Using a mirror and hotspotting - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 8 Old 10-28-2019, 01:49 AM - Thread Starter
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Using a mirror and hotspotting

Say a screen has a minimum 1.8 throw ratio, the projectors minimum throw is 1.85 and a mirror is used, the throw between the mirror and the screen isnt 1.8, but between the projector is, will any hotspotting be relative to the projector or to the mirror?
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post #2 of 8 Old 10-28-2019, 07:57 AM
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Originally Posted by bdht View Post
Say a screen has a minimum 1.8 throw ratio, the projectors minimum throw is 1.85 and a mirror is used, the throw between the mirror and the screen isnt 1.8, but between the projector is, will any hotspotting be relative to the projector or to the mirror?
The new throw length is the two added together.

The mirror will or can change your throw angle similar to using vertical lens offset. That angle can also be a factor at what angle a hot spot becomes apparent.

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post #3 of 8 Old 10-28-2019, 11:37 AM - Thread Starter
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Gotcha thx. Do you happen to know if there are any picture or diagram examples of first surface mirror draw lines? I havent been able to find anything and wonder how noticeable theyll be.

Last edited by bdht; 10-28-2019 at 11:42 AM.
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post #4 of 8 Old 10-28-2019, 04:44 PM
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I used a second surface mirror for about 3 years before buying a shorter throw projector. My mirror was a cheep wall mirror I bought at Big Lots for about $10. I was happy with the quality and could see some banding when the screen was all one color over a large area. The ghost image from a double reflection was never a problem as I was shooting backwards at the mirror at a very straight on angle just tipping the mirror enough that the projector didn’t cause a shadow with the image so straight on if there was an offset image it couldn’t be noticed.

The first time I tried it I lifted the sliding mirror out of a bathroom cabinet and tried it. It was easy to see it would work with just a few minutes of playing around with it. so I built a DIY rig to hold it all and be able to make fine adjustments.

I don’t tried. know of anything published on the subject except the thread I started and maybe a couple old ones others have shown what they tried.

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post #5 of 8 Old 10-30-2019, 12:02 PM
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Originally Posted by bdht View Post
Do you happen to know if there are any picture or diagram examples of first surface mirror draw lines?

Not sure what you mean by draw lines...

Noah

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post #6 of 8 Old 10-30-2019, 12:21 PM - Thread Starter
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Originally Posted by noah katz View Post
Not sure what you mean by draw lines...
https://www.twowaymirrors.com/decidi...urface-mirror/
https://www.avsforum.com/forum/68-di...l#post36579978
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post #7 of 8 Old 10-30-2019, 12:26 PM
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I see, thanks

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post #8 of 8 Old 10-30-2019, 03:07 PM
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Quote:
Originally Posted by bdht View Post
Gotcha thx. Do you happen to know if there are any picture or diagram examples of first surface mirror draw lines? I havent been able to find anything and wonder how noticeable theyll be.

I'm not sure about pictures or diagrams being available...

Draw lines are a function of how flat the surface is, i.e., a tolerance measurement. They can be or are orientated both horizontal and vertical.

Most of the rear projection rigs from companies like Da-Lite, Draper, etc., use first surface mirrors of 1λ flatness or what's often called typical grade, having surface quality and flatness characteristic typical of standard commercial grade soda lime float glass.

Moving up to λ/4 flatness, considered precision grade, is quite expensive as it requires extensive polishing to achieve that tolerance. Cost increase is usually a factor of 10. And as you may imagine, stepping up to λ/8 and higher is going to cost considerably more.

The naked eye cannot see the difference between a 1λ and λ/4 surface by just looking at it, instruments are required to measure the difference.

Other attributes to consider with a first surface mirror are percent wavelength reflectance at various spectrum nm values and defects like pinholes, scratches, circular defects, etc.

I wouldn't obsess over it too much as you will always have draw lines regardless of what grade you pay for as the reflected light will always react to any surface anomaly. A temperature difference on any part of the glass' surface causing expansion of that area can cause draw line type distortion.

Long story short, a glass first surface mirror of 1λ flatness from a company like FSM should do what you want to do.
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