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Thread: Bounce mirror alternatives!

  1. #1
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    Default Bounce mirror alternatives!

    I was thinking about first surface mirrors and how expensive they are and got thinking.
    What would the possibility be of using either stainless steel that is polished or chrome plated instead?
    Would they not be near 100% reflective?

    Jim

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    Cool

    More like 70-85% reflective. You'd be better off using a standard bathroom mirror in that case. (They get up around 90%)

    Search on E-bay... You can find deals on first-surface mirrors from time to time. A few years ago I bought 4 or 5 of them that were 8 inches by 10 inches for 19 bucks each including shipping. They were sold as hobby mirrors for making kaleidescopes and stuff, but they were first-surface mirrors with roughly 92% reflectivity.

    Adam

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    For single bounces, standard mirrored glass is a cost effective alternative.

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    I used bathroom mirrors for a show some weeks ago, it works great. BUT, they diffract a little the beam (separate the colors like a prism), and increase the divergeance... This is visible after a few meters of light travel.

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    That can work to your advantage though.

  7. #7
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    Quote Originally Posted by Buffo View Post
    More like 70-85% reflective. You'd be better off using a standard bathroom mirror in that case. (They get up around 90%)

    Search on E-bay... You can find deals on first-surface mirrors from time to time. A few years ago I bought 4 or 5 of them that were 8 inches by 10 inches for 19 bucks each including shipping. They were sold as hobby mirrors for making kaleidescopes and stuff, but they were first-surface mirrors with roughly 92% reflectivity.

    Adam
    If they are only 70-85% reflective where is the loss? I would expect loss on a normal mirror due to the reflection off the glass and also the refraction through it.
    The light cannot penetrate the surface and I wouldn't expect it to heat up as it should really be very reflective (the same as a 1st surface mirror).

    Jim
    Last edited by JimBo; 09-25-2007 at 11:22.

  8. #8
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    Cool

    where is the loss?
    The surface of the metal. That's why mirrors have glass on the front. It protects the smooth reflective surface. If you have a bare surface, it will rapidly pit and degrade. (No, this damage won't be visible to your eye, but it will be enough to screw up your optical surface.) Even austenitic stainles steel is not immune to this pitting. Sure, it might look good enough to allow you to see your reflection, but even a 50% reflective mirror will do that. To get really high reflection values, you need to protect that surface.

    Here's an example where a group wanted to build a solar collector. They decided to use polished galvanized steel sheets as the mirror surface, protected by a 1 mm thick glass laminate. Even so, the best they could get is 83% efficiency. (Scroll to the bottom of the .pdf and look at the table to see the mirror efficiency.)

    Bottom line: Polished metal mirrors need a coating in front of them to protect the surface. Glass works, and it's cheap, but you get losses. Thin films are better, but they're more expensive. Dielectric first-surface mirrors are much more efficient, and are also far less suceptible to casual environmental damage, but they're quite expensive. (Note that they're not going to hold up in a hailstorm, but they'll last nearly forever in an indoor environment with proper cleaning.)

    Adam

    Edit: consider this real-world example: Ever burn your hand on the shiny chrome bumper of a car that's been sitting in the sun? (Me too!) Now, ask yourself, "How did that bumper get hot if chrome is such a good reflector?" Well, it turns out that it's probably only 60-70% reflective, and the rest of the sun's energy was transfered to the metal as heat.
    Last edited by buffo; 09-25-2007 at 12:31.

  9. #9
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    mixedgas is online now Creaky Old Award Winning Bastard Technologist
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    if you are so darn cheap that you cant afford first surface mirror, go to a tile store and buy rear surfaced glass mirror tile. Its lousy as a rear surface mirror, but if you turn it around and strip the black coating off the back, it becomes poor man's aluminized first surface mirror, however over time it will oxidize and get cruddy due to lack of overcoat. oly 65-70% reflective and may melt if you hit it with a yag beam of more then a watt or two.
    BUt cheap..............

    acetone and methanol will strip the cheap carbon black paint they put on the back of it if you soak it with a paper towel on the glass for a while.
    you pay big bucks per milliwatt on the laser, why eat photons with cheap mirrors? Sorry to be insulting, but my pet peeve is that first surface mirror both dielectric and silvered is cheap from places like One Stop Laser.

    Steve Roberts
    Last edited by mixedgas; 09-26-2007 at 08:09.

  10. #10
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    when i was working for a major touring company i found out that they don't use first surface mirror on the bounce mirrors what they do is cut up 1' mirror tiles and it seems to work fine
    i had even offer them a source for cheap first surface and i was told that
    A) they could never be as cost effective as a $1 mirror tile
    B) they would be destroyed half way through a tour (going in and out of road cases every night)
    --
    John
    VJ AIWAZ

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