Our Li-Cor Radiometer / Photometer (pictured below) and its special Quantum sensor, measures PAR very accurately. It allows us to get a much more meaningful measurement of how much plant-usable light that a grow light is giving out. However, even PAR measurement is not perfect! The above chart shows the McCree curve spectrum which is the amount of photosynthesis response a plant has over the light spectrum. But we can accept that PAR is a far superior way of measuring plant usable light compared to measuring lumens.
A healthy plant leaf can withstand quite a lot of light but only for a short period of time. The ppfd could be 1500-2000 umol/m2/s for perhaps an hour or 2 (like the midday sun might provide at the equator) and no damage would occur. Some strains of plant will yield their maximum when given around 1500 umol/m2/s. Over this and there is risk of leaf bleaching and burn. Most high power grow lights get HOT. If you have a plant close enough to a hydroponics grow light that it is getting more than 1500 umol/m2/s then you will possibly find that the leaf temperature is too high for maximum photosynthesis anyway!
The measurement of the amount of light that we give to our plants is called PAR (Photo-synthetically Active Radiation). This is a measurement of the amount of photons that a plant can use to photosynthesise with in a given area in a certain length of time. It is measured in something called PPFD (Photosynthetic Photon Flux Density) and the unit of measurement of PPFD is micromoles per square metre per second (umol/m2/s). It is measured between the wavelengths of 400nm to 700nm. There is no weighting given to one particular colour or wavelength range. All photons in the range 400nm to 700nm are counted the same:
Plant leaves can only really make good use of a certain amount of photons in any given 24 hour period. Plant leaf cells contain special little “factories” called chloroplasts which is where all the photosynthesis happens. Just like any factory, they have a limited production rate. Chloroplasts can only capture light photons and use carbon dioxide and water to make sugar at a certain rate, then they need time to recover.
Advanced technology allows quick determination of a light source's ability to promote photosynthesis. The availability of two devices has made this possible: A 'photosynthesis meter' (a PAM fluorometer, which allows in-situ determination of rates of photosynthesis) and LEDs (Light-Emitting Diodes of high efficiencies when combined with lenses to create intense light fields.)
The following sections describe light bandwidths, the light sources used in the procedures (in great detail), and the photosynthetic responses of a coral's zooxanthellae to these light sources. If this is not of particular interest, skip to the Discussion section.
Abstract: Light transmittance regulated by canopy openness influences the microsite conditions for natural regeneration. The successful transition from seed germination to subsequent seedling recruitment (, early seedling survival and growth) determines the natural regeneration potential. However, there is little information on the effect of varying light transmittance on seed germination and seedling recruitment of Siebold & Zucc. (Korean pine). We aimed to determine the optimum light requirements for this transition process in to propose practical measures for improving its natural regeneration. The transition process was studied under five light transmittance regimes (100%, 60%, 30%, 15% and 5% of full light) over two consecutive years (2010 and 2011). The highest germination percentage in both years occurred at 30% light transmittance. Generally, mean germination time () declined with increased light transmittance. Seedling survival exhibited no significant differences between treatments for 1-year-old seedlings, but was higher at 30% than at 5% light transmittance for the 2-year-old seedlings. In contrast, seedling height, root collar diameter and total biomass were highest at 60%-100% light transmittance for both 1- and 2-year-old seedlings. Furthermore, the light transmittance also influenced the growth characteristics of seedlings through regulating . These results suggest that growth of seedling requires a higher light transmittance (60%-100%) than that required for seed germination, even though 30% light transmittance was favorable to the earlier emergence with larger specific leaf area. Silvicultural measures such as thinning are recommended to increase light irradiance in the forest understorey with the aim of improving the natural regeneration of
The following charts demonstrate the rate of photosynthesis - the relative electron transport rate (rETR) - in a coral when illuminated by various LED light sources described above. Other energy dissipation pathways (NPQ and NO) are shown as well. See the Glossary (above) for definitions and the Methods and Materials (below) for further information. See Figures 19 - 24.
The standard method for determining light requirements for corals' zooxanthellae has been examination of absorption characteristics of photopigments such as chlorophyll , chlorophyll etc. (good) or action spectrums (better). Both are not without problems. Absorption characteristics are usually based on pigments extracted in solvents. Spectral characteristics shift slightly according to the extraction solvent used, and photopigments, when combined, also change these characteristics slightly. A better way is to examine the action spectrum of zooxanthellae isolated from a stony coral. This is usually done with a monochromator, where a beam of pure color (hue) illuminates a culture of dinoflagellates and a reaction is determined (such as oxygen evolution). A chart of wavelengths versus reaction is then made. See Figure 1. This method also suffers from deficiencies - the two Photosystems (I and II) absorb light wavelengths with difference efficiencies, hence a monochromator might stimulate one photosystem, but not the other, and photosynthesis might not proceed efficiently (although 'spill-over' - also called State Transitions 1 or 2 - could perhaps overcome this problem - something very much in the discovery phase for zooxanthellae.) See Kirk (2000) for further details on action spectrums.
Figure 20. The rate of photosynthesis (and energy dissipation routes) when a "UV" LED is used as the light source. These rates have been corrected for photosynthetic radiation not sensed by quantum meters.
These results may seem perplexing if we take the adage 'a photon is a photon' to be correct, and the color of light (or more correctly, its energy level) does not make a difference in photosynthesis (which is true.)