meas ured us in g a thi c k layer (thi c kn ess ab out 2 mm) at th e irradi ated side. Th e detection tak es place pe rpe ndi c ular to th e plan e of the phos phor, t he excitati on is at a n a ngle of 50° with th at plan e (see fi g. 1). Th e e xcita tio n wa vele ngth ( A exc) or regio ns a re iso la ted from a hi gh pressure merc ury la mp by inte rfe re nce filt ers, the arc being foc used on th e phos phor with a qu artz le ns. In thi s way a hi gh excita ti on de nsity is reached , but generally well below th e excitation region where s aturation e ffects s ta rt. This is es peciall y ad vantageo us wh en a rela ti vely in se ns iti ve th erm oele me nt is used as a d etector.* P aper p re se nt ed at th e W ork s hup Sem in a r 'Slandard izat ion in S pec trophutometry a nd L u m in esce lwe Meas ure rn e nl s' held a t the Nat iona l Burea u uf Standards. Ga ithersbu rg , Md .. Nove mbe r 19

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