Photosynthetically active radiation (PAR) quantifies photons in the wave length range that supplies photosynthesis. You need a PAR meter to measure this way. A standard foot candle meter usually measures additional wavelengths, so the reading isn’t as precise for what plants specifically need. However, the ratio of 5:1 foot candles to PAR peak photon density may be a rough approximation. If 300 peak photon density is the most that you should need for constant lighting over the course of a day, that would be roughly 1500 foot candles. I think it is important to remember that many orchids are sensitive to day length and giving them more or less day length than they are genetically programmed to want may inhibit best growth or blooming. The range from 11.0 - 13.0 day length will satisfy any Cattleya. An indoor light grower can give this exactly. A green house grower in northern latitudes is mostly stuck with long day length in the summer. In the winter, they can give supplemental light to keep to a minimum of 11.0 hours.

The daily light integral for my Cattleyas varies from about 10 to 12 moles/m2 over the course of the year. This is consistent with a few other reports of PAR light exposure for higher light orchids in green house conditions. Ray’s calculations work in an indoor plant room with LED lights.