• IanA78 commented on Tom Hargrave's instructable Build Your Own 3120 Lumen LED Grow Light2 months ago
    Build Your Own 3120 Lumen LED Grow Light

    I read your full instructions AND all the comments with some interest. I have a PhD in physics and a new interest in grow lights. Specifically , I want to provide some extra light to a terrarium in a north facing spot in my house that receives only a small amount of indirect sunlight. An important point here, often overlooked or sidelined, is that these lights produce a light that is more pleasing to the (human) eye. Whenever mentioned it has been a case of 'but the human eye is different to a plant's needs', but I am interested in simultaneously pleasing both my plants and my eyes! Sure, it must be true that by concentrating power output to the specific wavelengths that are used in the various photosynthetic processes must result both in better plant growth and lower operational cost, but such lights have the drawback of making the room look like aliens are visiting! That might be fine in a basement or cupboard for certain types of horticulture, but it is not going to cut it in my dining room! The discussion here about LUX vs. Lumens is very true (i especially enjoyed the tangent about rays of sunlight being almost parallel by the distance of the Earth - something I assumed in so many problems I solved at uni!), but being a physicist, the photosynthetic response curves were completely new to me. Now I assume that the various elements of photosynthesis evolved long before the current variety of plants and these peaks in absorption (as indicated on the graph posted by 'Redbinary') are the same across the diversity of plant species? i.e. Algae, moss, flowers and vegetables all have very similar peak absorptions in the red and blue that are often quoted here and elsewhere.From what I understand about photonics (e.g. from the photoelectric effect) is that so long as some of the photons are in the energy range needed for absorption (chlorophyll A & B absorption peaks), then the light will have some effect on the plants. Increasing the intensity of photons at these wavelengths will increase the effectiveness of the grow light but with the drawback that the emitted light becomes less aesthetically pleasing. However, if the intensity of the whole spectrum were boosted by the same amount, the intensity of PAR photons increase, yet the overall light remains white(ish) but gets brighter. What is potentially of worth here then, is that it MAY be possible to deliver enough red and blue wavelength (PAR) photons to benefit the plant but where the other wavelengths, far from being useless, result in a light which isn't painful to look at! Given the low cost of LED lighting, I'd pay a bit extra for the light and room to look nice, especially since I have very limited space and only one 18 inch square terrarium to illuminate - my considerations for a nice room with healthy plants are very different to those of a commercial grower who wants maximum yield at minimum cost and it seems that many of the detractors on this post are in the latter category. Such disagreements are like comparing apples to oranges. So, rather than saying this light is for reading or other tasks (as suggested in one comment), so long as these LED lights provide some photons at the wavelengths of interest, you could say it is a 'supplemental to natural light' grow light. Only experimentation will tell you whether or not the number of white LED fixtures necessary to noticeably boost growth would result in a set up so bright that your room looks like a floodlit stadium!With the last comment being some months ago and it now being the height of summer, I hope that you have been able to test out these lights and that you may be able to let us know whether your set up provided a high enough intensity of photons at the key wavelengths to have an effect on your plants.

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