  Clouds play a significant role in the fluctuation of solar radiation received
by the earth's surface. It is important to study the various cloud properties,
as it impacts the total solar irradiance falling on the earth's surface. One of
such important optical properties of the cloud is the Cloud Optical Thickness
(COT). It is defined with the amount of light that can pass through the clouds.
The COT values are generally obtained from satellite images. However, satellite
images have a low temporal- and spatial- resolutions; and are not suitable for
study in applications as solar energy generation and forecasting. Therefore,
ground-based sky cameras are now getting popular in such fields. In this paper,
we analyze the cloud optical thickness value, from the ground-based sky
cameras, and provide future research directions.
