Grow lights for plants indoors

Excerpts from
http://www.kulekat.com/led-home-lighting/do-led-grow-lights-work.html


Photosynthetically Available Radiation (also called Photosynthetically Active Radiation and abbreviated to PAR) defines the spectral band between 400 and 700 nanometres (nm) as illustrated in the diagram where plants find light suitable for photosynthesis.
The standard unit of measure for PAR is µmol (micromoles) per square meter per second which gives the photosynthetic photon flux density (PPF or PPFD) – essentially how much light is hitting a given area. Where lumens per watt are used to determine the efficacy of regular lighting (i.e. intended for humans), PPF holds sway in horticulture.

How much light do plants need?

Now generally speaking, the more light the better where plants are concerned; however there is a distinct point beyond which they are unable to make further use of extra light. This is rather boringly called the Light Saturation Point and for your average plant this is around about 500 µmol/m². For comparison, the maximum amount of PAR available on a clear summer day is 2000 µmol/m².
Subjecting plants to light in excess of their particular saturation point (this obviously varies for individual species) is not only wasteful but in fact counter productive as they will actually grow less well. An easy way to adjust the PAR is to simply alter the height of the lights above the plants.
For example, the saturation point for lettuce is about 300 µmol/m² and a typical 130w LED grow light can deliver between 200-1400 µmol/m² as the distance is varied from 3 feet to 6 inches. The optimum height for growing lettuce would thus be at about 2 feet.
To sum up then, what plants require for healthy growth is light that radiates at four specific wavelengths (two types of chlorophyll times two absorption peaks each). To be technical, chlorophyll “a” absorption peaks are at 430 nm and 662 nm, and chlorophyll “b” sits within “a” at 453 nm and 642 nm.

Likewise, because LEDs accurately target the wavelengths preferred by plants they both don’t need to be and in fact don’t seem so bright to us, even though they are actually delivering a greater amount of “useful” light.
That perception is further bolstered by the fact that HIDs emit a lot of light in the green/yellow part of the spectrum which increases their perceived intensity to the human eye (unlike plants we favor light around the 550 nm mark). But the fact is that they are just wasting all this additional light since it is of little benefit to plant life.
If we take a 400w HID as an example, about 260 watts of that will actually be completely outside the Photosynthetically Available Range (PAR) so at least 65% of the light (and the energy used to produce it) is completely wasted.
So with less than 40% of the light generated by HPS and MH lamps being absorbed by plants, and barely 10% of the electricity you pay to run them being actually converted into light in the first place this is not looking promising for conventional lighting. On these figure about 4% of the input energy ends up doing some good for the plant (it’s not quite that bad to be honest, but it’s fun to push the data around to draw extreme conclusions).

spo--you can use combinations of blue and red LED or it might be cheaper to just use a full spectrum LED and get the benefit of a SAD light box.

 Light intensity and distance
It’s worth also reiterating here that because LED lights have a very low heat output, the Inverse Square Law of light (mentioned above) plays to their advantage. Let’s say you have a set of grow lights positioned two feet from your plants that delivers, for the sake of argument, 400 lux (the intensity of light per square metre). If you halve that distance to one foot then the light delivered will be equivalent instead to 1600 lux – a massive increase in light density without any additional expenditure.

To be at all effective, each individual LED should consume at least one watt of power. Below this level there is insufficient penetrative power. There is a balance to be maintained between overall light coverage and individual light intensity. Consider for example replacing a 50w halogen lamp with 10 x 5w lamps – the intensity is simply not there.
So if (to use an example that may be familiar to some) a panel is rated at 13.8 watts using 225 LEDs then you can easily calculate that each one is 0.061 watts – about 16 times below the minimum threshold, or equivalent to using a 6w light bulb in place of a 100w one

To go back to our Edison 1W LED by way of example, the same Edixeon S in the form of a 3w bulb is indeed brighter at a total of 85 lumens, but the lumens per watt figure is one third that at just over 28lm/w. Put another way, 3 x 1w LEDs gives a total of 150lm (3*50lm)which is a lot more light than 1 x 3w LED outputting 85lm. The same effect is observed as the wattage increase through 5 and 7 watt LEDs.

But you can compare what is used in any given product with a respected standard such as this Edison High Power 1w LED. Note that this lists luminous flux for LEDs emitting light at different wavelengths, so we can easily see that at 620~630nm we get 50 lumens (per watt, obviously since this is a 1 watt lamp).

Many 1st generation LED grow lamps were bi-band (just red and blue) which experience has shown to be insufficient for most plants. Although these two wavelengths satisfy the bulk of a plant’s needs, most still require trace amounts of light from other portions of the spectrum. However, many growers reported good results even with these early models – once considered state of the art and yet still the subject of heated debate – plus ça change…
More modern (so-called 2nd generation) system are tri-band and incorporate an orange component and there are now 5 band lights that match both chlorophyll absorption peaks in the red and blue zones plus orange.
You should be looking for anything above 90 watts, so an LED grow light 120 watts or more should suffice depending of course on your specific requirements and situation. The reason incidentally that it is common to find LED grow lights 120w advertised online is that this is approximately equivalent to most standard 400w HPS setups (in much the same way that in the world of domestic energy saving light bulbs a 6w LED is shorthand equivalence for a 50w halogen lamp).