Visual discomfort in a day-lit interior environment is usually represented by the degree of discomfort glare perceived by the user. The paper reports on the evaluation of several models for the prediction of discomfort glare from daylighting, using data collected over a 10-month period, in a purpose built test cell. The Daylight Glare Index (DGI) was evaluated according to different interpretations of the background luminance (L b ). The evaluation of the models led to a better understanding of the effect of the adaptation function and a modi ed DGI is explored in light of these ndings. The Uni ed Glare Rating (UGR), which is the CIE model for the prediction of discomfort glare from arti cial lighting, was also evaluated to identify the differences between the two approaches and to test the potential for the development of a general UGR that would include both lighting environments. List of symbolsL s source luminance (cd/m 2 ) L b background luminance (cd/m 2 ) L a average luminance (cd/m 2 ) L w window luminance (cd/m 2 ) E un average vertical illuminance from surroundings (lux) E s average vertical shielded illuminance from source (lux) v solid angular subtense of source at the eye of the observer (sr) V solid angular subtense of source modi ed for the effect of the position of the observer in relation to the source 1 (sr) pGuth Position Index 2 f con guration factor of source in respect to the measurement point 3 n number of glare sources

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