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  1. #1
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    Mirror thickness

    Does gold mirror thickness matter? I did a test on my mirrors for heat and after a 10sec firing at 100% power @50 watts the mirrors surface did not raise 1deg. The mirrors are 1/4" thick and I plan on replacing them with a mirror that's only 1/8" thick is that going to hurt the performance any?

  2. #2
    Moderator ynneb's Avatar
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    Paul I am guessing here, but the thinner the glass I would expect the less light absorption, and therfore less heat generated, Its a bit like when light is absorbed the deeper down you go into the sea. The light must change into heat.
    While we are not talking about great changes in thickness, because the light is so intense a small difference in thickness may change its apsorption/heat properties.
    Hence, thinner is more efficient at reflecting than thicker of the same material.

  3. #3
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    That is what I was thinking to but why the thicker material? Is that for 500 + watt lasers, I know some of those laser need water cooled mirrors.

  4. #4
    Moderator ynneb's Avatar
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    Still guessing, but I guess there will always be heat put into the glass no matter how thin it is. For higher powered lasers it may be a case that if they make the glass too thin, even though it is more efficient it will still absorb too much heat and then break. Possibly it is better to make thicker mirrors for higher powered lasers, even though they are less efficient, they are more capable of holding together.

    Shesh....I dunno, just hypothisizing

  5. #5
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    Projectors and laser systems are "first surface" mirrors. The reflective material (usually vapor deposited gold or aluminum) is on the "front" of the glass. The light never goes through the glass at all. The thickness is only for mechanical stiffness.

    robotic regards,

    Tom
    = = = = =
    "It's easier facing loss if you care little about losing face."
    - - Dennis R. Ridley

  6. #6
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    Quote Originally Posted by ToyMaker
    Projectors and laser systems are "first surface" mirrors. The reflective material (usually vapor deposited gold or aluminum) is on the "front" of the glass. The light never goes through the glass at all. The thickness is only for mechanical stiffness.

    robotic regards,

    Tom
    = = = = =
    "It's easier facing loss if you care little about losing face."
    - - Dennis R. Ridley

    Ha yes BUT mechanical stiffness from what? Heat, to much heat will warp the surface, so having said that what is the heat range for a given thickness? That's what I'm trying to figure out.

  7. #7
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    I don't have any hard fast numbers but some time ago (OK a long time ago) I worked on film projectors that used aluminized first surface mirrors. The projectors used 600 watt quartz-halogen lamps and the mirrors were 125 thou (1/8 inch) thick. Air flow cooling kept the mirrors (and just about everything else exept the bulb) within 30 degrees of ambient. I don't recall that warpage was ever an issue.

    robotic regards,

    Tom
    = = = = =
    "Maybe a great idea blew your socks off; a great song can sock your blues off."
    - - Dennis R. Ridley

  8. #8
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    Well the best I can remember is that the mirrors are pyrex glass that are ground to a parabolic shape.....dimensionally pyrex is pretty stable....but I'm sure that having a high powered laser beam heating up the reflective surface can tend to cause the pyrex to change shapes....I wonder what the coefficient of expansion for a pyrex blank is...hmmmmmm???

  9. #9
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    Geesch....there are all kinds of mirror blanks....some with a lower coefficient of expanion then pyrex....and reflectivity is way up...hovering at 100%....guess what doesn't get reflected heats up the reflective surface and the mirror body.

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