How Does VIS-NIR Reflectance Linearity Vary in Mixed Color Pigments?

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Discussion Overview

The discussion centers on the physics of reflectance in mixed color pigments, specifically in the visible (VIS) and near-infrared (NIR) regions (400-1000nm). Participants explore the underlying physical properties that influence reflectance readings and the linearity of these readings when pigments are mixed.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether reflectance readings in the VIS and NIR regions represent specific physical properties of atoms, noting that NIR reflectance is attributed to vibration overtones and asking what causes reflections in the VIS region.
  • Another participant suggests that there are various processes that scatter light in both the NIR and VIS regions, indicating that the topic is complex and not easily simplified.
  • A later reply mentions that processes at the energy of visible light involve electron transitions between allowed energy levels within atoms.
  • One participant discusses the concept of linear unmixing for estimating pigment ratios in mixtures, questioning whether linearity is consistent across the entire observed range or varies in specific regions.
  • Another participant emphasizes that the perceived color hue of pigments depends on their spectral reflectance across the visible spectrum and that different pigments may not have similar reflectance spectra.
  • References to the fields of colorimetry and spectrophotometry are made as relevant areas for further exploration.

Areas of Agreement / Disagreement

Participants express differing views on the complexity of the processes involved in reflectance and the linearity of unmixing in pigment mixtures. There is no consensus on the specific mechanisms causing reflections in the VIS region or the consistency of linearity across the spectrum.

Contextual Notes

Participants note that the field of pigment chemistry is vast and complex, with many factors influencing reflectance that are not easily generalized. The discussion highlights the need for specificity in questions regarding reflectance and pigment interactions.

XonDoi
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Hi everyone,

I am a student studying color pigment fingerprints and how we can improve their identification. Especially in cases where these are mixed with each other to create variants. We are using the VIS and NIR regions (400-1000nm) to obtain our observations. I have a few questions regarding the physics behind the reflectance (at molecular level) in these regions:

1. In the VIS and NIR regions, do the reflectance readings represent specific physical properties of the atoms? For example, I've read that in the NIR region reflectance is caused by vibration overtones. What causes these reflections in the VIS region?

2. The second question might be a bit off topic. When two pure elements - say pigment blue and pigment green - are mixed together, linear unmixing can be used to estimate the ratio of the elements in the mixture. There is some linearity (not taking into consideration scattering... etc). Is this linearity equal throughout the entire range being observed (400-1000nm)? Or are certain areas like absorbance/reflectance bands in VIS region more linear than others (NIR regions).

I'd appreciate any insight especially that which can be sourced/cited.

Thanks:)
 
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You are seeking simple answers where I am afraid there are none.
(1) there are many various processes that scatter NIR and more processes that scatter visible.
(2) the linear unmixing typically uses the entire spectrum of interest and finds the best admixture of pigments to RMS fit the desired spectrum when all colors wavelength are included. There is no reason certain colors cannot be excluded...again it depends upon exactly what you wish to accomplish.

You need to be much more specific. This is an old and very well investigated field of study. Perhaps you should start be looking into metamerism
 
hutchphd said:
(1) there are many various processes that scatter NIR and more processes that scatter visible.

Hello, could you name any of these processes such that I could look into them?

Also my original question, if rotational spectroscopy is used for the Far-IR, rotational-vibrational in the Long-IR, Vibration overtones in the Near-IR... What causes these reflections in the visible region?

I am not a graduate in physics and hence my knowledge in this field is limited to the basics
 
Many of the processes at the energy of visible light involve the transition of electrons between allowed energy levels within atoms.
 
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XonDoi said:
1. In the VIS and NIR regions, do the reflectance readings represent specific physical properties of the atoms? For example, I've read that in the NIR region reflectance is caused by vibration overtones. What causes these reflections in the VIS region?

For condensed matter (liquid and solid pigment), not really- there is indeed absorption and reflection, but the absorption band is not a single frequency. The specific hue of a specific pigment depends on the detailed chemistry used and the enormity of the field 'pigment chemistry' can't be overstated:

https://onlinelibrary.wiley.com/doi/10.1002/14356007.a20_243.pub3

XonDoi said:
2. The second question might be a bit off topic. When two pure elements - say pigment blue and pigment green - are mixed together, linear unmixing can be used to estimate the ratio of the elements in the mixture. There is some linearity (not taking into consideration scattering... etc). Is this linearity equal throughout the entire range being observed (400-1000nm)? Or are certain areas like absorbance/reflectance bands in VIS region more linear than others (NIR regions).

Again, there is no simple answer to this- the perceived color hue of a particular pigment depends on the spectral reflectance across the entire visible spectrum; there is no reason to assume that two different pigments have similar, shifted, reflectance spectra. Also, depending on the chemical formulation, those two different pigments may have the same binder- for example, it turns out that all US military paint has a spectral feature around 9.8 um (IIRC) while Russian paint has a spectral feature around 10 um (IIRC), regardless of the visible color appearance.

The fields you are asking to learn more about are 'colorimetry' and 'spectrophotometry'.

Does this help?
 
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