| Description: | It has been observed that the zenith radiance measurement is susceptible to contamination
from light scattered off of the internal optics. This problem is most serious when
measuring clear sky radiance in the presence of direct sun. Under this worst case condition,
it affects short wavelengths (less than 500 nm) at a few percent level, moderate
wavelengths (500-800nm) between a few percent and 10-20%, and longer wavelengths up to 80% or more.
Chui and Marshak have demonstrated some success at identifying isolated affected data
using the TSI to find cases of scattered clouds. In general though, one must treat the
zenith radiance data with considerable caution whenever direct solar radiance is likely to be
present. It is anticipated that the SWS zenith radiance for overcast skies is not
significantly affected.
The instrument has been renovated by adding a sun shade to prevent direct sunlight from
striking the optics. An additional set of baffles further restricts the field of view,
further reducing the potential for even off-axis bright clouds to affect the zenith
measurement. Data taken after June 2009 is considered to be free of contamination. |