Lumens, PPFD calculations

I know the PPFD formula which is

Input power [W] x efficiency [dimensionless quantity] = radiant power [W]
luminous flux [lm] / LER (luminous efficiency of radiation) [lm/W] = radiant power [W]
radiant power [w] x QER (quantum efficiency of radiation) [µmol/J] = photon flux [µmol/s]
photon flux [µmol/s] / area [m^2] = photon flux density [µmol/s/m^2]

We also now the LER/QER values, say for Cree CXB3590 3500k, 36V etc.

So, it is easy to calculate the PPFD, for a given area.

But there is (IN MY OPINION) a problem.

Luminous flux (lumens), say what you get from the CREE tool for a specific LED, driven at a specific amperage, it says merely how much light is "emitted form a light source".

It does NOT say how much light is actually hitting your plants and it entirely disregards distance from plants to your light (this is major!), whether you have lenses, reflectors etc.

I can say that a CREE CXB3590 36V CD bin "emits" about 10,000lm driven at 1750mA but isn't this entirely meaningless?

Has someone measured, eg. lux, the actual illumination of a surface at a given distance, WITH or without lenses? Maybe someone has a table somewhere? I understand I can do this myself except that I don't have a lux meter right now. (And yes I also understand that lumens, lux etc. is meaningless for plants, but I would just have a more realistic idea...say if I calculate PPFD with the above formula that it actually takes into account lenses and the distance from canopy).

With the above PPFD formula, with the CREE tool, I can calculate that I get 549 PPFD for a 0,7m2 area with my 4 lights each at 37W (largely dimmed back)...but I feel this could be well off, either upwards or downwards (lens increases intensity, distance decreases intensity etc.) So basically, without having actually measured...I am still clueless in regards to PPFD

flexy123 said:

It does NOT say how much light is actually hitting your plants and it entirely disregards distance from plants to your light

Click to expand...

There was an argument about this recently and I tried to point out that the only factor that needs to be taken into consideration in this regard is reflective losses. A reflective enclosure will partially negate losses invoked via the inverse square law. If for instance an enclosure was 100% specular reflective then the inverse square law would have no effect. A lens or reflector will increase intensity at the center point, while hopefully also increasing outer readings. There's no magic involved of course. Any particular method of reflecting light will never increase the PPFD as calculated from manufacturer data. It will always be somewhat less assuming the manufacturer data is accurate. A closer estimation will take into account the transmittance of the lens or the reflectivity of the reflector.

In the end, PPFD is an estimation, especially helpful in comparing the efficacy of various sources against one another. You can calculate PPFD based on accurate spot readings. Nothing too complicated but not as simple as taking a single measurement or even a few measurements and averaging them. Using this simplified example you can see two readings that would average out to 75, however the actual average will be closer to 55.