LED related links
 

DavidCampen, Ray, and I were talking about PAR meters in another thread. After that, I was looking into more info about comparison of different light sources. I came across a few interesting links (most of them were found on other non-orchid "plant" forums). A lot of people here aren't looking at these other forums, so I decided to post this info here.

Basic information about light
First, a lot of people are mislead to think lumen, lux, footcandles are important. These measurements are only important for human eyes, but they are not the best metrics for plants growth. This document explains pretty well:
http://www.gpnmag.com/sites/default/...13%20FINAL.pdf
I was also confused until David pointed out the problems in my understanding.

Comparison of Efficacy
The following document compares the long-term cost of LED vs HPS lighting. There are lots of info here. For a given amount of light intensity (relevant to plants), current LED can be slightly more efficient than HPS, but the initial cost of LED fixture is still higher. So over 10 years or so, the total cost of LED rigs can be fairly competitive (but HPS is still cheaper). A lot of interesting info here:

Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures
(updated with a newer, published version, now it contains the comparison to T8). The balance will change quickly with increasing cost of electricity, and changes in LED technology.

This youtube video (Dec 2014) tests 6 LED fixtures, and explain some methods to compare efficiency:
https://youtu.be/24M2Ch0w6qo

Here David compares fluorescent vs other light source:
http://www.orchidboard.com/community...tml#post554104

Effect of light spectra on plants
Here is a poster comparing different types of LED to plant growth:
http://cpl.usu.edu/files/publication...b__2576523.pdf
I'm surprised how well Cool & Nuetral White LED in the Fig. 8. The effect of light frequency on stem elongation (photomorphogenesis) could be species specific, and it probably won't apply to orchids.

light measurement:
http://www.orchidboard.com/community...par-meter.html

A comparison of quantum (PAR) sensors:
Apogee SQ‐120 vs. Kipp & Zonen PQS 1 vs. LI‐COR LI‐190 vs. Skye SKP 215

Accuracy of Quantum (PPF and YPF) sensors:
Spectroradiometer vs Li-Cor LI_190SB (PPF) or Sky SKP 210 (YPF)

Conversion factor between lux (or foot-candle) and PPFD (PPF/Klux column):
http://cpl.usu.edu/files/publication...b__6740181.pdf
Similar info (but limited types of light source):
Conversion - PPF to Lux

Blue drift in old PAR quantum sensor

Longevity
This link talks about the design of LED fixtures (relevant for DIY). As you increase the driving current, efficiency and longevity of LED decrease. But you need more LEDs to get a certain amount of light (higher initial cost) with low driving current. So one needs to balance these factors.
Driving current vs longevity
A bit more technical details

The above article has a link to this:
Cree Product Characterization Tool,
which calculate the lumen, lumen/W, etc. for different Cree LEDs driven at different amount of current and different temperature.

"T8" LED bulbs

Pros and cons of different types of LED bulbs which can be fitted to T8 florescent light fixtures (e.g. Direct fit which uses florescent ballast vs ballast-bypass):
http://www.premierltg.com/should-you...8-led-tubes-2/
http://www.ledsmagazine.com/articles...plexities.html
http://www.ledsmagazine.com/articles...led-lamps.html
http://www.ledsmagazine.com/articles...l-success.html


Test results of several T8 LED bulbs:
http://apps1.eere.energy.gov/buildin...iper_21_t8.pdf
Many of them are not efficient, but one based on COB can achieve 143lm/W, which is excellent.

I updated the link to the 2nd pdf in the original post. Now they include more LED models, and T8. Table 3 is something what most people care.

There are fair amount of variation among LED fixture. The PAR photon efficiency (basically number of photosynthetically useful photons emitted per given amount of electricity watt used) is between 0.89-1.7 micro-mol/J. The consumer-grade LED grow light (California Lightworks, Apache, Blackdog etc) are around 1 micro-mol/J, which is pretty close to HPS.

Florescent light (T8) is 0.84 micro-mol/J, so typical LED is about 19% more efficient. Efficiency of T8 is probably similar to T5HO, but slightly less than T5 normal output. So the comparison applies to T5HO, too.

Efficiency of Induction light (iGrow) was a bit lower than I expected.

Table 1 is also interesting, it compares individual LED (blue vs red vs cool white). Blue is has the highest photon efficiency, but when you consider red is about 20% more efficient than blue in terms of photosynthesis, then red LED is better. Firgure 4 is also interesting in comparing warm white vs cool white. I've been using warm white because their spectra seem to be better (more red). But ww is quite a bit lower than cw in terms of photon efficiency. But if you calculate efficiency in terms of YPF, I expect the difference to become much smaller. I wonder how well newer COB (chip-on-board) type LED will compete.

I added links to some data measuring the efficiency of T8 LED bulbs. One of them (Sunritek ST-PT12) has amazingly high efficiency. It is almost as good as DIY LEDs.