O-13
The effects of small particles, lighting intensity and background contrast on
visual perception of turbidity. The objects of the study were to learn more about the effect of particle size
on human visual perception of turbidity, and 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). The colors were chosen
to resemble pale and dark beers. Threshold determinations were carried out by
the Ascending Method of Limits. Panelists were shown three vials at a time in
randomized order; in each case one of the three vials contained particles and
the other two were particle-free controls of the same color. The process was
repeated with geometrically increasing concentrations of the added particles
within each sample set (one particle size in one solution color). 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 ×
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), which had much lower contrast with the color of the beads (off-white)
than the black fabric. 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). Much lower
illumination produced intermediate results (0.18 to 1.20 NTU). The effect of
changing the viewing background to one with a lower contrast with the particles
resulted in far higher threshold levels (on the order of 40 times the particle
concentration) than those obtained with the same sample sets with a high
contrast 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.
Karl Siebert received a Ph.D. in biochemistry from Penn State in 1970. He
joined the Stroh Brewery Company in Detroit where he spent 18 years and held
positions from Research Associate to Director of Research. In 1990, Dr. Siebert
joined Cornell University as Professor of Biochemistry in the Department of Food
Science and Technology. He served five years as department chairman and now has
a 100% research commitment. Dr. Siebert served on ASBC technical subcommittees
and was a member and chairman of the Technical Committee. He is serving his
second stint on the journal editorial board (1980-1992; 1996-).
Christine F. Fleet and KARL J. SIEBERT. Food Science & Technology Dept., Cornell
University, Geneva, NY 14456-0462.