Abstract:
Skylight polarisation pattern in general and the
positions of the polarisation neutral points in particular are
important parameters in atmospheric science. The construction
and operation of a low cost partially automated polarimeter for
studying the polarisation pattern of skylight in the principal
plane of the sun, and some results obtained for the skylight
polarisation pattern using the polarimeter are presented in
this paper. The polarimeter mainly consists of a collimating
tube with a linear polariser and a detector consisting of a light
dependent resistor controlled by a microprocessor. The degree
of polarisation for a given angular distance from the sun in the
principal plane of the sun was measured using the maximum and
minimum currents passing through the light dependant resistor
(LDR) computed through voltage measurements across the LDR.
Positions of the polarisation neutral points and the maximum
degree of polarisation were determined graphically using plots
of the degree of polarisation as functions of the angular solar
distance. The error in the neutral point positions was estimated
to be 2°, while that of the degree of polarisation was estimated
to be a maximum of 18 %. The measurements taken at different
times on relatively clear sky days during a period of over one
year provided consistent results in fairly good agreement with
similar studies carried out using more sophisticated instruments
as well as theoretical predictions for a molecular atmosphere. An
attempt has also been made to account for any deviations of the
quantitative results from those of other studies.
