Yellow from an RGB LED, but not from ink mixing for example?

Morning,

I've just had a thought - I'm using an RG LED in an application, as in - Red and Green, no blue. Looking around at sources like this - http://uk.farnell.com/rgb-led-lighting-applications shows quite clearly that turning on both red and green at the same time gives yellow.

However, if you mixed red and green paints - you'd just get a dirty brown, not a nice yellow? Why this is?

I guess it's because you're combining two different wavelengths together or? hmm...

Thanks!

One is additive, one is subtractive.

Ah, i'll have having a good read of that later - something I've never really thought about until now...

Thanks!

Simply put -

If you mix primary ink colors together (Red, Blue, Yellow) you get black.

If you shine 3 primary color lamps (Red, Blue, Green) on a white wall, you get white!

Most computer images are stored in RGB and they have to be translated by the printer driver before printing.

...Now, what I find REALLY interesting is that there is no yellow coming from an RGB LED! It's your brain getting fooled. You can't "see" more than one color coming from one point. I haven't tried this, but I assume if you switch rapidly between red and green (say at 100Hz), you will also see yellow. This is unlike audio, where you can hear the whole band plus the singer coming from one speaker, all at the same time.

You can sum up the difference like this:

When you mix light sources of different colors together, you eventually get white.

When you mix paints of different colors together, you eventually get black.

EDIT: oops. A little late to this one. :slight_smile:

BTW, with practical experience, high PWM speeds make purer combination colors only if the LED is highly diffuse. I used a high-speed PWM with a bi-color LED and just saw both the red and green areas lit at the same time. Slowing the PWM down made it flicker a little, but the visual effect was more yellow and less red and green.

However, if you scuff the surface of that LED so the point sources become impossible to discern, then high-speed PWM looks better.

Now ya know. :smiley:

jtw11:
Ah, i'll have having a good read of that later - something I've never really thought about until now...

Thanks!

So you would rather wait to be spoon fed than find out an answer as it involves you in a bit of work.?

Not your brain, but your eye. You have 3 types of cones that are sensitive to different wavelengths.

And the three subtractive primaries are actually cyan, magenta and yellow. When you add those you get a slightly less muddy black than blue, red and yellow. 8^)

Grumpy_Mike:
So you would rather wait to be spoon fed than find out an answer as it involves you in a bit of work.?

If you had never heard of additive/subtractive mixing, how would you know to look it up?

There are I think two types of subtractive mixing, just to make it more complex!

When you have pigment particles as in paint, each particle absorbs various wavelengths
and reflects what is left over. The eye sees the result of adding the reflected light from
the several kinds of particle present. Yellow particles reflect red and green, Magenta
reflects red and blue, so mixing yellow and magenta particles gives mainly red with some
green and blue - the colours are muddy.

As a result pigment paints are not able to produce bright colours by mixing, you have
to use a larger range of pigments. An artists pallet includes far more than 3 paints!

Dye molecules are far far smaller than pigment particles and every photon sees all
the different dye molecules - the absorption spectrum is truly subtractive, a mix of yellow
and magenta dyes can only transmit red.

In old positive colour slide film the three layers in the film have yellow, magenta and
cyan dyes that are separate in space, but because they are in layers light has to pass
through each layer and thus they act in a purely subtractive way despite not being
intimately mixed.

Ink drops from an ink jet printer are not in well defined layers, so that colour mixing
is more like pigments than dyes, but depends a lot on the paper used (better mixing
while the inks are still wet makes better colour perhaps?). My printer uses two
shades of yellow, two of magenta and two of cyan to improve the colour gamut.

Inkjet printers also include lighter shades of the primary inks to avoid the speckled look. For instance, if a color required only a little magenta, the printer would have to print widely spaced dots of the usual dark magenta, and they would show. But with a lighter magenta, it can print more dots of the lighter shade.

Grumpy_Mike:

jtw11:
Ah, i’ll have having a good read of that later - something I’ve never really thought about until now…

Thanks!

So you would rather wait to be spoon fed than find out an answer as it involves you in a bit of work.?

Yes… that or I was working on something else that needed my attention, such as work, and would read the wiki pages later that day.

Anyway, to all those who replied with constructive comments - many thanks. The LEDs I’m using are highly diffused, as I have seen reference to issues with PWM and true colour mixing before.

Inkjet printers also include lighter shades of the primary inks to avoid the speckled look

Not always. There are color printers that use anything from 3 to 10+ colors of ink, with varying degrees of quality and resolution. Some have different inks depending on whether you have glossy or matte paper!

I didn't say that printers -only- have lighter shades, or that it is only to avoid the speckled look. Some printers have specific shades of orange and green inks, simply because it is difficult to get certain shades of those colors. Anyone can do a quick search for "color gamut" (now that you know what words to use). Your monitor is capable of much brighter colors than you can print on any paper with any ink, but a printed image may be capable of much darker blacks.

And yes, some inks are different for matte and glossy papers. There may even be lighter shades of black, both to avoid the speckled black look, and because trying to do darker shades of grey by just mixing color inks often results in a muddy brownish grey.

Add to all this, because our eyes detect bands of color, inks print with selections of colors, and light in various places has different compositions, something printed to look fine under one kind of lighting may look different shades under different lighting. Look up "metamerism".

And that LED that looks like different colors close-up, may start to look more white at longer distances.