reflectance standard it is possible to obtain the percent 
reflectance at any wavelength. A similar approach is used 
to obtain transmittance values except here the clear air 
transmittance is used as the standard. 
Photo Image Analysis: If one places a white styra- 
foam panel (similar in reflectance to barium sulfate) in a 
lake and photographs it, then the water in the image can be 
analyzed for reflectance by comparing the brightness of the 
water with the brightness of the standard panel. The density- 
log exposure curve for the film must be used in this analysis, 
but it is possible to obtain the percent apparent reflectance 
in the field by use of an aerial image (Klooster and Scherz, 
1974). The field reflectance values will always be greater 
than the laboratory reflectance because the field reflectance 
always has reflection of skylight from the water surface and 
atmospheric scatter while the laboratory reflectance does not. 
This shift between laboratory and field reflectance holds for 
both low altitude aerial photos and ERTS images with the 
amount of shift depending on meteorological conditions, time, 
and altitude. 
ERTS Image Analysis: The brightness of an ERTS image 
can be determines by use of 1 a microdensitometer or by use of 
satellite transmitted computer tapes. ERTS image calibration 
curves are used to obtain the percent apparent reflectance 
sensed by the satellite scanner. 
The lower part of Figure 1 shows a full ERTS frame 
of Lake Superior near Duluth, Minnesota. One will note the very 
turbid water in the SW end of Lake Superior near Duluth caused 
by clay runoff from several rivers in the area. This turbid 
water is present about 50% of the time. It is of interest to 
note that an engineer located an $8,000,000 water intake in the 
middle of this turbid water and when it began to operate in 1969 
the water was too turbid to use. ERTS imagery, if available 
during the design stage, should have been useful to the engineer 
in his decision of where to locate the intake. 
The turbid water in the SW end of Lake Superior was 
sampled over a year’s period and laboratory analysis run on the 
samples. As expected, the ERTS reflectance values are much 
higher than the laboratory values due to the skylight and atmos 
pheric effects. Figure 2 shows curves comparing turbidity to 
laboratory and field reflectance on three different days. While 
the laboratory curve is constant for all days, the ERTS curve 
shifts from day to day as the atmospheric effects change. Two 
simultaneous water samples collected at the time of the ERTS 
image will establish the position of the ERTS curve for a par 
ticular frame and allow that water in the entire image to be