Now that steradian (solid angle) is an International Standard dimensionless unit of measure (May 2019), it’s time to learn what it is and how it relates to Einstein, circles and squares.
Give love to the video source: https://www.youtube.com/watch?v=V1bOThT1GKI — love his accent and way of explaining.
Luminous Efficacy. Amazing to have this as a standard. It’s human ability to detect green monochrome luminousness. It’s a metric that feeds into “standard candle” and other otherwise subjective light detection abilities of humans.
“How good is your light bulb?”
“How accurate is your red-shift measurement of distance and acceleration of galaxies?”
It all ties back to the angle that green light enters the human eye.