A simple photoelectric transmi ssion densitometer can be converted in th e laboratory into an in strum ent for measuring spatial variations in spectral density of small areas of thin, n onturbid film s. This is accomplished by adding relatively in expensive co mpon ents that are rea dily available and easily assembled. The basic instrument can be one of a number of commercial densitome ters, consisting of stabilized electroni cs, an adjustable light source, and a photosensitive probe whi ch has

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... a wide ran ge and measures spectral tran smi ss ion density. For continuous s canning of r elatively small sa mpl e areas, th e foll owng additions are needed: (l) narrow· bandpass filt er s; (2) interchangeable fi eld illuminating slits; (3) a short·focal-length illuminating-cond en ser lens whi ch co mpromises between a relatively large num erical aperture and a reasonabl e depth of focus ; (4) a motor-driven stage with adjusta bl e speeds; (5) a li ght diffu ser an d li ght guid e; and (6) a conti nuous, relati vely fa st-res ponse data·recordin g system (such as a fas t· response strip-chart recorder). Tllis instrum ent has b een asse mbl ed and tested es pec ially for measuring patial variations of spectral density produced in irradiated photochrom ic film s; and al so for calibrating a train of irrad iated film sa mpl es, in terms of tran smi ss ion density at a given wavelen gth as a fun ction of irradiance, radiation abso rbed dose, or ph otochemi cal reacti vity. K ey word s : Den sitometer; mi crodensitometer; rad iochromic dye film s ; spectral transmission den sity.