Well most half decent grow light manufactures will publish this as part of their marketing…
Being in a metric part of the world foot candles are odd measurement for me, but a foot candle basically translates to about 10.764 lux. Which is also not a great unit to measure, however when combined with the relative spectrum distribution chart provide enough information for the average closet grower to make an informed decision… Hardcore growers and commercial large scale operations where every dollar spent has to be measured against every dollar recouped will use PAR and measure in umol/m^2 per sec. as @Northern_Loki has indicated.
If you are looking at building a light, any manufacturer worth their salt will publish a sheet that should contain the information you are after, as well as the relative spectral distribution, CRI, lumens/W across various temp ranges and also the photo-metric web i.e. the “shape” of the light output etc…
DIgikey and other suppliers usually have a direct link to these data sheets and some manufactures like Cree and Bridgleux also have free software tools that allow you to vary running parameters and simulate the results.
Here is the cree example that I took the graph from…
This is a bit of a “how big is a boat” question in some ways… I tend to use longer strips so that my lights rig covers the entire area of the tent and as I say I target 100, 000 lumens ish per square meter. However comparing them is a little like comparing apples and oranges because it doesn’t take into account the radiant intensity nor heat… so given that the inverse square law for light decay applies to all light sources… 145K lumens ends up being significant less when you take into account the distance you need to put the light to prevent it from frying everything… The chart in the first post indicates closer to 60K lumens for a 140K HPS by the time you have got the correct distance!
whereas the led strip lights can sit less than an inch from the canopy… And also scattered light sources are more effectively utilized by the plant vs intense point light sources… so it’s a little complicated huh? Lol