Questions on LED PWM

Some articles about it:

Looks like it is logarithmic. Another example is the magnitude value used for measuring star brigthness:

This means the perceived intensity p=2.5*log(r), with r=real intensity =>

r=10^(p/2.5). Maybe there are some additional non-linearities if your PWM rate is near 0% or near 100%, but for good looking results use a high resolution PWM and use a logarithmic lookup table.

Another interesting article about intensity correction:

Look at some of the LED driver devices, with PWM. That will give you an idea of ranges - often they have a RGB section, and a separate brightness control, so that you are not relying on the Colour bits for wide dynamic range. Depends if you want this to operate over a daylight/nightime dynamic range, or just a narrower range.

-jg

I suspect you will want to multiplex drive the display, just to save wires, drivers, etc. That means you will have a time pattern to fire each one. The only reasonable (IMO) control of intensity will be the length of activation pulse in that pattern (which can be delivered to only one end, if desired). This lets you fool with the intensities with a value to time converter somewhere.

Note that you can deliver the three colors in three different time slots, provided the multiplex frequency is high enough.

Op Mon, 06 Oct 2008 04:24:53 +0200 schreef Mike Silva :

Obviously, this depends heavily on the output power and the transition time. But if it is not enough, you could think about a simple routine that will transition between two 16-bit colours in several steps using

Depending on what you are displaying, you might find that the difference between 00000/00000/00000(all off) and 00000/00000/00001 is especially noticable, and that the color shifts at the lower end of brightness -- not enough resolution to keep the ratios right. An old trick is to use that extra bit to reduce the brightness of all three colors by a factor of 4, thus giving 7 bits of resolution at the dim end of the range.

That's a very clever idea and I'll be sure to look at it. Since I was going to manipulate the colors as 24 bit data anyway (and only transmit it as 16 bits) this sounds like a good fit. I was definitely thinking that that first level from all OFF would be too large a step, and that the colors at low resolution would be off (but since these will be very abstract patterns this latter may not matter at all, and may even add to the effect).

And thanks to all who posted links and other info.

If you want a reasonably smooth fade, 256 levels per colour would be an absolute minumum.

absolute minumum.

I agree (well, unless the changes are going to take place rather quickly like a second or two).

The eye is insensitive to small brightness differences, but quite sensitive to small changes in brightness. Best regards, Spehro Pefhany