Portions of the following has been excerpted from from copyrighted material. Material in this section should not be reproduced without specific permission of the North American Lake Management Society ( www.NALMS.org). The reader is encouraged to read the original material for more details on Secchi disk theory and methods. These are:
Carlson,
R.E. 1995. The Secchi
disk and the volunteer monitor. LakeLine.
15(1): 28-29, 35-37.
Carlson, R.E. and J. Simpson. 1996. A
Coordinator’s Guide to Volunteer Lake Monitoring Methods. North
American Lake Management Society. 96
pp.
Carlson,
R.E. 1997. The Secchi disk in black and white. LakeLine. 17: 14-15,
58-59. (7/20/97)
LakeLine is available through the North American Lake Management Society
Why a Black and White Secchi Disk?
The
original "Secchi's" disk was all-white. Today most disks
used in lakes have alternating black and white quadrants, while marine disks are
usually all-white. The reasons for the difference is not clear, and may be
more historical than theoretical. Early disks of the 19th century were all
white, but George C. Whipple (1899) stated that a disk with alternating black
and white quadrants was more easily seen (Fig. 1), and "Whipple's" disk became
the standard in freshwater situations.
 |
Figure 1. The "Whipple Sector Disk," as
illustrated in the 4th edition of The MIcroscopy of Drinking
Water (Whipple, 1933)
In the1899 edition he states: "The author’s experiments have shown that the limit of visibility may be determined most accurately by using a disc about 8 inches in diameter, divided into quadrants painted alternately black and white like the target of a level-rod, and looking vertically down upon it through a water-telescope provided with a suitable sunshade." |
In theory, the Secchi disk works as a contrast instrument. It disappears when the human eye can no longer see it, meaning
that there no longer remains any contrast between the disk and its
background. A white disk
should remain visible longest if viewed against a completely black
background. In many lakes,
the background is less than totally black because the light is reflected
off the bottom or back-scattered from suspended particles. In these situations, a white disk may disappear sooner than would
be expected than if the background were black. The black quadrants may
serve as a constant black background, thus standardizing the contrast. It may be that the all-white disk used in the ocean is acceptable
because the background color in the deep ocean is black, thus making the
black quadrants unnecessary.
The
newest trend in disk color is the all-black disk (Davies-Colley 1988). Developed and used extensively in New Zealand, the disk has the advantage that it can be used in shallow rivers and
streams. Unlike the horizontally-held Secchi disk, the black disk is
held vertically and is viewed over a horizontal distance using an
inverted periscope. Because
the black disk does not require much water depth, it can be used in
streams. The ability to use
black disk in rivers and streams could establish cooperation and
compatible data with stream volunteer programs. Lake monitors could use the black disk to discover which streams
entering a lake are the most turbid.
Davies-Colley,
R.J. 1988. Measuring water clarity with a black disk. Limnol. and Oceanogr. 33:
616-623.
Whipple, G.C. 1899, 1910,
1914, 1933. The
Microscopy of Drinking Water. John
Wiley and Sons. New York
and London.
Carlson,
R.E. 1997. The Secchi disk in black and white. LakeLine. 17: 14-15,
58-59. (7/20/97)