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  1. #1
    BetterSense's Avatar
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    Are CC values linear?

    I'm designing an additive enlarger light source with Red Green and Blue LEDs.

    I'm trying to decide if 8 bits (255 levels) of LED brightness is enough.

    Subtractive color heads often have knobs with values 0-130. The question is, are those 130 values distributed somewhat linearly? I will calibrate my light source once I have it built, but I need to know how much precision I will need for dimming each channel.
    f/22 and be there.

  2. #2
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    Sorry, no. It depends on the enlarger but they are often geometric and approximately 0.01D each (10-0.01 = 0.977). That means that 30 of them (10-0.3 = 0.5) is about one stop of attenuation and 120 is about 4 stops. So 8 bits is certainly enough to represent the dynamic range, but you need to figure out whether you will have sufficient precision at the lowest levels of illumination in each channel, i.e. at extreme filtration settings with big colour ratios.

    Presumably you're also building the timer which will control the 3 intensities and durations of illumination? Using PWM for the intensity control? If have only 8 bit PWM and you need more resolution at the lowest exposures, you can expose one channel at higher intensity (therefore more precision in the PWM) for a shorter time. However, that will cause problems with dodge/burn operations.

    Most cheap micros have 16-bit counters that would give you high precision PWM if you wanted it. Even an Arduino (ATmega328) has one and you can PWM with it if you write a bit of native AVR code - I can help you with that if you want - which gets you 16-bit PWM at 244Hz but only one or two channels.

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    You're already looking at one bazillion color combinations (256^3 = 16,777,216 to be pedantic) so I'm not sure what good extra precision will do. Though Polyglot sounds like he has some pertinent experience!

    The tricky part will be to give the software a good, usable user interface.
    Last edited by Kawaiithulhu; 07-26-2014 at 10:17 PM. Click to view previous post history.

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    The total number of available colours is not the problem, it's how far apart they are in the final output gamut. If a minimal change in the colour produces too much change in the image, it's got insufficient precision.

    On further thought though, assuming colour printing, you tend to have at most 30CC difference so (assuming 8 bit PWM) that leaves you with 7 bits of precision in the lowest intensity channel, which is about 0.5CC precision, which is plenty.

  5. #5
    AgX
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    Quote Originally Posted by polyglot View Post
    It depends on the enlarger but they are often geometric and approximately 0.01D each
    Those enlarger filter values are logarithmic not geometric.

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    They are logarithmic but instead of doing brightness level you can do with time. Exposure for the R,G,B lights of different times.

  7. #7
    AgX
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    Quote Originally Posted by BetterSense View Post
    I'm designing an additive enlarger light source with Red Green and Blue LEDs.
    Wouldn't just buying that 35mm Philips additive enlarger with incandescant lighting yield an outcome?

  8. #8
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    [QUOTE]The tricky part will be to give the software a good, usable user interface[/QUOTE

    I don't do GUI's. I do knobs and gages and buttons and switches.

    I know the atmega 328 has dual output compare registers on timer1. I can use those for green and blue. But I would rather use bytes internally if I can. If a cc is .1D I can calculate from there. For color printing it seems like I will be in the middle or upper range all the time...not like a subtractive enlarger at all where you leave one channel at 0.
    Last edited by BetterSense; Yesterday at 08:07 AM. Click to view previous post history.
    f/22 and be there.

  9. #9
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    Since you will most likely need more than one LED per color, you could trivially increase dynamic range by turning on/off the whole array, or only a subset of your LED array. 8 bits of PWM plus 4 bits from switching all or only every 16th LED will give you an 1:4096 range, or a density difference of 3.6, which is way more than any subtractive color head will ever give you.

    One thing you may want to look out for is actual light output from individual LEDs. There is a good chance that you will see quite a variation in light output between different parts, even from the same batch.
    Trying to be the best of whatever I am, even if what I am is no good.

  10. #10
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    Chan: that makes dodging & burning unreasonably difficult because you can't get nice smooth blends by moving the tool around.

    Quote Originally Posted by BetterSense View Post
    The tricky part will be to give the software a good, usable user interface
    I don't do GUI's. I do knobs and gages and buttons and switches.

    I know the atmega 328 has dual output compare registers on timer1. I can use those for green and blue. But I would rather use bytes internally if I can. If a cc is .1D I can calculate from there. For color printing it seems like I will be in the middle or upper range all the time...not like a subtractive enlarger at all where you leave one channel at 0.
    0.01D. You would definitely have too-coarse control with 0.1D (1/3 stop!).

    Knobs & text displays are easy on modern micros, certainly easier than driving a gauge accurately. I would suggest that a rotary encoder (for changing values like channel intensities or exposure times), small LCD (to display current settings), a few buttons (to select which channel and/or time you're chagning) is an easy approach. There's absolutely no need to keep things in just one byte though because 16 bit, 32 bit and even floating-point arithmetic is supported by gcc with zero extra effort on your part. It runs a little slower but for the purposes you have here, the micro is still about 10000x faster than necessary. Hell, you could write the whole program using double (64-bit IEEE floating point) and it would still run fine unless you trip yourself up on rounding modes.

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