What sort of light is good for plants. (LED users come hither)

I wanted to discover the spectrum and intensity requirement that will be generally useful to indoor-grown plants so I’ve been doing a lot of reading lately and I thought I’d share what I found out. I am by no means an expert on the subject so If I have made an error let me know.

Table of Contents:
How do we perceive light?
How do we measure light output?
Photosynthesis: plant structure
Photosynthesis: plant structure, photosystem II
Photosynthesis: plant structure, photosystem I
Photosynthesis: electron transport chain
Photosynthesis: photodamage
Photosynthesis: blue light responses
Photosynthesis: circadian clock
What pigments to plants need stimulated?
What wavelengths do those pigments use?
How much light do plants need?


I will start with: What is light?

Light is electromagnetic radiation. Electromagnetic radiation includes everything from gamma radiation to visible light to radio waves. It exhibits properties best explained by its wave-like characteristics such as interference, while also demonstrating properties best explained by its particle-like properties such as the photoelectric effect. Thus, it can be interpreted as either a wave or a particle.

Treating light like a particle, a light source emits many light particles called photons. These are distinguishable by their energy. When treating light like a wave, this is referred to as the wavelength, and is visible to us as a difference in color. Photons of the same energy have the same wavelength and the same color. Though they may be from different sources, they are still the same.

How do we perceive light?

Humans have two kinds of light receptors in the eye: rods and cones. Cones are less sensitive to light and thus require more light to be activated. Cones allow us to see in color because there are actually three types of cones designated S(hort) M(edium) and L(ong), each having different sensitivities which peak at light wavelengths of 420–440 nm, 534–545 nm, and 564–580 nm respectively. These correspond to the 3 colors (blue green and red) which, when combined, produce white light. Vision in light strong enough to activate the cones is called photopic vision or daytime vision.

Rods are able to function in little to no light and thus become dominant in those situations: a la night time. Rods are most sensitive to wavelengths of light about 498 nm (green-blue), and are completely insensitive to wavelengths longer than about 640 nm (red). Vision in darkness is called scotopic vision. There is a third type of vision called mesopic vision where the photopic and scotopic vision are both active and occurs in dimly lit environments.

How do we measure light output?

There are many measurements for light. I will try to cover the ones you will see most often.

Black-body radiator
A black body is a conceptual object that absorbs all electromagnetic radiation that falls on it. Because no light is reflected or transmitted, the object appears black when it is cold. However, a black body emits a temperature-dependent spectrum of light. It is not an indication of the spectral components but rather the sum. Black-body temperature, which is measured in degrees Kelvin, is used to describe the color of a light. For example, the sun approximates a blackbody of 5777K. But, a light source doesn't even need to approximate a black-body radiator to achieve the same perceived color and I haven't seen a single artificial light source which even comes close.

Radiant flux
Radiant flux is the measure of total power of electromagnetic radiation. This number is usually given for light sources near the ends or outside the visible spectrum (infrared and ultraviolet). Radiant flux is measured in watts (energy/time).

(mili)Candela
The candela (cd) or millicandela (mcd) is a measure of light in a particular direction, weighted by the luminosity function for photopic vision. The measurement is weighted according to the average sensitivity for human cones (photopic vision) which is visible light having a wavelength of 555nm. If you have seen the newer fire trucks or warning street signs, which are a greenish shade of yellow, then you have seen 555nm light. The luminosity function is a bell-curve so any other wavelength will require higher actual intensity to appear similar in intensity.

Lumen
The lumen is candela per steradian. A simple way to envision a steradian is to think of how one would measure of the angle of a cone from a point. If the point is the center of a bulb and the bulb is perfectly spherical then the surface area of the bulb which the base of that cone covers is a steradian. A globe has 4pi or 12.6 steradians so a candle, which generally has 1 candela of intensity, emits 12.6 lumens.

Lux
The lux is a measure of lumens per square meter. Lux is important because the intensity of a light source diminishes by 4 times (2 squared) the distance from a surface. If you have 1 lux at 1 meter from the source then at two meters you will only have a quarter lux. Another unit for this which is used sometimes in the US is called footcandles which can be approximated to 10 lux.

What about PAR PUR?

Photosynthetically available radiation (PAR) and photosynthetically usable radiation (PUR) is a weighted system (as candela/lumen are) of measuring light. It seems o.k. until you realize that a bright red light could have a high PAR rating but still not be sufficient for a plant to grow (properly). This measurement is about intensity relative to plants which is better than lumens but still does not speak to balance.

What wavelengths of light do plants need?

First, we must understand how photosynthesis works.

Plant Structure

Inside of plants there are mesophyll cells which contain chloroplasts. Inside each chloroplast there are thylakoids. On the thylakoids (stacked groups called grana) solar energy is absorbed by several structures consisting of both proteins and light-harvesting pigments. These protein-pigment structures, or complexes, are arranged so that they work like a satellite dish, or antenna, funneling their energy to a location where it can then be used to perform a function. In higher plants, this antenna is composed of two classes of protein-pigment complexes: those of the inner antenna, which contain chlorophyll a and beta-carotene, and those of the outer antenna, which contain chlorophyll a/b. There are two inner antennas: photosystem I (PSI) and photosystem II (PSII). PSI includes the PSI inner antenna as well as four additional exclusive antennas. PSII includes the PSII inner antenna as well as the major outer antenna called light-harvesting complex (LHC) II and three additional minor outer antennas called CP29, CP26, and CP24. In total, there are 10 distinct LHC proteins in higher plants.

Plant Structure (continued)

On the surface of negatively stained chloroplast thylakoid membranes, a repeating motif can be seen. Below is an electron micrograph - with layouts superimposed - showing the positions on such membranes of the PSII core complex, the LHCII trimers, CP24, CP26, and CP29. Also below we see PSI though; the last frame shows it upside down relative to the molecular image below.

Photosystem II:

The PSII protein-pigment complex has a large number of subunits. The core complex consists of the reaction center, called P680, and two proteins called D1 and D2. P680 consists of a specially bonded pair (dimer) of chlorophyll a with maximum absorption at 680nm. The rest of the core complex contains four chlorophyll molecules. Also within the PSII inner antenna are two subunits called CP43 and CP47. These contain 14 and 12 chlorophyll a, respectively. In total there are around 35 chlorophyll molecules in the PSII inner antenna.

Photosystem II: LHCII

Near the PSII core complex are two LHCII antenna complexes, each of which is a specially bonded triplet (trimer). Under normal conditions, each LHCII subunit contains seven chlorophyll a and five chlorophyll b. Plants can adapt the number of LHCII to the light available so in some complete PSII representations there may be only one or more than two per PSII core. Additionally, the chlorophyll a/b content can be adjusted as needed. Since three subunits make a single LHCII trimer, there are 36 chlorophylls in each LHCII antenna (shown in image).

Photosystem II: CP29/CP26/ CP24

CP29 contains eight chlorophyll molecules (also shown). CP26 contains at least 14 chlorophyll molecules and CP24 contains 10 chlorophyll molecules.

Photosystem I:

The PSI protein-pigment complex has fewer subunits than PSII. The core complex is similar to PSII in that its major components are the reaction center and two proteins, though they have different names; P700 and PSI-A and PSI-B, respectively. Like P680 of PSII, P700 consists of a dimer of chlorophyll a but has a maximum absorption at 700 nm instead. The PSI supercomplex contains 168 chlorophyll molecules. About 20 chlorophylls are positioned so as to connect the four proteins assembled in a half-moon shape on one side of the core that make up LHCI to the core. You may see trimeric representations of PSI which is an accurate depiction when dealing with bacteria but, in higher plants, it is actually monomeric (a single unit).

Electron Transport Chain:

The electron transport chain describes a series of redox (reduction and oxidation) events. Some portion of the chain becomes excited and oxidized then subsequently reduced. It begins in PSII. When P680 is excited, either directly by light or indirectly by a nearby pigment, it loses an electron. The PSII reaction center then separates a water molecule into its component gasses in an area called the oxygen evolving complex (OEC) which gives off oxygen as a waste product. An electron obtained from the OEC is passed through a portion of the D1 protein called tyrosine to P680 thereby reducing it. P680 passes an electron to pheophytin (It is worth noting that pheophytin is simply a chlorophyll without the magnesium center). Pheophytin passes the electrons by way of one plastoquinone (PQa) to another (PQb). Once PQb accumulates two electrons it leaves PSII to pass its electrons on to the cytochrome b6f complex. Cytochrome b6f then passes the electrons via plastocyanin (PC) to PSI where P700, excited by light or a nearby pigment, is reduced. In addition to the proteins used to transport electrons, cytochrome b6f contains chlorophylls of unknown function. This is probably related to the ATP production which occurs during the process between pheophytin and PC. From P700, the electrons are used to generate NADPH in the Calvin cycle. Thus, light-induced electron transport generates ATP and NADPH which are high-energy compounds but they are not suitable for long-term use like building plant biomass, or storing in seeds/tubers/fruits. For this, most plants produce sugars or more complex carbohydrates such as starches (although some plants produce large quantities of proteins or oils instead) using some of the materials from the Calvin cycle.

Photodamage

PSII Photoinhibition:

The exact circumstances which cause photodamage are not known but as long as plants are exposed to it, there is a chance for light to damage the photoreceptors. In addition, there is a threshold of intensity for photodamage. This damage nearly always occurs in PSII and nearly always in the D1 protein. Plants have a mechanism to compensate for this damage whereby the D1 protein is removed from PSII and replaced with a new one. As long as the rate of photodamage is low, the repair cycle can keep up with it and the system operates at near maximum efficiency but, if the rate of damage exceeds the rate of repair, the damaged PSII accumulate and the plant exhibits what is called photoinhibition. In addition to the repair mechanism, PSII has several prevention mechanisms. One of these mechanisms enable the plant to move the chloroplasts to reduce the amount of light they take in.

Cyclic Electron Flow:

Another photodamage prevention mechanism is cyclic electron flow in which the PSII OEC shuts down. The excited P680 is reduced by a beta-carotene instead of the normal process. The beta-carotene is reduced by cytochrome b559 (part of the PSII protein) which receives electrons from PQ.

Xanthophyll Cycle:

Another photodamage prevention mechanism is the xanthophyll cycle. The major pigments in plants beside chlorophylls are carotenoids. Carotenoids are divided into two classes; xanthophylls and carotenes. Under conditions of excess light, violaxanthin (a xanthophyll) is converted into zeaxanthin (also a xanthophyll) by the unneeded excitation energy of P680. This mechanism is called energy-dependent thermal dissipation (qE).

Nonphotochemical Quenching:

The last mechanism is through LHCII, which has the intrinsic capacity to dissipate excess light energy as heat in a process termed nonphotochemical quenching (NPQ). The Carotenoids are also involved in this process.

PSI Photoinhibition:

PSI is much less susceptible to photodamage but is more easily damaged by other means such as low temperatures.

Blue-Light Responses

Plants have two major groups of pigments; chlorophylls and carotenoids. Carotenoids function as both antenna pigments and repair mechanisms. The primary pigments are chlorophylls which come in two types: a and b. Chlorophyll a is the most common pigment and chlorophyll b the second-most common with a ratio of three to one in most plants. Chlorophylls have modified absorption peaks depending on their location. Take, for example, P680 and P700 which are both chlorophyll a but have a peak absorption difference of 20 nm. All chlorophylls absorb strongly in both the red and the blue regions of the electromagnetic spectrum but they perform best under red light. Although plants really only need red light for the electron transport chain to function, there are several plant responses induced by exposure to blue light which are necessary for plants to grow properly.

Stomatal Opening:

In the Calvin cycle, the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is used in the initial reaction with either oxygen or CO2. The former leads to a process called photorespiration and the latter to carbon fixation. The probability with which RuBisCO reacts with oxygen instead of CO2 depends on the availability of the two compounds. CO2 is preferred (about 80 to 1) but, as there is much more oxygen in Earth’s atmosphere, photorespiration occurs at about 1/3 the rate of carbon fixation. CO2 can only enter (and oxygen exit) through openings under the leaves called stomata. The openings are constricted or expanded by two guard cells which have a greater reaction to blue light than red (though the red and blue effects are additive). The degree of stomatal opening is greatest when exposed to UV light around 280 nm but, UV-B suppresses photosynthesis and damages plant DNA. The same effect can be obtained with a still relatively small amount of 430-460 nm light (about 1 micromole/m2/s).

Leaf Positioning / Phototropism / Chloroplast Movement:

The blue light photoreceptors, phototropins 1 and 2 (phot1 and phot2) and the cryptochromes, fine-tune the photosynthetic status of the plant by controlling several important adaptive processes in response to environmental light variations. These processes include stem and leaf positioning, leaf flattening, phototropism, and chloroplast movement. These effects are most pronounced at wavelengths between 440 and 460 nm. There is also an additive effect from red light in the range of 700 to 720 nm.

Growth Inhibition:

I know growth inhibition sounds bad but it isn’t. A plant grows taller to receive better light. It is advantageous to be shorter because that plant can spend more energy growing larger leaves and producing flowers, seeds, and roots. Hypocotyl length can be sufficiently inhibited with 35-40 micromole/m2/s of blue light. In the case of higher plants, a signal from phytochrome is also necessary for full activity of the cryptochromes.

Circadian Clock

Circadian clocks are universal mechanisms for sensing and responding to the environment. In addition to regulating daily activities, photoreceptors linked with circadian clocks are also involved in the seasonal regulation of processes such as flowering. Hence, many plants flower in response to seasonal changes in day length (actually night length). Those that respond to a long night period are referred to as short day plants (SDPs). When the night period reaches a critical length they begin flowering. Other plants respond to a short night period and are called long day plants (LDPs). When the night period reaches critical brevity they begin flowering. Still others are day neutral where the length does not provoke a response. In a further subdivision, obligate plants absolutely require a long or short enough night before flowering, whereas facultative plants are only more likely to flower under the appropriate light conditions, and will eventually flower regardless of night length. Some species also must pass through a "juvenile" period during which they cannot be induced to flower. Some examples are Poinsettias: obligate SDP, Hemp: facultative SDP, Clover: obligate LDP, and Capsicum: facultative LDP.

Phytochrome Response

In addition to time, the color of light also matters. An interrupted night period by red light (660nm) can be undone by subsequent far red light (730nm). Thus, we can deduce that this is due to yet another set of photoreceptors called phytochromes. Phytochromes are a family of plant photoreceptors which can exist in two interconvertable forms (Pr and Pfr). When Pr is exposed to red light it is converted to Pfr and when Pfr absorbs far red light it is converted to Pr though, the rate of Pfr to Pr is slower than the rate of Pr to Pfr. In addition, Pfr can also spontaneously revert to Pr over time in the dark by a process called dark reversion. Sunlight has a r-fr light ratio of 1.2, light under a canopy of leaves has a r-fr light ratio of 0.13, and light under 5 mm of soil has a r-fr light ratio of 0.88. In this way, using the ratio of Pr to Pfr, a plant can detect when they are shaded or exposed to full sunlight. Plants adapted for growth in full sun will display greater stem elongation when they are transferred to shade. They also develop smaller leaves and show less branching. Seeds of certain plants show a phytochrome response as well. Many small seeds with low amounts of storage reserves (such as lettuce) show such a red light requirement for germination. These seeds can be induced to germinate by exposing them to red light before a dark period. Far red light has the opposite effect - inhibiting germination. In this way we can see how it is possible to use r-fr ratios of light to our advantage.

What pigments to plants need stimulated?

Chlorophyll a is the basic building block of all chlorophyll. Chlorophyll a’ (the P700 dimer) and chlorophyll b are both made from it. Similarly, chlorophyll b can be reduced back to chlorophyll a. As a whole, PSI responds to a longer wavelength of red than PSII because PSII contains more chlorophyll b. Efficient electron transfer reactions require balanced excitation energy distribution between PSII and PSI so we need light that covers both ranges. Usually the overall chlorophyll ratio is 3 to 1 but experiments have shown that the ratio of each is regulated by the plant for optimal absorption and adjusted as needed so that with light saturation this ratio can increase to as much as 6 to 1. Still, if we use only light too far into the infrared, no photosynthesis takes place because PSII is not stimulated and if we use light too far into the orange then no photosynthesis takes place because PSI is not stimulated. The latter scenario also causes photodamage to PSII, as it cannot pass on its electrons, and therefore photoinhibition.

What wavelengths do those pigments use?

So, the first pigments we need to stimulate are chlorophyll a followed closely by chlorophyll b. For these we use red light because it is most effective despite most graphs wrongly showing the blue peak taller. Second is a general “blue” light to stimulate proper formation of the plant and the assortment of pigments which all respond in that overlapping range. The exact wavelength does not really need to be specific as the activated pigments have several similarly effective peaks between 440 nm and 460 nm. Third is a possible far-red for phytochrome action, if we desire. Lastly, we might require some general light. We do not need anything to specifically stimulate any carotenoids because they a) are a minor antenna pigment and b) need to be available for protecting PSII from photoinhibition. We also do not need anything to stimulate other pigments such as those found in algae and bacteria.

Chlorophyll a shows maximum absorption “within the living” (in vivo) at 670 nm. You might see this peak indicated at 662 nm but that is actually the absorption “within the glass” (in vitro). Chlorophyll b shows maximum absorption in vivo at 650 nm (but at 644 nm in vitro). Pr shows maximum absorption in vivo at 667 nm and Pfr at 730 nm. There are several reasons for the absorption changes in vitro. The cells can be damaged during extraction, components of the leaf which bind to the pigments naturally can alter the absorption, and the leaf itself can diffuse and alter the spectral content of the light.

How much light do plants need?

According to PAR, a daily average irradiance of 26 moles/m2/day (which equates to about 600 micromoles/m2/s for 12 hours) will effectively grow most species of higher plants. For comparison, the average annual irradiance is about 26 moles/m2/day in Washington, DC, 62 moles/m2/day in the summer in Phoenix, AZ, and 8 moles/m2/day in the winter in Madison, WI.

Moles are hard to work with when dealing with artificial light (usually we get something like lumens or radiant flux), so I’m going to use a roundabout way to guesstimate the requirement based on the sun’s irradiance (in actuality moles need to be converted separately for each wavelength but that would be a pain in the ass). Tucson receives about 1100 watts per square meter in the summer which is around 65 moles. Because 26 (our target irradiation in moles) is about 40% of 65 then 440 (40% of 1100) W/m2 should suffice. If we use this number as a basis for driving photosynthesis then we need to split it between chlorophyll a (670 nm) and b (650 nm) light in a ratio between 3 to 1 and 6 to 1. The first ratio being 330 W/m2 of 670 nm light to 110 W/m2 of 650 nm light, and the second being about 377 W/m2 of 670 nm light to 63W/m2 of 650 nm light. The amount of blue light required varies between species from 8% to 20% of the amount of red. Thus, somewhere between 35 W/m2 and 88 W/m2 of 440-460 nm light is required for proper blue light responses. Finally for the far red responses, you will want to adjust the ratio of red to far red. I won’t go into detail other than to say it would probably be easier to wire some of the red lighting to turn off while the far red lighting was active.


When should I provide light?

There are other effects of altering the day length than just those to flowering, which I talked about above. For some species, it is not necessary to grow plants under alternative light dark periods. Radish, cucumber and wheat, for example, can be grown quite well under continuous irradiation. However I’m unsure about other plants. I once read of a guy successfully growing a hemp plant in a 5 gallon water bottle with a single 5 W red LED. If you choose to experiment with this idea then be sure to know what you are getting into.

Sources

General Photosynthesis:
http://academic.kellogg.edu/herbrandsonc/bio111/metabolism.htm#tissuepatterns
http://dwb.unl.edu/Teacher/NSF/C11/C11Links/photoscience.la.asu.edu/photosyn/education/photointro.html
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf
http://plantphys.info/plant_physiology/
http://plantsinmotion.bio.indiana.edu/plantmotion/starthere.html
http://preuniversity.grkraj.org/html/7_PHOTOSYNTHESIS.htm
http://www.cartage.org.lb/en/themes/sciences/BotanicalSciences/Photosynthesis/Photosynthesis/Photosynthesis.htm
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A4434
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=stryer&part=A2679
http://vcell.ndsu.edu/animations/photosystemII/movie-flash.htm
Book: Molecular Mechanisms of Photosynthesis ISBN 978-0-632-04321-7
Wikipedia
Various forums

PSII:
http://deepblue.lib.umich.edu/bitstream/2027.42/31112/1/0000008.pdf
http://photosynthesis.peterhorton.eu/research/lightharvesting.aspx
http://www.bio.ic.ac.uk/Research/barber/psIIimages/PSII.html
http://www.bio.ic.ac.uk/Research/barber/PSIIreview/greenplants2_electron.html
http://www.bio.ic.ac.uk/Research/barber/PSIIreview/greenplants9_SubunitLhcb.html
http://www.jbc.org/content/273/27/17154.full.pdf+html
http://www.lbl.gov/Science-Articles/Archive/PBD-CP29.html
http://www.ncbi.nlm.nih.gov/pubmed/12044165
http://www.plantcell.org/cgi/reprint/20/4/1012.pdf
http://www.plantphysiol.org/cgi/reprint/80/2/429.pdf
http://www.pnas.org/content/104/31/12767/F1.expansion.html
http://www.publish.csiro.au/?act=view_file&file_id=SA0403616.pdf
http://www.uniprot.org/uniprot/Q9XF89

PSI:
http://www.bio.ic.ac.uk/Research/barber/psIIimages/PSI.html
http://www.esrf.eu/news/spotlight/spotlight45/spotlight45psI/
http://www.esrf.eu/UsersAndScience/Publications/Highlights/2003/MX/MX02
http://www.ks.uiuc.edu/Research/psres/plantps1.html
http://www.nature.com/nature/journal/v426/n6967/full/nature02200.html
http://www.queenmaryphotosynthesis.org/nield/LHCIPSIchlamyJBC03/ps1_green_alga_fig3.html

Phytochromes:
http://plantphys.info/plant_physiology/photoperiodism.shtml
http://www.escholarship.org/editions/view?docId=ft796nb4n2&doc.view=content&chunk.id=d0e21651&toc.depth=1&anchor.id=0&brand=ucpress
http://www.mcdb.ucla.edu/Research/Tobin/lab/Research/research.html
http://www.mobot.org/jwcross/duckweed/phytochrome.htm
http://www.plantcell.org/cgi/reprint/18/1/4.pdf
http://www.plantcell.org/cgi/reprint/17/7/1941.pdf
http://www.plantphysiol.org/cgi/rapidpdf/pp.109.150441v1.pdf
http://www.plantphysiol.org/cgi/reprint/63/4/680.pdf
http://www.plantphysiol.org/cgi/reprint/98/4/1511.pdf
http://www.plantphysiol.org/cgi/reprint/152/1/177.pdf

Phototprotection:
http://protein.bio.msu.ru/biokhimiya/contents/v69/full/69111592.html
http://www.biomedcentral.com/content/pdf/1471-2229-9-71.pdf
http://www.plantcell.org/cgi/reprint/13/5/1193.pdf
http://www.plantphysiol.org/cgi/reprint/92/4/1196.pdf
http://www.plantphysiol.org/cgi/reprint/120/3/727.pdf
http://www.plantphysiol.org/cgi/reprint/132/1/352.pdf
http://www.plantphysiol.org/cgi/reprint/149/2/1061.pdf
http://www.life.illinois.edu/govindjee/papers/CarFin1.html

Blue Light Responses:
http://www.plantphysiol.org/cgi/reprint/68/5/1170.pdf
http://www.plantphysiol.org/cgi/reprint/102/4/1219.pdf
http://www.plantphysiol.org/cgi/reprint/122/1/99.pdf
http://www.plantphysiol.org/cgi/reprint/129/2/774.pdf
http://www.plantphysiol.org/cgi/rapidpdf/pp.109.149526v1.pdf
http://www.plantphysiol.org/cgi/rapidpdf/pp.109.150441v1.pdf

PAR:
http://www.plantphysiol.org/cgi/reprint/46/1/1.pdf
http://www.plantphysiol.org/cgi/reprint/46/4/568.pdf
http://www.plantphysiol.org/cgi/reprint/49/3/421.pdf
http://www.plantphysiol.org/cgi/reprint/125/1/85.pdf
http://www.pnas.org/content/98/24/14168.full.pdf+html
http://www.plantphysiol.org/cgi/reprint/46/2/247.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1302477/pdf/11751324.pdf

Solar Irradiance:
http://ag.arizona.edu/CEAC/research/archive/solar-radiation_kania.pdf

Electromagnetic Spectrum:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html

For veg I use fluoros and 200W CFL made for vegging plants with the best PAR for plants...and for flower i use HPS with 40% more "blue"...I've heard people using MH bulbs all the ways through and also HPS all the way through...reason I use the CFL's is my plants veg like crazy under them + less costly than HPS/MH...but i flowered under CFL's and uder HPS and there is a significant diffrence in size of budz and resin production...at least thats what I experienced.

If u have the opportunity, try to mix say 2 x HPS + 1 x MH and u get the whole spectra....but im just guessing I really dont know if thats the best.

//Keep it real! Peace, CaL!