Measurement of internal stray light within Dobson ozone spectrophotometers

The Dobson ozone spectrophotometer is the primary ground-based instrument making measurements of total column ozone. Knowledge of the ozone layer prior to the satellite measurements is derived from these instruments' more than 80-year record. Internal stray light within the instrument limits its ability to make accurate measurements at high total ozone amounts and high solar zenith angles (SZAs). Additionally, a recent analysis of comparative Dobson observations of the Umkehr effect (an SZA-dependent series of measurements on the clear zenith that are used to produce an ozone vertical profile) has shown that internal stray light produces incompatible results between compared instruments. We present a method of making a measurement of this internal stray light, as well as other approaches to evaluating the approximate level of the internal stray light. These techniques can also be used to improve the results of direct Sun observations at high latitudes and high ozone.