Just got a new magazine from my local Hydro Store. Its called "Garden Culture" it is issue #5 of 2015.
There is an article by Theo Tekstra and he makes an excellent point regarding LED's
Here are some excerpts from this article I find will add to the conversations about LED's
"PPF VS PPFD
Let's look at the output specifications first, forgetting lumens, because we're using grow lights. Lumens are for humans - not plants. So, what defines the total output of a fixture is the total output of photons in the PAR region (400-700 nm), measured in micromoles per second (photons per second). This is also called the photosynthetic photon flux, or PPF.
Light intensity on a PAR spectrum is called PPFD. Now there is only one "D" difference from PPF, but that makes a big difference. PPFD is intensity, measured in micromoles per second, per meter squared (µmol s-1 m-2)! So, remember: PPF is total output. PPFD is intensity at a certain spot, and depends on where you measure it under the fixture.
If you have a lamp with a PPF of 1000 µmol s-1, and you spread this light over two square meters, you would get an average of 500µmol s-1 m-2 intensity on that surface (total light divided by surface) Its like Lumen and Lux, but for PAR spectrum and measured in photons. Lumens is the total output of a lamp, Lux is the intensity at a certain distance from that lamp, with the light spread over a certain surface (Lux is Lumens per square meter).
Measuring Total Output of a Lamp
To measure the total output of a lamp or fixture, we use an integrating sphere or photogoniometer. These (calibrated!) instruments integrate all of the light, and give you an accurate measurement of the total output of a lamp or fixture. Measuring the light under the fixture on a grid, and integrating the values is very accurate, specifically with a low number of measurements on a small surface.
Now lets take an HPS lamp as an example. The double ended HPS lamp does let's say 2000 µmol out of the reflector in total. So spread over a 2 square meter surface I would get about 1000 µmol per second, per square meter intensity. Easy, right? But now I hold a light meter about 40cm away from the lamp, and I measure more than 4000 µmol s-1 m-2. How is that possible? That is twice the PPF of the lamp?
No, it isn't. 2000 µmol s-1 concentrated over just half a square meter gives you that intensity (ppf/surface) So a measurement under a lamp at a certain distance, specifically if it is a deep lamp with a concentrated beam (as in lensed LED's) say nothing (at all!) about that light or fixture.
PPFD at 30CM
Now look at the (Chinese) LED specifications. Some actually say PPFD of x at y cm from the fixture (which you know now is absolutely rubbish information), but some even go as far as to call this PPF (in a footnote they say @ 30cm from fixture)
So with my 270W plasma light I measure 3000 µmol s-1 m-2 close to the glass, so it replaces 1.5 1000W HPS fixture, right? Wrong. You fell for the hype again.
How Do I Compare?
You need about as much LED light as you need HPS light to get the same yields. As LED is not twice as efficient as HPS (equal to, or at most a little better in a limited spectrum) these fixtures do not replace a 1000W HPS lamp at just 40% of the power. When you want to replace 1000W HPS with LED fixtures, you need 1000W LED. Then look at the difference in price.
LED fixture manufacturers that specify the output by the PPFD at a distance don't know anything about lighting, or do know, but want to fool you. Either way, you shouldn't trust them. A 400W LED fixture uses 60% less energy than a 1000W HID lamp. So does a 400W CFL or a 400W incandescent lamp. 60% less energy? Yes. But I also promise you 60% less yield in a high intensity lighting production room.
Don't just go for the hype, keep thinking!"