Light mount height?

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Light mount height?

Postby Tim » 14 Jul 2010, 12:33

I'm building my first scanner, and I realized I didn't know what height to mount the lights at. The new standard seems to have about a 42" high or so light column, and my cradle and base are basically the same. Is there any reason the have the lights closer to the platen or farther away? I guess farther means less brightness per square inch on the book due to the inverse square law, but perhaps more evenly diffused as well. I'm using 2 650 lumen incandescent flood lights for now because that's what I had around. Ideally the lights should shine straight down on the center of each half of the platen too, right?
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Re: Light mount height?

Postby Antoha-spb » 15 Jul 2010, 01:52

Hi Tim,

You'd better try different positions and distances and pass all the results through scan tailor or similar software to know what results are satisfactory and what are not.

I got one diy scanner where four 12V halogen lights are used to lit a platen. The distance is very small - about 10 centimetres. Another scanner rig (that is not yet finished) uses two fluoriscent lamps with a diy parabolic reflector behind them to address most of the light onto the platen. The distance is ca. 50 cm.
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Re: Light mount height?

Postby daniel_reetz » 15 Jul 2010, 11:52

AnothaSPH pretty much nails it. Light height is a balance between four things:

1. light uniformity
2. brightness
3. glare
4. scanner size

light uniformity is easier to get with greater distance, but brightness suffers.
brightness is a function of how many lights, and how close.
as for glare, the higher up the lights are, the easier it is to "move" their glare image off the glass.
scanner size -- depends on how much room you have, or how big you are willing to make it.
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Re: Light mount height?

Postby Tim » 15 Jul 2010, 17:42

Thank you both. That makes a lot of sense. I did end up creating a pretty sad looking jig that would allow me to move the light up and down for some quick and dirty testing. I got a little frustrated that even being careful on the rest of the scanner my craftsmanship skills aren't up to my desired accuracy level, so I resorted to this to get some information.
Lighting 1.JPG
Yep, that's duct tape
Lighting 1.JPG (187.91 KiB) Viewed 3463 times

The cheap flood light I have in there does a pretty good job of avoiding glare. It has a kind of softening coating over most of it that seems to work pretty well. I did indeed find that too close to the platen results in uneven light and glare, but I didn't have to get very far away to avoid both of those. With the bulb about 16" away from the center of the page, it gives pretty good results. I think I will put another one in and see how much better it does. Even with just the one light I get pretty good results from OCR which is my primary aim. In this case the light arm is at about 28" from the bottom of the vertical light/camera column. That gives about 14" of clearance from the platen to the bottom of the light cover, which is plenty to lift and change pages for anything this scanner can fit. I'll document my build a bit more in another thread when it's finished, but you can see I followed Rob's design closely and I only made a couple very minor innovations.
Tim
 

Re: Light mount height?

Postby StevePoling » 17 Jul 2010, 11:42

You can get pretty good illumination from a light source 92 or 93 million miles away if it's not cloudy. Since I built my scanner in my basement in the dead of a Michigan winter, that's easy to forget. However, it should inform our efforts to remember that artists prefer a north-facing window in their studios. I chose a maximal height (limited by fears of setting the ceiling afire) and zillions of watts of halogen incandescence.
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Re: Light mount height?

Postby Mathue » 18 Jul 2010, 22:19

Tim wrote:I'm building my first scanner, and I realized I didn't know what height to mount the lights at.


On the rig we're building the lights are at the 'new standard' height, the variation is largely the lights. We're using two contrast enhancing compact fluorescents (I forget the lumen's rating)

I have two Bayco 10-1/2" Brooder Clamp fixtures http://images.lowes.com/product/convert ... 6832lg.jpg They collect the light extremely well and direct it without a hotspot.

I'm currently considering a more direct mounting as the spring clamps brings the reflectors out a little wide

right---00538.jpg
Lighting sample.
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Re: Light mount height?

Postby n9yty » 30 Jul 2010, 19:31

I like the design . . Although similar to the one Rob posted, I think you should put it on a new thread to make it easier to find, and if you have time to put some measurement or construction notes in there to help others. :) Like me. :)
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