I have an application for which I want to normalize a lat-long map by the solid angle subtended by each pixel, but I'm a bit lost with this. I did read that to have an equal area I have to multiply the values in the map by the vertical cosine fallof (cos phi), however as I'm curious to understand better, do you know a good source to fully understand why is this or can you explain to me why is this? I mean I know that the mapping is not equi-area, but why is the correction by cos phi (if it is cos phi)?

Thank you!