The data from Kanpur
The main principle of Sun Photometry/ Solar Radiometry is to estimate the aerosol optical depth , at a near monochromatic wavelength for  radiation  incident normally on the top of the atmosphere and traveling the entire atmospheric path. When the radiation is nearly monochromatic and the wavelength  lies outside any major absorption bands, the ground reaching solar flux, is related to the extra-terrestrial flux  at the top of the atmosphere through the    Bouguer-Lambert-Beer law . The ground reaching solar flux depends on the mean and instantaneous Sun-Earth distances, the normal incidence columnar total optical depth of the atmosphere at a particular wavelength, and the relative air mass. The relative air mass term accounts for the increased distance traveled by the solar radiation incident away from zenith. The airmass term (m) depends on the solar zenith angle and can be calculated from the knowledge of the time of observation, the solar declination angle, the ephemeris transit time (or equation of time) and the geographic co-ordinates of the observing station (like latitude and longitude).  Thus if continuous measurements of V are made, as a function of time during a day, a plot of ln (V) against m will show the points lying very close to a straight line. The slope of the straight line estimated using linear regression analysis yields the total optical depth and the intercept extrapolated to meet the ordinate (i.e., for m = 0) can be used as a self-calibration for the radiometers. This procedure is also widely known as the Langley technique in literature.It yields the columnar total optical depth  of the atmosphere, which actually is the sum of optical depths due to molecular extinction and extinction due to aerosols. By subtracting the molecular contribution, the aerosol optical depth can be estimated.

The Langley plot developed from the Data observed at IIT Kanpur for the month of November and December
for the year 2002 is available.