Values of scalar illuminance are included in the IES Code for Interior Lighting for certain areas. There is evidence that cylindrical illuminance might be a better correlate with the impression of the brightness of an interior. Experimental work is discussed and reported that suggests that the geometric mean of vertical and horizontal illuminance is also a significant parameter when comparing observers' impressions of the equivalent brightness of objects. Some of the experiments described show that hemispherical illuminance is a very good correlate to the impression of the brightness of objects where a single direction of view is considered.