a led is named after its length and breadth and not in which it is put. these are jst marketing techniques I believe. florescent tubes are named after their dimension in terms of tube diameter in 1/8 of an inch. that is for eg a t8 tube has a diameter of 1"...
Back to your query----
check this thread out- viewtopic.php?f=16&t=245
you can check the following link for your query-http://www.plantedtank.net/forums/10-li ... cency.html
Also check this video out-https://www.youtube.com/watch?v=THuDHw7BLhI
if you ask me how I work for planted tanks.; my reply would be "a mix of knowledge, experience and lumen/sq inch, plants' response and my eyes". I dont rely on any one of the factors completely. these days lumen/sq inch is quite popular. I used to follow it too but there are things to be considered. Alone lumen/sq inch cant possibly determine if a tank is low/high light tank imo.
for example -
Case 1) suppose I am putting in lights of 642nm(near to red) @ 100lm on Chlorophyll b. the relative absorption rate is approx 30%, which we can say energy from 30lm is absorbed and rest is not.
Case 2) now I am putting in lights of 453nm(shade of blue) @ 100lm on Chlorophyll b. the relative absorption rate is approx 80%
, which we can say energy from 80lm is absorbed and rest is not.
so, to get 80lm worth energy absorbed, i will need to put in approx 267lm of 642nm (which we can achive with 100lm of 453nm.
Also, for this scenario(267lm of 642nm), I am only being able to make use of 80 lemens and rest 187lm is wasted which only makes the tank look brighter but not high light.
but if i use 267lm of 453nm you can calculate the relative absorption rate...
amount of light available for photosysthesis is PAR(Approx 400 to 700 nm ) and amount of light suitable for photosynthesis is PUR. there is also a catch, PUR value will keep varying from pigment to pigment for any given lumen count and is nearly impossible to measure, even if measured, impossible to implement. So, for the time being if you are new to planted tanks, follow plain simple lm/sq inch w/o getting into too much details. lights is a huge chapter, you can always experiment and study on, once you have a better grip on the subject...