This will be a doozy to explain, but I’ll do my best!
I work in the film industry and we often times use footcandles as a unit of measurement for how bright a light is. There are lots of apps out there made by the manufacturers that will tell you how many foot candles their light will produce at a given distance.
The issue I found is that is only half of the equation. Based on how you setup your camera you need to also know how bright you even need the light to be, or how many foot candles you need to expose the shot properly. None of the manufacturers apps take this into account.
I set out to make an app where you could type in all of your camera settings, it would tell you how many foot candles were needed to expose the shot properly, then display a list of all the lights that would match those requirements.
The most difficult part was deciphering the formulas to figure out all of this. I also realized early on that I had to just ask the user for one foot candle reading at a specific distance, then I could use a formula to calculate how bright the light is at other distances.