Rating and classification of the ergonomic performance of visual display screens in the presence of ambient light sources, as described in ISO DIS 13406-2 and ISO 9241-7, requires assessment of the reflective properties of the display unit. In this paper, we introduce a novel approach and instrument for measurement of the complete 2-dimensional Bidirectional Reflectance Distribution Function (BRDF) of visual display screens and we propose methods for evaluation of suitable reflectance characteristics. The novel instrument is based on an optical transform technique rather than on conventional mechanical angular scanning with a directional receiver and a new illumination scheme allows extension of the standard BRDF evaluation method to non-opaque multilayered objects. Typical 2-dimensional BRDF results are presented and compared with in-plane BRDF results. Evaluation of BRDF data implies that the haze, as induced by e.g. antiglare treatment of the front surface, plays an important role in the visual performance of displays. If the haze component of reflected light is not properly taken into account it leads to measurement inconsistencies. We introduce the 'haze-spread ratio' as a simple measure of reflection haze and we review a variety of other quantities, e.g. 'gloss', 'distinctness-of-image' etc, for characterization of object appearance. Critical details of geometrical alignment of the sample with respect to the instrument and basic instrumentation requirements are identified, procedures for calibration and reference via suitable working standards are listed and discussed together with the instrument specifications that make the evaluations reproducible. (C) 1998 Elsevier Science B.V.
