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Re: [APD] light measurement

Lux/lumens is based on our peak light preception, 555nm, which is "green". 
Plants use 400-700nm which is what the light meters measures.

>but I am interested in 
>the edge cases. Think of me as the lazy guy at work who wants to see how 
>little he has to do before he gets into trouble, but instead of trying 
>to dodge work I'm trying to dodge electricity usage.
Well why not use the bulbs that aquarist use and simply raise the distance from the plant carefully? Then you'd have a distance(something an aquarist can actually measure) and new bulb(which is also something an aquarist can find).
You will not have a unit really, just a min distance using X brand light for a certain plant species under certain conditions(say non limiting nutrients/CO2).
At low light, both Tropica and myself have shown a plant can utilize light gathering more efficently when ample CO2 enrichment is done. A plant can allocate more nitrogen and resources to light gather molecules rather than Rubisco for CO2 uptake when you add higher levels of CO2. This means the plant needs less light energy to survive. You maximize the light use efficency by providing non limiting nutrients and CO2, basically you limit light.........at most levels that aquarist use, we are light limited, which is a good thing. 
I've grown pearl grass easily at 54 w PC lights on 60 gal tank at 23" depth this way. 

>> These run about 550-2000$, not something hobbyist care to indulge in.

>I might spend the money on it just to publish the results here. That 
>would be a great deal of entertainment for me, and useful for others as 
>well. Nothing better than being able to add to the body of good 
>scientific data in the aquarium trade. We need it.

We do(need)? I mean what good is the unit if no one else can afford the method to test it?
Give that some thought. 
 Even if you tested it(which I have the ability to do), variation based of distance from the light of the actual plant in question changes a lot, plants grow closer to the light, so that distance changes, shading from other plants, wood, rock etc also changes through time. 
I also provided a chart of plant's abilities to adapt to low and high light here some years ago(2001 or 2002). Light intensity changes on even an individual plant is quite large.
Consider the light hitting the lower leaves versus the top for example, where is the lower limit of light? Perhaps somewhere in the middle............ 
In order to do the study, you need to ensure good CO2/nutrients. Many aquarist still struggle with that and that is where folk's success is made or broken .........unless they go to extremes. Which goes back to why watts/gal works well...........it's very broad and general, but targets the middle ranges and perhaps a little on the high side lately with more efficent lighting that has come out since PC, HQI, electronic ballast, T5, T8 etc have become much more common.  

>Of course I would probably get an Apogee if I have to fund it with my 
>personal cash, but perhaps there is an aquarium club that is interested 
>in getting a spherical Li-Cor.

A regular Li-Cor light meter is fine, the depth of most aquatic plants is roughly 1 meter or less in most cases. The sphere is useful if you want the light hitting at most every angle etc and cost more. 
Nothing wrong with being curious, testing etc, but few folks will ever buy such a meter to see if their light is low/med/high enough, then if you are at the razor's edge, you set yourself up for problems(light will be limiting if you don't add enough CO2/nutrients etc), while interesting in other aspects, in terms of practical horticulture, the watt/gal rule still stands up pretty well as a middle of the road rough estimation, although I've felt folks have been adding too much light for about the last 7 - 8 years since PC lighting became popular.
The other thing, you can simply try the light set up and see if it works with good CO2, ample nutrients etc. I think many are surprised at how low the light level can go when this is done.
Many use CO2 mainly for high er light set ups arather than low light set ups since folks often say that lower light does not require CO2, but.............every low light tank will definitely gain dramatic results, often better than the higher light tanks, by adding CO2............
Since uptake is slowed/CO2 demand less, the lower light CO2 enriched does very well and the plants do not need trimmed 1-2x a week.
Many folks pre mid late 1990's or so used NO FL's in the 1.5-2w/gal range, they grew most things fairly well and used less CO2 generally speaking. 
More light is not better. Nor is it's measurement an easier matter for aquarist nor even the scientist. Ivo's chart is fairly good IMO, but I have still never had an issue with the watt/gal rule nor has anyone offered a good reason to change. Folks measure NO3, CO2, GH etc, few will ever test light.........
>Jerry Baker

Tom Barr


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