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I'm essentially trying to diffuse a daylight spectrum bulb and have very little depth to work with. I opted for bouncing the light out of a matte white chamber with a fancy shape so it distributes evenly and it works great but the light makes 2.4 bounces on the average so how much light the surface reflects makes a big difference in efficiency. eg. 80% reflection is 58% efficiency and 90% reflection is 78% efficiency. (reflectivity ^ average_number_of_bounces)

I'm currently using stacked up whitest copier paper that I could find but from what I researched, its only ~80% reflective. I don't know much about paint but google says that typical white paint is 70-80% reflective, that would not be an improvement. There is this cool diy paint on youtube that is supposedly over 98% reflective https://www.youtube.com/watch?v=dNs_kNilSjk and if I had some time on my hands, I'd go for it but unfortunately I don't.

That said, what's the best way to make a flat surface really really white (like 90%+ reflective) with stuff that you'd be able to find in a well stocked hardware store or perhaps easily ordered online? My budget is <€100 to cover 2 square meters.

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  • You might want to specify what sort of daylight bulb. I've used halogen and seen fluorescent, but there are "daylight" LED lamps too
    – Chris H
    Dec 3, 2023 at 10:47
  • Just FYI there's an updated video by NightHawkInLight that uses more accessible materials and a method of applying the paint that doesn't take 8 hours: youtu.be/KDRnEm-B3AI?feature=shared
    – Elmy
    Dec 4, 2023 at 6:22

2 Answers 2

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What you're making is fairly similar to an integrating sphere used in my line of work. Coatings are traditionally barium sulphate or magnesium oxide in the form of fine powder. The former is used as an art pigment, often mixed with titanium dioxide (another bright white pigment), and pure pigments are available online. So the trick might be to get those to stick with the minimum of binding agent. Wearing a respirator or good mask, I might coat the surface with a thin layer of spray varnish, then dust with the pigment powder, tipping it off when dry. It won't be very robust.

Easier options include the non-shiny side of aluminium foil. The reflectivity is a bit better than paper, but it's not as diffusing, so you might want to give it a very light spray of white spray paint on the most direct line of sight to the output (or anywhere that's on a 1-bounce route to the bulb). Note that halogen bulbs get hot enough to melt aluminium foil in direct contact. You may even be able to use the shiny side to reflect light that's going to have at least 2 more bounces.

There's probably no point trying silver paint.

Finally you can try making a diffusing reflector from a mirror, glass or plastic so long as the reflective metal coating is on the back. Scuff up the front surface with fine aluminium oxide or silicon carbide sandpaper (e.g. emery paper). Start with just a little sanding and test, expecting to sand a bit more. Work wet, test, dry, and wear a mask.

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  • I'd try the foil first, then see about ordering mirror tiles (but I've got a tile cutter) or mirror plastic sheet.
    – Chris H
    Dec 3, 2023 at 10:45
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You could use a high quality commercial interior matte white paint. Use matte to expose the most surface area to the light and cut down on hot spots. With the reflectivity of the top white paints in the 90%'s it would be the easiest and most affordable approach.

A quick search shows five major manufacturers with paints that have an LRV (light reflectivity value) over 90. The brightest appears to be a "Ultra Pure White" from one company (rhymes with bear) that has an LRV of 94.

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