Re: linearizing PWM'd LED output for greyscales
- From: RHRRC <h.lewis@xxxxxxxxxxxxxxx>
- Date: Mon, 20 Aug 2007 11:17:25 -0700
On 19 Aug, 19:16, Mike Harrison <m...@xxxxxxxxxxxxxxx> wrote:
Does anyone have a reference/suggestion for a curve to linearise the apparent brightness of a LED
with respect to its PWM drive signal. i.e. to map 256 'grey' levels to 4096 PWM ratios such that the
'grey' scale appears linear to the eye (and so things like antialiasing work well).
As the drive is PWM, at constant current, I don't think the LED characteristic itself is especially
relevant, as the instantaneous intensity doesn't change, so I'm guessing this is essentially an
optical perception question. Is there a major difference between colours (Orange, ~600nm in my case)
? I would imagine that white at least might be different to a single-wavelength source.
I've messed with some log based curves in Excel, which certainly work way better than a linear
mapping, but I'd like to try to get things closer for an app that uses antialiasing to smooth motion
on a fairly low-res bar-graph type display, and I think it can probably be less jerky than I'm
seeing at present.
provided you are above the flicker threshold frequency (decently
above to avoid Broca-Sulzer effects etc) at constant frequency the
effect is linear with on-time - the Talbot plateau effect -provided
the led(s) do not significantly vary in efficacy with duty cycle.
Provided you keep in the photopic the effect is independant of color.
google for 'Temporal Vision'
.
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- From: Mike Harrison
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