Basically it's a measure of how much light gets into the camera, in lumens*s/m^2. It's defines the practical limit for how fast you can run a video camera and get decent exposure.
Naturally, the closer to the 555nm green peak, and the more monochromatic your light source, the brighter it appears to be. This is a part of why night vision systems are green.
You measure a lumen by taking the area of your light source spectrogram in respect to the luminosity function.
I did this years ago with some manufacturers LED light output curves to reverse engineer electrical->photon efficiency vs cost from their data sheets. The main purpose was to figure out how close we were to LED lighting taking over.
I did that research back in 2012 and predicted 2015 as the point where LED was more efficient and cheaper than CFL but I was off by a year or so. Hilariously that paper got me a C- because it was only tangentially related to the teacher's pet subject and never saw the light of day.
