Hello pirg, I calculate the efficiency from the data sheets. I trust Cree, Osram and Bridgelux data to be reasonable accurate and it is very helpful to understand what differences to expect from on bin to another.
For white LEDs we have to rely lumen data and convert to PAR W by using the LER. Sometimes we estimate the LER from the curves (as Mr Flux has done here) for each color temp, and in the case of the Cree 2700K and 3000k we got the LER directly from the manufacturer. So we need to figure out how many lumens are expected at each drive current and junction temp. For this we extrapolate from the manufacturer graphs and charts, adjusting for bin, color temp, temp droop and current droop. Once we have lumens converted into PAR W, we then have to figure out dissipation power at each given current. This changes slightly based on junction temp as well because the Vf shifts downward as junction temp rises.
As you can imagine, this is a good job for a spreadsheet. The tedious part for me is figuring out the current droop factor and Vf at each current. This requires pixel counting
So as an example, lets say we drive a CXA3070 3000K AB bin at 1.4A. If you use sufficient good cooling with a reasonable ambient temp, you might be able to get a Tj of 50C. According to the PDF that is about 7350 lumens using the typical figures. Dissipation will be about 52.2W (1.4A * 37.3 Vf). So that is 140.8 lumens/W. Divided by the LER (325) we get 43.3% efficient or 22.6 PAR W.