Imagine a bucolic scene on a clear sunny day at the equator, sand warmed by the tropical sun with a typical irradiance (
) of about 1000 watts per square meter. As discussed earlier we could express this quantity as illuminance in lumens per square meter (
) – or as a certain number of photons per second (
) over an area of interest (
).
(1) 
How many 
per unit area can we expect on the camera’s image plane (irradiance 
)?
In answering this question we will discover the Camera Equation as a function of opening angles – and set the stage for the next article on lens pupils. By the way, all quantities in this article depend on wavelength, which will be assumed in the formulas to make them more readable.