Solar luminance distribution in the principal plane for different wavelengths at two locations in Sri Lanka

The colour of the sky has been the subject of many studies related to solar luminance distribution. This paper presents the angular sky luminance distributions measured at the ground level for white light and three spectral bands centred at blue (450 nm), green (550 nm) and red (650 nm) in the principal plane of the sun at two locations in Sri Lanka. The luminance measurements were taken by an LDR detector along with three colour filters. Results reveal that the ground level angular luminance distributions of white, green, and red light are similar with high peak levels of direct sun luminance which sharply decrease away from the solar disc followed by a relatively small gradual rise for angular solar distances exceeding 90º. In contrast, the direct sun luminance for blue light shows a small peak directly under the sun which too decreases away from the solar disc and remains with little variation up to 90º but rises to comparatively high levels of luminance for angular solar distances above 90º. The measured blue peak luminance under direct sunlight is higher in Kandy which is at a higher altitude with a shorter air column above the site compared to Mahiyanganaya where the peak luminance is smaller due to more light being scattered by the longer air column at the lower altitude in Mahiyanganaya.