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Re: White Paint

Date: Thu, 04 Mar 1999 10:35:15 -0800
From: MICHAEL SERPA <mserpa at bayweb_net>
Subject: White paint


        Being a grower of many different kinds of plants for most
        of my life, I have tried just about everything under lites
        at one time or another. I've probably read every publication
        on the subject, in English, on growing plants under lites.
        One common item,in these writings, has always been a list
        on reflectivity values. And flat white paint has always been
        near the top of the list. It's cheap, it's easy and when the
        paint 'yellows' get out the paint brush!  It ain't rocket
        science and your plants will bearly know the difference. Your
        wallet will!

Hearty agreement with Michael on this posting!

Having not been flamed, properly (recently), I can't resist adding a
comment or three.

The total reflectivity of a good white paint nearly equals that of
silver, and is certainly better than any aluminized surface (mylar,
foil or sheet metal). The difference between a diffuse (flat white or
brushed metal) reflector and a specular (polished metal or mirror)
reflector is the direction of the reflected light. 

Over water, that just might have some effect, for light nearly
parallel to the water surface is reflected away, while 96% or so of
that incident nearly directly on the water does penetrate and go on
into the water.

Diffuse reflection scatters in all directions, so some of the light
must go back toward the tube or off at a shallow angle. Really
efficient specular designs (The McDonald's Golden Arches cross
section, for example) could, in theory, direct more of the light in
proper directions to go into the water better. Be sure and re-polish
them weekly, though. :-)

White paint has the huge advantage that the *shape* of the reflector
is totally non-critical, where shiny reflectors need careful
shape-control for any real efficiency advantage.

On plastic:

Unless specially coated, or otherwise treated to resist ultra-violet
light, the bonds cross linking plastic molecules are destroyed and
plastic will get brittle and fragile if exposed to strong UV light. I
have been using brown plastic rain-gutter material, with a brilliant
white interior, for at least 5 years with no obvious degradation. The
lights have usually been lower-level CW that put out little UV,
anyway, but the material was designed to take full sunlight and last
for many years. It works.

If you bought a plastic hood that is suspect, just line it with
aluminum foil and the problem will never appear. Replace the foil as
needed, and protect from spray.

That's my US$0.02 for this morning,


Wright Huntley, Fremont CA, USA, 510 494-8679  huntley1 at home dot

One big difference between a Libertarian and a Demopublican is the 
Libertarian knows it's not a waste to vote against a Republocrat.