As far as LED's go, it appears there's a few ways to go about it. Huw Finney; http://www.textklick.demon.co.uk/ledhead.html used 32 Lumiled's for his, Zone VI (IIRC) uses 24 for theirs and Heiland, judging from the photo in the magazine article, uses 840.
All a person has to factor in is their time...how much extra time do you have available to learn how to build one, compared to how much it costs to buy one already made, and then you have to make a choice from only two companies.
Note to self: Turn your negatives into positives.
Steve, true enough. I'd be happy to help with circuits. That is the easy part. If I were to design an LED head (which you can tell is not high on my list), I'd be concerned about the thermal issues and the optical issues. Maybe Heiland used 840 LEDs to increase diffusion and decrease hot spots; though it sure adds to assembly costs.
Originally Posted by Steve Smith
Is anybody out there tried the 395 nm LEDs? Also, those of you printing with an LED head, what kind of printing speeds are you getting?
Carry on. -LG-
I’m no EE, but the biggest problems I’ve seen with these LED heads is accurate and consistent dimming of either blue or green for a uniform, even light that gives one specific grade. I would be interested in a simple LED head that operated all blue or all green, by toggling a switch, for split-printing only.
That's probably the simplest way to do it. If I was planning a variable blue/green light I would drive the LEDs with pulse width modulation. That means that they are driven with a high frequency (say 1kHz) square wave with a variable duty cycle.
Originally Posted by resummerfield
The terms 'pulse width' and 'duty cycle' refer to the amount of time the LED is on. or the ratio of on time to off time.
e.g. 50% on, 50% off is obviously half power. 67% on and 33% off is two thirds power. 99% on 1% off is just about full power.
I would arrange that the blue is opposite to the green so if blue was 25%, green would be 75%, blue 50%/green 50%, blue 75%/green 25% and so on. That would give an accurate, linear variable control from full green to full blue with all intermediate settings.
Another advantage with this method is because the LEDs are switched either on or off, there is no unknown in between state which may cause non-uniformity.
doing this, one would probably want to drive at twice the nominal recommended steady state current to maintain brightness. One also needs to compensate for brightness vs temperature, slightly different for blue and green. Looking at the Avago website, it is apparent that there is a brightness decay over lifetime, too, but maybe this is too small to worry about for this application. I found it interesting that Avago bins its parts into many grades for brightness and that these bins can vary in brightness by over 10X and within a grade by over 30%. Color can shift by about 10 nm over temperature and a couple of nanometers per bin. I don't know how much color shift can be tolerated for a lightsource, but I would think this would matter.
I'd still want to know if 425 nm is blue enough for higher, harder, grades of paper.
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