We present CCD photometry in UBVRI of several thousand Galactic field stars in four large ( > 1 degree ^ { 2 } ) regions centered on diffuse interstellar dust clouds , commonly referred to as “ cirrus ” clouds ( with optical depth A _ { V } less than unity ) .
Our goal in studying these stars is to investigate the properties of the cirrus clouds .
A comparison of the observed stellar surface density between on-cloud and off-cloud regions as a function of apparent magnitude in each of the five bands effectively yields a measure of the extinction through each cloud .
For two of the cirrus clouds , this method is used to derive UBVRI star counts-based extinction curves , and U -band counts are used to place constraints on the cloud distance .
The color distribution of stars and their location in ( U - B , B - V ) and ( B - V , V - I ) color-color space are analyzed in order to determine the amount of selective extinction ( reddening ) caused by the cirrus .
The color excesses , A _ { \lambda } - A _ { V } , derived from stellar color histogram offsets for the four clouds , are better fit by a reddening law that rises steeply towards short wavelengths [ R _ { V } \equiv { A } _ { V } / E ( B - V ) \lesssim 2 ] than by the standard law ( R _ { V } = 3.1 ) .
This may be indicative of a higher-than-average abundance of small dust grains relative to larger grains in diffuse cirrus clouds .
The shape of the counts-based effective extinction curve and a comparison of different estimates of the dust optical depth ( extinction optical depth derived from background star counts/colors ; emission optical depth derived from far infrared measurements ) , are used to measure the degree of clumpiness in clouds .
The set of techniques explored in this paper can be readily adapted to the Sloan Digital Sky Survey data set in order to carry out a systematic , large-scale study of cirrus clouds .