Increasing Winter Greenhouse Light with CHEAP Mirrors

[Hotwired]

Up in NY we get some short days in Winter and a low Winter sun. I maximize my sunlight by installing cheap "back-of-the-door" mirrors behind the benches on the north walls.

http://imageshack.us/a/img832/1691/271ga.jpg
http://imageshack.us/a/img87/1220/219wg.jpg

13"x49" mirrors are available at home depot for $5.97
http://www.homedepot.com/h_d1/N-25ec...1#.UL4ahoYgs_8

Model # 72923
Store


SKU # 279348
Store SO SKU # 279348

Hotwired NY 5b
http://www.hotwiredgardens.com/pdf/G...d_starting.pdf

[Cole_Robbie]

Please forgive me in advance for criticizing your idea, but it would work better if you painted the mirror with flat white paint.

Mirrors are actually quite bad at reflecting light that is useable to plants, at least when compared to mylar or flat white paint. They are made to reflect the spectrum of light that humans see. The light also has to pass through glass twice. It goes through the clear glass, hits the reflective back of the mirror, and passes back through the clear glass again, losing intensity each time. Cheaply made mirrors also have a lot of flaws that are not easy for the human eye to see, and you lose more light with each minor flaw.

If you want more light, paint everything flat white. Every dark surface in the greenhouse will absorb light, while white will reflect it.

[Hotwired]

I looked at that idea, as well as aluminum foil. Mirrors use rear surface Aluminum Coatings. Aluminum reflect 89%, while the glass transmits 96%. http://img40.imageshack.us/img40/4215/alumspect.jpg Actually I'm reflecting 82% (.96x.96x.89). As far as spectrum reflected, the soda-lime glass blocks about 40% of UV light below the 380μm range, which is outside the PAR curve. I'm starting seeds inside for transplant outside, so I really want more blue light. http://imageshack.us/a/img560/3693/daylightcflcurve.jpg

I ran a spectrograph on gloss white paint and found it reflected between 60% and 35% within the PAR curve. I didn't run a curve for flat white. I have to admit that, even though the reflectivity is much lower, the diffusing effect would provide better coverage but significantly less lumens.

As a retired optical engineer, I have spent significant time evaluating and running curves on commercial growlights, and as a byproduct of that effort I looked at different mediums for reflection. Actually Aluminum Foil works extremely well, except it is difficult to keep clean.

And by the way, a combination of daylight 6500k and Soft White 2800k fluorescent bulbs provide the best spectral output relative to PAR, and definitely the most bang for the buck.

Hotwired NY 5b

[greentiger87]

It's great to hear an empirical perspective on it. Two of the other reasons flat white are often suggested over mylar or aluminum foil is because the metal reflectors also reflect infrared extremely well, allowing for "hot spots" or general temperature increase. I'm guessing that's only a significant issue for those using MH or HPS lights indoors in an enclosed space.. which is a fairly specialized population of growers.

[Cole_Robbie]

The "hot spot" thing is a myth. But at the same time, there is a real benefit with consistent diffusion.

I am fanatical about everything in the greenhouse being white, which makes me a great customer of the paint company. I buy a lot of white duct tape, too.

[greentiger87]

Would crumpled/textured foil provide both diffusion and greater reflection?

[Hotwired]

With white paint, it's about diffusion. Diffusion is a good thing because it allows light to reach surfaces of the plant that normally wouldn't be lit as brightly. I use mirrors, because they are cheap, can be cleaned, and have the highest reflectivity of any material. Crumpled aluminum foil is a good solution as well. I have a friend who covers his multi-cell trays with aluminum foil, then punched a hole with a pencil to plant his seeds. To me that's a bit over the top, but once the seedlinge emerge I have to admit that it really helps to get light on the leaves. I'm not sure how much good it does to light the bottom of the leaves.

[dokutaaguriin]

Originally Posted by Hotwired

With white paint, it's about diffusion. Diffusion is a good thing because it allows light to reach surfaces of the plant that normally wouldn't be lit as brightly. I use mirrors, because they are cheap, can be cleaned, and have the highest reflectivity of any material. Crumpled aluminum foil is a good solution as well. I have a friend who covers his multi-cell trays with aluminum foil, then punched a hole with a pencil to plant his seeds. To me that's a bit over the top, but once the seedlinge emerge I have to admit that it really helps to get light on the leaves. I'm not sure how much good it does to light the bottom of the leaves.

You may confuse some insects with light on the underside of the leaf.
Jeff