This is really cool and very clever. But i want to raise one thing.

> designed a special color reference chart that can be printed on a card

My rudimentary understanding of physics makes me suspect this sentence is a simplification.

A normal printer use Cyan Magenta Yellow Black to print. A photo of such a print would already destroy alot of spectral information for the same reason the individual rgb sensors do.

So i suspect those colored dots are a very careful and deliberate concoction of very particular inks with very specific spectral color bands.

I suspect alot of effort went into finding, mixing and algoritmically combining the right inks.

I'm guessing it works similarly to a how a narrow band florescent lamp makes only materials that reflect a very specific frequency be visible, which makea alot of prints and pigments look wierd. (If you do the opposite; use ink with very specific spectral band, you can instead measure the lamp)

Insanely clever. (Whatever they did)

Not just that, but it would presumably be sensitive to light emission spectra too. As inks can only reflect wavelengths of light that hit them, if the emission spectra has spikes or gaps - think LED or florescent - the reflected spectra will be a function of the light source[1].

Perhaps there's some accounting for this, and I'm curious to learn what it is, because it's a phenomenally complex problem.

1. You might think the sun is a standard source, but it's usually modulated by the atmosphere[2].

2. Unless you are in space.

https://ieeexplore.ieee.org/ielx8/83/10795784/11125864/supp1...

We utilized a professional photographic inkjet printer (ImagePROGRAF PRO-1000, Canon), equipped with 11 ink cartridges and a chroma optimizer (PFI-1000 LUCIA PRO Ink, Canon), and used the manufacturer-recommended genuine paper (Photo Paper Premium Fine Art Smooth, Canon) for printing. To reproduce the desired reference colors for the spectral color chart, we also implemented a customized printing calibration process while maintaining the International Color Consortium (ICC) profile. The actual printed colors (output) showed notable distortions compared with the intended colors (input), which were particularly influenced by the type of paper (print sheet). For customized printing calibration, we mapped the exact relationship of the CIE xy chromaticity values between the digital color input and printed output values. After the printing process was completed, we measured the reflectance spectra of all reference colors from the printed spectral color chart (Fig. S1) using a spectrometer and a diffuse reflectance standard (equivalent to using CIE illuminant E). We confirmed that the CIE xy chromaticity values obtained from these measurements were in excellent agreement with the desired input values within the SWOP v2 gamut (Fig. 1(e)).

Based on supplementary material "We utilized a professional photographic inkjet printer (ImagePROGRAF PRO-1000, Canon), equipped with 11 ink cartridges and a chroma optimizer". Not your typical office printer but nowhere near as exotic as you might expect.

Wouldn't it be nice if they just told us so we didn't have to speculate? This is cool stuff and I'm glad I know about it but, as someone interested in this field of study, I'd love to try this out. But I guess I should stop being surprised when even a company like IEEE can't be bothered to write an article with any actual information. Just a bunch of simplified summarized crap.

I was initially surprised too, but I think there is a some trick...

I'm not sure what happens on paper, but when you have ink disolved in water the abortion is not linearly proportional to the concentration, it's exponential. For example, consider a red ink and 5 magical selected frequencies and the absortions at 1% of concentration are: 99%, 99%, 50%, 10%, 1%

If you double the ink at 2% you get 99.99%, 99.99%, 75%, 19%, 1.99%

So increasing or decreasing the ink concentration may give you information of different frequencies, even with only one ink and only one sensor. In this case mostly about the 3rd and 4th. With more concentration you may kill all the light in the 3rd and measure the absortions ratio between the 4th and the 5th.

One problem I see here is how to order them, but I guess it's possible with a few sensors and a few inks. Each sensor sees all the frequencies, but with different weight. I'm not sure if it's possible to solve this, but perhaps you need some initial approximated model of the inks(???).

Now, when you put the inks on paper and have a unreliable light source and perhaps other technical problems, ...

In conclusion, I think it's possible to use different saturation and mixes of the inks to get different spectral distribution of the light that bounce on the card. Then use the three sensors to get three averages and try to use big linear algebra book to reconstruct what happens in between. But I should read the paper to be sure.

I would assume that with time, you could just print it on a generic $30 inkjet, and calibrate the card, sensor, and whitelte balance to a stored reference image.

It won't be as accurate, but it might be enough to offer some insights into whether liquid photographed in the article is in fact whisky, not urine (which to me seems to be a much more noble demonstration subject).

Ink is perfectly capable of being a phosphor, in which case it'll up or down convert wavelength X to wavelength Y.

Printing can use so-called spot colors.