On Mixing Colors of Light

[Charles Link]

And what about my question about Magenta? Does that have a wavelength?

I had to google that=magenta is not typically in my every day vocabulary, but I see it is a mixture of blue and red. Magenta is something a prism spectrometer would also split up into basically a blue grouping and a red grouping. I do think it can be worthwhile for the next generation to address items like even magenta in the discussion, rather than to have the mindset that everyone must know that since it seems to have been around for as long as I can remember. Cheers. :)

 

[Charles Link]

A photodiode is non-linear. That's how it works

For EE's who work with voltage, (or electric field strength), perhaps they might consider a photodiode to be non-linear, but for an optics person, the photodiode, basically counting photons, with a photocurrent proportional to the photon count, (proportional to second power of the electric field amplitude), is about as linear as you can get. I've measured their linearity over several orders of magnitude (of incident light level), and could not detect any noticeable non-linearity. That is getting off-topic, but since the topic came up, I think it is worth addressing it.

 

[renormalize]

For EE's who work with voltage, (or electric field strength), perhaps they might consider a photodiode to be non-linear, but for an optics person, the photodiode, basically counting photons, with a photocurrent proportional to the photon count, (proportional to second power of the electric field amplitude), is about as linear as you can get. I've measured their linearity over several orders of magnitude (of incident light level), and could not detect any noticeable non-linearity. That is getting off-topic, but since the topic came up, I think it is worth addressing it.

I am confused by your exchange with @sophiecentaur. This link https://en.wikipedia.org/wiki/Optical_heterodyne_detection states:
"The comparison of the two light signals is typically accomplished by combining them in a photodiode detector, which has a response that is linear in energy, and hence quadratic in amplitude of electromagnetic field. Typically, the two light frequencies are similar enough that their difference or beat frequency produced by the detector is in the radio or microwave band that can be conveniently processed by electronic means."
So yes, a photodiode is indeed inherently linear in detecting energy (counting photons) as you say you measured, but it is inherently nonlinear (i.e., quadratic) in detecting amplitude. It is this nonlinearity that is responsible for mixing the amplitudes of optical signals with slightly differing frequencies that results in a measurable beat frequency. That's why @sophiecentaur correctly says that photodiode nonlinearity is responsible for producing the observable RF difference signal.

 

[Charles Link]

I am confused by your exchange with @sophiecentaur. This link https://en.wikipedia.org/wiki/Optical_heterodyne_detection states:
"The comparison of the two light signals is typically accomplished by combining them in a photodiode detector, which has a response that is linear in energy, and hence quadratic in amplitude of electromagnetic field. Typically, the two light frequencies are similar enough that their difference or beat frequency produced by the detector is in the radio or microwave band that can be conveniently processed by electronic means."
So yes, a photodiode is indeed inherently linear in detecting energy (counting photons) as you say you measured, but it is inherently nonlinear (i.e., quadratic) in detecting amplitude. It is this nonlinearity that is responsible for mixing the amplitudes of optical signals with slightly differing frequencies that results in a measurable beat frequency. That's why @sophiecentaur correctly says that photodiode nonlinearity is responsible for producing the observable RF difference signal.

In optics, we would never consider a photodiode to be non-linear. That's where in some cases, we are crossing areas of specialization. I was very much a spectroscopist, and almost always worked with wavelengths. He seems to have done a fair amount of work with mixing colors, (more than I have), and has probably been familiar with the CIE color map for many years, something whose details I only got very familiar with in the last 3 weeks. We seem to disagree greatly on how to address an audience that may be largely made up of beginners. I'm trying to take it all in stride.

 

[sophiecentaur]

Typically, the two light frequencies are similar enough that their difference or beat frequency produced by the detector is in the radio or microwave band that can be conveniently processed by electronic means."

Exactly. The intermodulation product is not at a frequency of visible light but a beat frequency in the microwave region. The mechanism by which two different coloured light sources can give the perception of another different coloured source is entirely because of the analysis curves of the eye and the way the three signals are processed in the brain.. The photodiode bit is a complete red herring.

@Charles Link has still not acknowledged the absolute distinction between spectrum and colour.

 

[Charles Link]

The difference in frequency is often created by an acousto-optic modulator. That allows for the original reference frequency along with one upshifted or down-shifted by the acoustic frequency. The two beams are combined onto a photodiode, and it is really a question of semantics=I know the photodiode as a linear device.

 

[sophiecentaur]

We seem to disagree greatly on how to address an audience that may be largely made up of beginners.

I can't think of an 'audience' for this topic except students of a specialist field in Colour reproduction systems. Anyone delivering useful lectures to such an audience would be a specialist in the field and I can't see why Charles Link keeps talking in terms of presenting his ideas to 'beginners'. It could only confuse or baffle an audience of straightforward Physics or Engineering students or possibly Broadcast Engineers.

 

[Charles Link]

I can't think of an 'audience' for this topic

From what I can tell, we don't have much of an audience in any case. We are arguing about some details that could largely be due to me being very much a spectroscopist versus someone who may have had much experience in the color television industry. We can thank our lucky stars though that we can both see good enough to be able to argue what we might see when we see a color scene. I have known at least a couple of people who were not able to see, and life for them was very challenging. One was bright enough though, that she was able to tutor calculus. (She went blind around the age of 24, and at 60 she was still tutoring calculus to high school students). I thought I made some worthwhile posts throughout the thread, but perhaps I am mistaken. In any case, I am a retired spectroscopist, and that was my major area of specialization from early-on. Cheers. :)

 

[DaveC426913]

I can't think of an 'audience' for this topic except students of a specialist field in Colour reproduction systems. Anyone delivering useful lectures to such an audience would be a specialist in the field and I can't see why Charles Link keeps talking in terms of presenting his ideas to 'beginners'. It could only confuse or baffle an audience of straightforward Physics or Engineering students or possibly Broadcast Engineers.

Which is why Wiki would be the perfect place to store this. Anyone at any level can avail themselves of this knowledge as they see fit. That's what it's for.

The OP has said he prefers to discuss with PF denizens, because they might have a commensurate level of interest and knowledge - yet in the same breath, he is saying he's targeting it to beginners.

 

[Charles Link]

The OP has said he prefers to discuss with PF denizens, because they might have a commensurate level of interest and knowledge - yet in the same breath, he is saying he's targeting it to beginners.

I'm glad I posted what I did, but I'm not getting very much constructive feedback. Perhaps the posts (mine) are lacking in good quality content, so I may be asking for too much. In any case, it's going to be 20 degrees here in Chicago for the next week or two or more, but at least the coffee is good at Starbucks. Cheers. :)

 

[renormalize]

I know the photodiode as a linear device.

Yes, to intensity, but not to amplitude, which is why photodiodes can be used for optical heterodyning. Do you understand and acknowledge the distinction?

 

[Charles Link]

Yes, to intensity, but not to amplitude, which is why photodiodes can be used for optical heterodyning. Do you understand and acknowledge the distinction?

Of course. Many r-f engineers work with electric field amplitudes, but in Optics we are working with photons=energy. I'm well familiar with the classical E&M, and in Optics we simplify the formula/units and say intensity $I=n E^2$, where $n$ is index of refraction and $E$ is electric field amplitude.

Meanwhile the photodiode response is typically specified as amperes(photocurrent)/watt which is the same as Coulombs/joule and the response is wavelength dependent, largely because the photon has energy inversely proportional to wavelength. ($E_p=hc/ \lambda$). Silicon photodiodes are used very much in the visible and cut off in the near infrared at a wavelength around 1.0 microns. Typically a photodiode (the older larger ones) receives a couple microwatts of incident light that results in a couple microamps of current. Nowadays in cell-phone cameras I think they are working at far lower levels of power and photocurrent.

 

[sophiecentaur]

Yes, to intensity, but not to amplitude, which is why photodiodes can be used for optical heterodyning. Do you understand and acknowledge the distinction?

There are many examples of devices with a non linear response but the design criteria will not not include very low loss at optical frequencies. A photodiode (with very low loss) would presumeably be great at counting photons. That same low loss (quantum efficiency) will make it good in the hetrodyning mode for detecting and analysing low levels of light. Two birds with one stone.

 

[Charles Link]

@sophiecentaur I would like to respond further to your post 118. I do think the students do benefit greatly to think in terms of wavelength(s), and are hardly poor if they go in that direction. I also would encourage them to not only have an understanding of what a prism spectrometer is, but to also know some of the details of the workings of a diffraction grating type spectrometer. See https://www.physicsforums.com/insights/fundamentals-of-the-diffraction-grating-spectrometer/

Even if they do begin to think in terms of wavelength, I still think they could also benefit by learning about the CIE color coordinates and map, which is discussed in some detail on the first page of this thread, beginning around posts 16 and 20. I'm not sure where the job market is going these days, but IMO one really doesn't go wrong if they make up their mind early-on to try to be as complete a physicist as possible.

 

[sophiecentaur]

I do think the students do benefit greatly to think in terms of wavelength(s), and are hardly poor if they go in that direction

Students are already familiar with the quantities involved in optics and general wave theory. Of course, they think in terms of wavelength and frequency. How else?

Why would colourimetry be taught in a Physics course? Colour vision is more PsychoPhysics than Physics. We all have to teach to the test if you want good exam results.

I have suggested using a computer monitor and a simple image editing package to see (on a subjective level) how R, G and B values relate to recognisable colours. Have you tried that exercise? You wouldn't even need to get up from the chair you're sitting on to do it. GIMP is vast and it is free.

 

[Charles Link]

I still encourage readers to read the OP thoroughly as well as the "link" in post 9: See https://www.physicsforums.com/threads/color-theory-what-does-blue-yellow-make.1008903/

Even though I have worked for many years (now retired) doing things like spectroscopy, this topic was really very new to me, (Mixing Colors, whether light or pigments), and it really isn't IMO written up very well in very many places.

 

[DaveC426913]

... and it really isn't IMO written up very well in very many places.

So write it up then. No one can do a better job than the guy who is motivated to do so.

Start with Wiki. They will automatically provide feedback.

 

[Charles Link]

I have suggested using a computer monitor and a simple image editing package to see (on a subjective level) how R, G and B values relate to recognisable colours. Have you tried that exercise? You wouldn't even need to get up from the chair you're sitting on to do it. GIMP is vast and it is free.

See https://www.luxalight.eu/en/cie-convertor that I linked previously in post 78. These days I am working with a simple Chromebook so I am very limited with what I am able to download. I did find the program in this link useful though.

Edit: I anticipate when you type in 580 nm or 590 nm into this program and get yellow on your screen, it is actually coming from a combination of a red LED and a green LED, with wavelengths (perhaps somewhat broadband) in the range of 550 nm and 650 nm respectively. It even gives you the color coordinates, (x and y), so it is basically telling you the proportions (approximately) of red and green that it used to create the color displayed on the screen. Note that the blue coordinate z=1-x-y.

 

[sophiecentaur]

These days I am working with a simple Chromebook so I am very limited with what I am able to download.

GIMP is available on a Chromebook. Worth the effort to find the colour picker or the colour 'dropper', whichever you can find. Load a CIE chart Into GIMP and play away.

 

[Charles Link]

Worth the effort to find the colour picker or the colour 'dropper',

I found the color picker. I'm still working on it though to get color coordinates etc. The software is not self-explanatory.

 

[sophiecentaur]

I found the color picker. I'm still working on it though to get color coordinates etc. The software is not self-explanatory.

It was a long time ago that I used GIMP but I seem to remember a little 'dropper' icon which breaks out a magnified sample of the image (on the mouse position) it shows the RGB values of the central pixel. A CIE chart is interesting to scanover with the dropper.

 

[Charles Link]

It was a long time ago that I used GIMP but I seem to remember a little 'dropper' icon which breaks out a magnified sample of the image (on the mouse position) it shows the RGB values of the central pixel. A CIE chart is interesting to scanover with the dropper.

I'm starting to figure it out thanks. If you work with RGB, those are each numbered 0 to 255, but as you change those, it changes the other values in the display, like CMY, etc.

See https://fixthephoto.com/online-gimp-editor.html

 

[sophiecentaur]

but as you change those, it changes the other values in the display

What does that mean? As you move between areas on the CIE chart, why shouldn't more than one value change?

Did you notice that there are very few objects in a scene with one of RGB being near zero? Saturated colours are rare in everyday scenes.

 

[Charles Link]

See https://fixthephoto.com/online-gimp-editor.html

I'm still working on it, but I'm beginning to figure it out. :)

 

[Charles Link]

I've gotten very little feedback from the section on page 3 of posts 75-78, ( just one person responded there), but I think a couple readers might find that section of interest. Using the software in the link in post 75, it is easy to show that my original post, the OP, is indeed what you can get, with the one correction being that the yellow is from around 580-590 nm, rather than at 600 nm.

 

[sophiecentaur]

a couple readers might find that section of interest

Did you consider that what you are saying may not be as 'right' as you think it is.? After a lot of posts between you and me, you still don't seem to have taken on board what I have been saying. You just seem to ignore very important points that I have made. Your terminology and basics are still the same as when you started on this thread. You have to allow yourself to have your ideas changed but I get the feeling that you just don't want to be wrong. Just 'bending' what you read here to fit your ideas won't get you anywhere.

 

[Charles Link]

Did you consider that what you are saying may not be as 'right' as you think it is.? After a lot of posts between you and me, you still don't seem to have taken on board what I have been saying. You just seem to ignore very important points that I have made. Your terminology and basics are still the same as when you started on this thread. You have to allow yourself to have your ideas changed but I get the feeling that you just don't want to be wrong. Just 'bending' what you read here to fit your ideas won't get you anywhere.

It is certainly possible it isn't completely correct. The CIE coordinates and their vector space assume a linearity of the human response, and in that sense it isn't a perfect model. Using the CIE map though, what I proposed in the OP and post 75 is in agreement.

Perhaps I would do well to move onto some other topic though. The E&M with its vector calculus and things like magnetostatics might be worth revisiting, but that will have to be in some other new thread= and there is always the possibility that I erred in the computation of the addition of a couple of simple vectors, (in posts 75-78), but do you know that $\nabla ( a \cdot b)=a \cdot \nabla b+b \cdot \nabla a +a \times \nabla \times b+b \times \nabla \times a$ ?, (where $a=\vec{a}$ and $b=\vec{b}$), a vector identity that can be useful in some E&M work. I don't know everything either, but I do try to make the posts somewhat interesting. Hopefully I didn't bore you too much. Cheers. :)

 

[DaveC426913]

Something that I only recently cottoned on to is that, in the early days, camera film had really bad red sensitivity so spectral measurements had very little information about red (or IR). The universe looked very different in them thar days.

That's also why darkrooms are lit with red light, even in TV and films. The photo paper is insensitive to far red, though we can can pick it up with our eyes.

 

[sophiecentaur]

That's also why darkrooms are lit with red light, even in TV and films. The photo paper is insensitive to far red, though we can can pick it up with our eyes.

Very historical. I can’t bring myself to ditch my old film cameras but it wasn’t a good medium.

 

[DaveC426913]

Very historical. I can’t bring myself to ditch my old film cameras but it wasn’t a good medium.

Still got my Pentax K-1000 from college.