You should be able to get the basics from the RCA(Burle) Handbook. But the basics are: excite the phosphor assembly with its operational source. Units will be in watts. Measure the out put in lux. Convert Lux to lumens. The external efficiency is by convention in terms of lumens per watt. If you need to turn it into a true external Q.E. Then you will need to convert lumens to watts by dividing by the luminous efficacy. The RCA handbook will give you the equations to do that.