My summary of the various information sources would go something like:
Here is a chart of conversion factors from the Sylvania reference:
Larger version here
There is a reasonable risk of running up against Snake-Oil sellers when designing a biological growing lighting system.
PAR refers to light which is photosynthetically useful.
Wikipedia has things to say about it. From there:
This graph purports to show you what matters.
Essentially, any light between 400 and 700 nm is useful and light towards the 700 nm (red) end is somewhat more so.
However, also from there are these two graphs. The lower one is very roughly equivalent to the one above BUT it has a deepish central dip AND the top one below shows you why.
Real lighting people like Sylvania weigh in with mumbo jumbo like this which while it is useful and does give some insights also (for my liking) leans too far towards the enjargoned-mystical. eg measuring photons in units of mols because the quantity can be divided by 6.023 x 1-^23 (Avogadro's number) is about as meaningful as measuring the quantity of sandflies present in units of years because they can be divided by 365.
Allegedly scholarly stuff thereon
Related images with associated web pages
Here is a page related to "grow lights" which was cited by stack exchange question Heat Transfer of Aluminum Plate with LEDs in April 2012 - as this is about heat transfer from LEDs and not electrical engineering [tm] it appears to be off topic,but it does give some light specifics.
This stack exchange question How to tell the lumen output of these LED's asks questions about the lumen output of deep blue LEDs for use in an acquarium light. The OP is confused by the use of mW output rather than lumen for blue spectrum LEDs. As this question is conversion of light in mW to and from lumens for aquarium use and not electrical engineering [tm] it appears to be off topic, but the concepts covered may have some relevance for you when designing "grow lights".
Now that this question has been moved to Gardening & Landscape you'll find quite a few other variably relevant answers here. Looking through all of them is probably useful, but here are some references that seem especially good.
List of many related images - each linked to a relevant website
In this post nigelc referred to a free access paper done in cooperation by the University of Florida & NASA, to produce variable wavelength distribution LED lights for use in studies at the Kennedy Space Centre & elswhere.
Their electronic circuitry is arguably more complex than most people would wish and probably unnecessarily so, but their general observations and results are liable to be useful.
Paper: "Design and fabrication of adjustable red-green-blue LED light arrays for plant research"
In: BMC Plant Biology 2005, 5:17 doi:10.1186/1471-2229-5-17
CC by SA 2 ("free") licence.
PDF version <- best redability. Monochrome.
HTML version - separate figures and tables - makes printing or subsetting of details easier.
Aka added 3 4 5 ....
Excellent discussion on plant growth & lights wavelengths:
now much quoted and now vanished link led to a paper named "What light do plants need".
This appears to be the original paper -
Which regions of the electromagnetic spectrum do plants use to drive photosynthesis?
now on a UCCS page.
Original found! - heliospectra had moved sites.
Also from heliospectra - this is a poster promoting their HS & HB LED grow-lights but gives much useful information.
LED lamp enhances plant growth,alters morphology and modulates photochemistry.
And another heliospectra poster
Variation in responses of Arabidopsis thaliana accessions to low proportions of far red in LED emitted light
Looks useful. 2 pages
Chlorophyll absorption and photosynthetic action spectra
The above is from the useful looking site Plants in Action
Welcome to the electronic edition of Plants in Action, a plant physiology textbook published by the Australian Society of Plant Scientists, New Zealand Society of Plant Biologists, and New Zealand Institute of Agricultural and Horticultural Science. This is a free, peer-reviewed on-line publication resource for plant science teaching and research.
Plants in Action explores the principles underlying plant biology in natural and managed communities. It is prepared for teaching and learning by higher level undergraduates in tertiary institutes. Postgraduate students as well as professional plant scientists will also find much useful source material in this textbook that is richly illustrated with original data.
Illinois.edu 17 pages from a text book - looks useful.
Absorption of Light and Fate of Excitation Energy in Plant Cells
BIOLOGICAL ACTION SPECTRA
- ... Photobiologists studying different systems depend on similar photochemical and photobiological principles, and similar questions arise about vastly different light-controlled biological systems. ... A central question is what wavelengths of light elicit a particular response, and the goal of action spectroscopy is to answer that question. An action spectrum is simply a plot of biological effectiveness as a function of the wavelength of incident light, and it can provide fundamental information about the system under study.
This is brief but possibly very important.
What are full spectrum LED grow-lights?
Brief. Worth reading (even though its a promo for their LEDs).
The following extracts are worth noting:
In order to be able to provide your plants with everything they need, including light for both vegetative and flowering growth, you will need a full spectrum light for indoor farming.
... people have turned to greenhouse LED lighting, but many neglect to still make use of the full spectrum that is necessary for growing plants through a natural life cycle.
Many people make the mistake of only using one kind of LED grow light. That simply can’t provide your plants with what they need for both stages of growth. A full spectrum LED grow light, in comparison, will provide a wide range of blue, red, white, and to top it off, ...
... Many growers are using LED grow lights for indoor farming and one set of blue LED lights. The problem with this is that it lacks the ability to provide your plants with the full range that they are used to having in nature. During an outdoor growth cycle, the kind of light that it receives fluctuates and moves as the days and months progress for indoor gardening. Replicating that inside of your grow room is an important part of successful farming harvests. Leave no stone unturned and no helpful wavelength left out.
Also look useful: