An equisolid angle projection is a specific mathematical mapping used in fisheye lenses where each pixel or unit area on the sensor represents an equal solid angle in the object space. This mapping is highly valued in radiometric and photometric measurements because it preserves the relative intensity of light sources across the entire wide field of view.
Heard on the job
The sky camera uses an equisolid lens so we can accurately integrate the total irradiance across the hemisphere.