12-9 |
The effects of small particles, lighting intensity and background contrast on visual perception of turbidity |
K. J. SIEBERT, Food Science & Technology, Cornell Univ., New York State Agricultural Experiment Station, 630 W. North St., Geneva, NY 14456-1371 and C. F. Fleet, Boyce Thompson Institute for Plant Research, Cornell Univ., Jonathan Comstock's Lab, Tower Rd., Ithaca, NY 14853-1801. Very little has been published on visual observation of turbidity. The objects of the study were to determine the effects of illumination intensity and viewing background on turbidity perception thresholds. Synthetic, spherical polymer beads of three known diameters (0.15, 0.30 and 0.70 µm) were each suspended at a range of concentrations in liquids of each of three colors (clear, pale amber and dark brown). Threshold determinations were carried out by the Ascending Method of Limits. Samples were presented in a viewing box lined with fabric under controlled illumination. Panelists indicated which vial in each group of three they considered turbid. Thresholds were determined for each of the nine sample sets (3 particle sizes x 3 colors) with each of three lighting intensities (1190, 590 and 18 lux) using a black fabric background. Thresholds for each set were also determined with bright illumination with two other background fabrics (shiny white and dull white). The turbidities of the samples were determined by nephelometry. Turbidity perception thresholds found with the black background and bright illumination ranged from 0.21 to 2.19 NTU. Reducing the illumination intensity by half resulted in lower thresholds (0.17 to 0.81 NTU), corresponding to greater sensitivity. Much lower illumination produced intermediate results (0.18 to 1.20 NTU). The effect of changing the viewing background from black to either of the lighter materials resulted in far higher threshold levels (on the order of 40 times the particle concentration) than those obtained with the same sample sets with the black background. Dimmer illumination is more sensitive for perception of low level turbidity than bright illumination. The viewing background has a huge effect on turbidity perception, with higher contrast producing much more sensitive detection of turbidity.
Session 12, Sensory Evaluation: Methods and consumer testing
|