Effect of illumination intensity on visual perception of turbidity

Colloidal size (0.15, 0.31, and 0.70 μm diameter) synthetic polymer microspheres were each suspended in liquids of three different colors (clear, tan and dark brown) at a range of concentrations. The latter two model solutions resembled colors found in light and dark beers. Samples were presented to panelists in a viewing box. Visual threshold determinations were made for each of the nine sample sets using the Ascending Method of Limits and bright illumination (1192 lux). Thresholds were much more similar expressed as turbidimeter observations than as either weight or number concentration, and ranged from 0.206 to 2.19 nephelos turbidity units (NTU). Panel mean thresholds increased as color intensity increased. Thresholds tended to be lower with the largest particles. Thresholds for each sample set were also determined with reduced illumination. Intermediate lighting intensity (592 lux) resulted in the lowest detection thresholds (0.176-0.829 NTU). The threshold results obtained with the lowest illumination (18 lux) were intermediate between those with the higher two illuminations (0.183-1.20 NTU).