Folks, I am confused -- there have been a few mentions of LED as an affordable option on this site, but I had thought they are very expensive new tech usable only for essentially fairly protected environments (like ceiling recesses, due to their expense) and very, very long lifetimes (to come even close to cost recovery). Those are not what I would call characteristics of the DIY scanner world, much less the portable DIY scanner world.
I'm sorry, you have this all wrong. Let me try to explain. As a backgrounder, I may suggest that you check out the thread that Ceeann linked earlier:
The thread contains a complete design for an LED lighting system, from scratch. That LED lighting system has scanned around ten thousand pages in several countries around the world, sometimes running on car batteries, sometimes on wall power. The LED I use there (which is also linked in that thread) is this one: http://www.dealextreme.com/details.dx/sku.2394
Perhaps I don't truly know what the Cree Q5 mentioned by Daniel is, but the links I came up with had them in flashlights generating 180-240 lumens (whatever that really is -- perhaps just a tenth or a seventh of a standard 100-watt incadescent bulb?) for $20 and up. Thus that kind of LED does not seem cost effective or bright enough for scanning. The same goes for all the LEDs now hitting Home Depot, which range up in the $100+ range for relatively small brightnesses, as I recall, topping out at 150-watt incadescent equivalent.
Let's talk about portable scanning requirements.
1. The lights must be small.
2. For all my portable scanners, they were designed to operate without wall power.
2a. They may need to be LOW power and DC (like running from a car battery).
3. The lights should be kind to books.
For 1, the advantage of a light like this is that LEDs are very small in comparison to halogen, incandescent, and fluorescent bulbs.
For 2, all the lights you described fail.
For 2a, almost all lights of the type you describe fail.
For 3, LEDs are the *most* suitable.
LEDs are realatively small and I gather fantastically cool, good for portable and regular scanning, but it does not seem that sufficiently bright ones are even being manufactured, much less affordably, for DIY scannign purposes.
Please let me know what I am missing here. Thanks.
You have the wrong idea regarding brightness. You can see that the LEDs I am using get so bright that they interfere with the cameras
. It is misleading to think about LEDs in terms of the quoted "lumens". The total area of a page is maybe 1ft^2, and the light from even a single Cree Q5 can illuminate that page such that we can get a picture at 1/30th of a second. Most of the light from incandescent bulbs is wasted in our setups. Also, in my 3rd generation scanner, I was able to use the LEDs in a very dim mode for working light, and then turn them on very brightly to illuminate the page in full while the camera fired. This is impossibly wasteful with incandescents.
You're right, though, that if you go buy a $100 LED bulb from Home Depot you are wasting your money. However for portable scanners and projects like DonnaA's (where wall power cannot be guaranteed, which is the case almost anywhere other than in the US and Europe, LED is the only way to go.
If you haven't built an LED lighting system, I can see where the workings of power LEDs might be mysterious, but trust me, the technology is here and it works very well.
EDIT: just to be absolutely clear, I still think that incandescents, halogens, etc are great options for scanners that will be run at home and definitely represent the best trade-off between price and luminous efficacy.