Thanks... I guess this LUX thing was a bit of a waste. I appreciate the links - that is exactly what I was looking for.
I did a quick measurement before replacing the bulbs in my 96w Tek light - LUX before change from the bottom of the tank was 5,500 - with new bulbs, same spot it measured 9,500. The bulbs were 12 month old T5HO's.
Could this be utterly meaningless in reference to how much PAR has diminished over the same period and how much effective light is reaching the plants?
Well, it's no secret that bulbs get dimmer as they age so the LUX readings simply confirm that. It follows that the photon flux that a bulb is able to produce will also diminish as the bulb deteriorates. Since you are comparing the same bulb over time this is logical, but since you have no direct correlation between LUX value and PAR value there is no way to determine absolute values of the change in PAR. In other words you have no way of knowing how many photons are striking 1 square centimeter of a plant surface at coordinates X-Y-Z distance from the centerline of the bulb. You simply know that the number is lower now than it was a year ago.
In this sense it's not completely meaningless but since you have no meaningful numbers as regards PAR you are simply confirming qualitatively what you already know.
Where it really becomes meaningless is if you were for example to compare the LUX values of several bulbs of different spectral characteristics. The LUX meter would give you a higher reading for the bulb containing the higher green content whereas higher photon flux is likely to emanate from the bulb with more blue content simply because a higher frequency wave will deliver more photons to a surface per unit time compared to a lower frequency wave. Also a bulb with a broader spectral curve (having more area under the spectral curve) will generate more photons even though it may not "look" more luminous. So there are many ways in which the LUX meter readings will simply be rubbish and will be no more meaningful than what you can see with your own eyes.
But even this is muddied by the fact that at a given X,Y,Z coordinate in the tank there is reflection, refraction, attenuation and a host of factors that affect exactly how many photons per second actually reach that X,Y,Z point, and only a PAR meter will tell you that. Even with the PAR meter it's necessary to build a 2D or more accurately a 3D photon flux profile across the dimensions of the tank to have an idea of the photosynthetic energy distribution of the tank using a specific bulb set.
As Tom says, it's better to sell the LUX meter to someone who hasn't read these threads and to get the PAR meter.