LED Lighting systems for book scanning

[daniel_reetz]

Hi Kentsin,

I'm just beginning working on LED lighting. I can't recommend any of those LEDs, because I have no idea of their true performance. I wouldn't get the small ones at all, and the larger ones may work, but I would work with Cree Q5 LEDs... I don't think the generics will be as good. You should get a single one to test if you can. I haven't yet had the chance to measure the actual output of my LED system yet or try it with the cameras in place. I hope to get a chance to do that soon, and it should help quantify what is necessary to get the job done.

The big thing is that you can't trust the manufacturer ratings, and they are measuring the luminance at the surface of the LED. When the LED is far from the book, the light will be very diffuse and due to the inverse square law, it will also be very dim.

[jimb]

How many of the Cree leds are you expecting to use? at what amperage?

[daniel_reetz]

At least without optics, these four are barely sufficient for slow shutter speeds (where slow = less than 1/30sec).

So a minimum of 8, driven normally at a low current (say, 150ma each) ~3v but then briefly driven hard, to something like an amp each. It's going to require a beefy supply.

[daniel_reetz]

It's clear to me now that I can use the light in a more intelligent way than just putting a zillion high-intensity LEDs in front of the page. These pages illustrate some designs that I am considering.

http://www.adept.net.au/lighting/led/
http://www.stemmer-imaging.co.uk/pages/ ... nation.LED
(particularly the "coaxial" illuminators)

Discussion of different types of illumination from Edmund Optics:
http://www.edmundoptics.com/techSupport ... icleid=264

[daniel_reetz]

Well, I've been absent for a while because I'm moving and having some serious health problems. But I haven't abandoned my mission; far from it. This weekend I got some help from Wolfgang Teder, a brilliant neuroscientist/EEG expert that I'm friends with. I built the base of the first book scanner in his workshop. He helped me design a high-current intermittent LED lighting system for the portable book scanner.

System description video showing the LEDs kicking ass:

YouTube video should be here shortly:
http://www.youtube.com/watch?v=cSWj0c6j5P8

Higher quality Vimeo video will be here, but it will be a while:
http://vimeo.com/user1097911

Circuit:

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A transformer drops 120VAC to ~36VAC. A bridge rectifier and capacitor complete a basic linear power supply. The positive output of the supply is connected to a series array of Cree Q5 LEDs. These are connected to the drain of a high-power MOSFET MTW14N50E. The drain is also connected to ground through a 470 ohm resistor. This resistor serves to keep the LEDs on constantly at a low level. The source of the MOSFET is connected to ground, and the gate switches the whole thing on when fed 2-5VDC. This input also has a small pulldown resistor to keep it off when nothing is connected, and a Zener diode to prevent overvoltage. Very robust. In this iteration, the 5V input is from a voltage dividing tap across the power supply, made from a couple resistors. In the final circuit, it is likely that I will drive it with Arduino.

Things I learned:
How to switch with FETs (well, a little anyway).
Not to use the current-regulating drivers from DX (they weren't driving the LEDs anywhere near hard enough; pointless things).
How to fuck up a dead simple circuit, and recover.
Good soldering irons (Weller, Hakko) are totally worth it.
How to calculate the necessary transformer size for a given application and throw together a mean linear supply.

Relevant electronics principles being explained to me by Wolfgang:

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[Lagerfeld]

I'm think of using this thing as a LED lightning system: http://www.kanlux.info/514,produkt.html
(or in PDF http://www.kanlux.info/pliki/strona/07620.pdf). The only thing that worries me is that there's no info about the lightning power in Lumens, so i have no idea , should i use one or two devices, smaller or bigger ones. Any ideas would this thing be suitable as well as powerful enough?

[charlies]

Um, for USA folks, maybe this?

don't know about the 'blue' leds for color scans...

[charlies]

um here, again:http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=66308

[Lagerfeld]

Yes! This unit has quiet the same parameters. I started to think about LED projectors, when i saw Atiz scanner pictures- lightning system actually looks quiet the same- sort of two LED projectors. But would two units by 270 lumens each be enough?

P.S. $14.99 - cool price for the device! In my country it costs $45

um here, again:http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=66308

[charlies]

Well, I got one; no good for book scanning; will return it. Daniel is on the right path with the Cree (though dealextreme.com says Q5 are on backorder... they are only around $5 each with US delivery included - they say they have 5 packs of P4 for about $18, but output is not as good, and do they truly have them?). I am not quite ready to build, yet, so I await Daniel's lighting update. As an old colleague once said: sometimes cheap is good, sometimes nasty. Constantly seeking smart cheap, with all of your help... Thanks.