Light Fixture - Input Current

Traditionally it is the input power. light out is equal to this minus the losses that come out as heat instead, which in some designs can be most of the input !  

The conversion between lumens of light into watts of light is a colour dependent  thing, as for the eye brightness response is not spectrally flat - most of us peak in the green at about 500 -700 lumens per watt, and tail off to less than  half this in the blue and the reds, and of course we see next to nothing in the UV and IR.

Traditional filament lamps have a black body response, and generate about 100 lumens per watt, as most of the radiant energy is in the IR not the visible. LEDs can do much better, running quite cool, 

Filaments have a nice resistive construction, electronic drivers for LEDs tend to have an awful power factor.

Mike.

Normal convention is the input power, or sometimes the nominal input power rating of the replaceable lamp it's intended to contain. In neither case did it directly relate to the light output.  There can nevertheless be an efficiency consideration - e.g. with traditional fluorescent fittings an amount of power would be lost in the control gear - e.g. a 38W fitting might draw maybe 42W from the circuit - 38W going to the tube and 4W being lost as heat from the ballast etc. Later, high frequency ballasts could get the same amount of light out of a tube with less power, so despite still having some control gear losses, so the power drawn from the circuit might actually be lower than than lamp's nominal rating.

Sometimes on consumer packaging for 'low energy' replacement lamps you'll see an "watts equivalent" - e.g. this 7W LED produces an equivalent amount of light as a 100W traditional GLS lamp - but that 100W is purely for ease of comparison and doesn't relate to actual power consumption in any way.

Efficiency (or efficacy) in terms of input Watts to light output is still pretty terrible - not as truly awful as old incandescents which might be well below  5% but still most of the power ends up as something else rather than visible light (usually heat).

   - Andy.

AJJewsbury said:

or sometimes the nominal input power rating of the replaceable lamp it's intended to contain

When CFLs came out, it seemed sensible to describe them in terms of the filament lamps they were replacing, but there must be plenty of younger people around now who wouldn't have a clue about a 100 W bulb, so perhaps there is no longer any need to talk about equivalent power.

Even talking about the wattage of an LED lamp can be misleading because they have been slowly getting more efficient.

Why not start by measuring the actual current taken by the lamp using a clamp meter and then estimate the losses,pf?