Naman, I've already discussed light with Vaughn in person and mentioned making such plots for his tank using the PAR meter. However, as time passes, the plants grow, reptile tanks do not.

Unlike the watt/gal/day thing, this is a far more rational approach and measures things through time and space for his specific tanks and plants. Something none of the links or rules can muster nor ever will, which is why botantist use PAR/PPFD etc. Things change as does uptake of both nutrients, CO2, flow rates through time and space.

PPFD also changes throughout the day and light cycles, plants do grow and change in space after all. The above method does not measure PPFD throughout a day nor at all for that matter. PAR is just the total PPFD integrated into one unit that plants use. They seem to adapt pretty well to most PFFD. Since we know the light intensity, we can easily measure that throughout the day. But taking measurements "where" and "when" specifically and for what plant species?

Do we assume they are all the same?
Is W/m^2 the same as PAR? I do not think it is, but it does illustrate the inverse square law through space and from the point source which is what Vaughn quickly saw when he used the meter and I showed him various points around the tank at Kyle's place.

However, w/gal, or w/gal/day are poor predictors of uptake. I discussed the issues with light measurement in depth in the Barr report article I wrote several years ago about such issues. So called watt/gal rules, whether they use watts per day per gallon or watts/gal based on a 10 hour day conversion charts are very rough estimators and offer little insight into the demands of the plants and how they influence uptake for CO2 and for nutrients. Which is why researchers and aquarist should measure things and not just claim them to be so and "better".

That's why Vaughn came over and borrowed the meter and I gave him some thought questions to ponder as he uses it for a few weeks. He will learn far more this way

The only thing the Watt/gal/day thing does is allow the Watt/gal rule to use more light for less time or a little less light for longer time vs the same intensity all day long for 10 hours. Total intensity is still the same over time from the bulbs.

However, bulbs are all different(Age, distance from the surface, clean or not, water spray etc), as are all tanks, as are all plants, and nutrient status, as are all ballast and bulb types(T5, T8, T12, HQI, MH's, LED)

All these errors are accounted for by measuring light in the tank at various points through space and a time in situ. Which if you sit down and think about it, it a lot more rational than w/gal or w/gal/day.

Which is also why I suggest this.
I also measure CO2 very critically now with a CO2 data logger, so that + light in situ tells you far more about nutrient uptake and behavoir.

Regards,
Tom Barr