Artificial lighting (again again)

[IanW]

I do 18 hours a day Joel and it's still far faster seedling growth than you'd find from growing under natural sunlight. I don't know if any more would benefit or not.

[JoelR]

I currently have mine set to 14 hour day lengths but have considered putting the light on at night instead of during the day to combat winter temperatures which were getting too low in the winter. My light is an "envirolight" and not a T5 and emits quite a bit of heat. I found the soil warming cable struggled to keep my seedlings above 12C on the coldest nights. I suppose one answer might be to leave the light on constantly.

[AnTTun]

I was thinking about 12 hours (a day) light. Thats about how long we have sun here during summer. Tho my plants dont get all of it because of surrounding walls. Will plug cord into timer that will leave the lights on from 8:00 - 18:00 and see what happens next month or so. Then will decide if time should be shorter or longer.

Thanks for the compliment JoelR, I know I could have done it much better but I decided to try this quickie first. If it works, will build another one. Better of course

[IanW]

I use 4x 4ft T8s in a heated greenhouse, I don't find the heat they kick out to be problematic, but no doubt it helps heat the greenhouse somewhat.

[iann]

Mylar sheeting is quite tough, much more reflective than foil and can be easily PVA glued into position. I haven't compared results without using it but I'm convinced more light is bounced around.

That's exactly my point. The human eye is pretty poor at distinguishing small differences in light intensity, say between two similar surfaces with 70% reflectivity 90% reflectivity, and it is absolutely useless at distinguishing between the reflectivity of matt and specular surfaces. Specifically, people over-estimate the reflectivity of mirror-like surfaces and under-estimate the reflectivity of matt surfaces.

Mylar is the brand name of a translucent plastic, but as with many things the name has come to be applied frequently to aluminised reflective sheets which may or not be the same material. High quality mylar sheets from reputable manufacturers come with specifications for close to 90% reflectivity, when new. Any figures you may have read about high numbers are just bogus (I've seen 99% and even 100% reflectivities quoted by sellers of reflective sheeting ). Aluminium mirrors are available with higher reflectivities, up to 98% or so, but these require a solid surface which can be coated with a protective layer and sometimes a precise thickness of dielectric to increase the normal reflectivity through constructive interference. These techniques cannot be used on mylar. Uncoated aluminium corrodes quite quickly, usually with a fairly transparent layer of the oxide but sometimes with a milky chloride. This reduces the reflectivity of sheeting in a real world situation by several percent and eventually quite a bit more. The corrosion cannot be cleaned off.

Reflective plastic sheets are also available backed with opaque white and sometimes black coatings to further delude the unwary. Unbacked reflective plastic sheeting passes a quantity of light that would be scary to most people, and you can comfortably see plants in a well-lit grow box through mylar sheeting.

Again the human eye deceives, and this is actually just a few percent of the light passing through, which we already knew because the reflectivity is less than 90%. The opaque coatings may impress by blocking this frightening sight, but they are useless in terms of adding extra light inside. If 10% of the light passes through the reflective film and is all reflected back by the opaque backing (it isn't!), then 10% of that 10% will get back though the reflective coating. Bask in the glory of your extra 1% of light

Because the reflective coating is on one side of the film (the outside!) and only a few microns thick, any time it is touched the reflectivity is reduced and quite quickly reduced to zero where the material is scratched (double-coated films are available which are useful as filters but not any better as mirrors, although you should always remember to point the shiny side towards the light). You can see some scratches like this at the corners in my picture. Actually not the end of the world, they will amount to less than 1% of the surface even when quite badly scratched, but any attempt to clean a dirty sheet dramatically reduces the reflectivity.

White paints are available with up to about 90% reflectivity, but these can be quite expensive. Standard white emulsion paint is about 80% reflective when clean. This is less than a perfect new sheet of mylar, but it is quite practical to maintain 80% reflectivity while most mylar in use will not be even 80% reflective. One obvious improvement is to add high quality solid reflectors above the tubes, where you can have more than 90% reflectivity for about half the light on its first bounce. Quite expensive though. The standard reflectors supplied with fluorescent lighting are worse than white paint and I suggest you remove them. Definitely remove diffusers from in front of tubes!

So use what works in your situation. Sheeting is a quick and easy solution where you don't have or don't want a solid box. Just don't delude yourself that it is any better than white paint. And remember that a great deal of the light reaches the plants without reflection, and most of the rest with just a single reflection. As a rule of thumb for calculating the light intensity in a fully enclosed box, backed up by measurement, I assume that the total light output gets reflected once so typically about a 20% loss.

[AnTTun]

Tenants have arrived some time ago:



In the meantime, I have added one regular 100 W lamp controlled by thermostat because fluo lamps don't heat enough. I have set thermostat to 25 C and will try to go up some more in the future. All lamps are timer controlled and go off after 12 hours. Approximate electricity consumption is around 60 kW per month.

[Trevor]

Here's one of my grow-out/propogator boxes. It uses the 36W 4' tubes. While it can fit four I think I'm only running 3 in them at the moment. Four was bit too bright and hot I thought as the surface of the mix was drying out too fast. I use two cool whites and a Grow Lux or similar aquarium tube to supplement the red end of the spectrum a bit. Height of the lamps is adjustable and in winter or with sensitive seedlings I'll drape opaque plastic around the edge to contain warmth and humidity. The inside of the canopy is painted white and it extends below the tubes a fair way to reflect/trap as much light as possible. I have two of these units, one with heats pads and one without. The pads are only needed for seed germination, for grow out there is enough warmth in the unit from the lamps alone to keep things growing in winter, but our winter is probably a little milder than yours....

prop_o.jpg (28.83 KiB) Viewed 2489 times

[AnTTun]

Ummmm Trevor.. doesn't aquarium tube cover blue spectrum?

[Morgans Beauty]

here my semi-professionell solutions with 2x 1,20m tubes, 45cm above the plants and seedpots, aluminium foil as reflection surface, heating pad under the seedpots. currently everything looks great

growingstation1

growingstation2

at the window

[Trevor]

Not all of them Anttun, the marine actinics are in the pure blue spectrum, but the freshwater gro tubes for aquariums with aquatic plants have high spikes in both blue and red....so glow a kind of pinkish mauve...
Scroll down to Plant Growth Lights on this page for a bit of an explanation..
http://aquaria.net/articles/lighting/FAQ/fluorescent/