Measuring luminance

Factual Questions
1

I decided to try a compact fluorescent bulb. I got a 23 watt bulb that claimed to be as bright as a 100 incandescent bulb. I put it into my wife’s bedside lamp (it didn’t fit in mine). She immediately claimed (and, to be sure, it was obvious to me) that it was much dimmer than the old bulb.

So I decided to measure it, since there was another lamp in our bedroom identical to my wife’s bedside lamp. I used the method I used in HS of making an oil spot on a piece of paper and finding the position between the lamps where the spot disappeared. The actual distances turned out to be 44 inches to the CF bulb and 110 to the incandescent.

Since the luminance falls off as the square of distance, I conclude that the CF bulb has only 4/25 of the luminance of the incandescent. This seems way off, but I guess judgments by eye are non-linear. Is there anything wrong with my computation? And should I conclude that this was a bad sample or that the whole business of CF bulbs is a fraud?

The Canadian government just announced that after a certain date–2011, IIRC–sales of incandescent bulbs will be banned. This will result in a run on them in 2010 and discarding a lot of old lamps a year later. So that the world can be made safe for SUVs presumably.

2

In photographic parlance that is about 2.5 stops (a stop is a factor of 2). Not an enormous difference but rather more than would be expected. Often these bulbs have a considerable warm-up period so try leaving it on for a while and repeat your test.

3

You know, I’ve heard of cases where sellers “claim” things that aren’t true. Could it be possible that this seller might be that kind of scammer?
P.S. Was it a Lights of America (LOA) brand bulb?

4

It was not Lights of America. I have forgotten the brand name, but it had “green” in it. Should have been brown. It claimed it was 23 watts, whereas most CF bulbs that claim to give the light of 100 watt incandescent lights are 26 watts. But that is hardly enough to explain the disparity. It has made me wary of buying CF bulbs. Although I have a floor lamp with a double toroidal bulb that claims to be 55 watts and seems to give as much light as a 300 watt halogen bulb. It also has a built-in dimmer switch.