I usually hang around in the business & marketing, audio and edit areas, but came across this word today that is a great conversation starter. Efficacy. We do a bunch of medical stuff and typically this refers to the ability of a drug-receptor complex to produce a functional response. Who knew it had a definition for lighting!
Main article: Luminous efficacy
In lighting design, "efficacy" refers to the amount of light (luminous flux) produced by a lamp (a light bulb or other light source), usually measured in lumens, as a ratio of the amount of energy consumed to produce it, usually measured in watts. This is not to be confused with efficiency which is always a dimensionless ratio of output divided by input which for lighting relates to the watts of visible energy as a ratio of the energy consumed in watts. The visible energy can be approximated by the area under the Planck curve between 300 nm and 700 nm for a blackbody at the temperature of the filament as a ratio of the total energy under the blackbody curve. Efficacy values for light from a heat source are typically less than two percent.
I had a feeling that's what is was. There's a few 10 cent words there you can use along with C-47 to make the day go faster.