This time I dug into territory of plant lighting. Early summer I got some young chilis from IRC acquaintance of mine and, well, three of five were still alive when autumn fell and it was time to check how I should have grown them in the first place.
The most obvious thing lacking was light. During the summer they did ok’ish just with sunlight, but during the rest of the year there’s not much sunlight to speak of here in Finland. Time to go artificial.
I already knew most of the plant lights just produce unnecessary heat and make plants look good, but do not help the plants grow. Quick dig into the Web confirmed this.
From http://www.bonsaifi.net forums I got what I was looking for: some hard data on what wavelengths chlorophyll actually uses (from Botany Online):
To put the long story short, I bought a E27 form factor compact fluorescent tube optimized for plants (info via bonsaifi.net again) and ordered some 400 leds of proper wavelengths from China and it was time for soldering and comparison.
The reference light is branded as “Megaman” and has a spectrum that looks like this:
The leds came from Chinese company branded as HB. They are the only Chinese manufacturer I am yet to find who sell small quantities directly to customers and who have English web page with prices. The delivery was very fast and service was good, earning my warm recommendation.
As UV and deep red frequencies are hard to come by and/or expensive, I took leds in 1:2:2 proportion on wavelengths of 402nm:455nm:639nm. The UV ones (402nm) cost four times the deep blue/red ones, so I took a chance and hoped this combination would work.
The combined electricity usage of the led light is around 20W and this is how it looks in action (never mind the casing, or lack thereof, its just in prototype stage):
The light is actually nowhere close to white, it’s red-blue-violet to a human eye. Whatever white you see there is just camera sensor having difficulties. The plant leaves look dark blue in that light as expected - there is no green light to reflect away, just wavelengths the leaves absorb and use.
And here is a combined image showing the LED light passing the real test with flying colors. On the left is the fluorescent tube light, on the right the led one; time difference between the two shots is five days: