Recreating Spectral Response from 6-Channel Color Sensor

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Discussion Overview

The discussion revolves around the feasibility of recreating a spectral response using a 6-channel color sensor (AMS AS7262) with overlapping channels. Participants explore the challenges and methodologies related to spectral measurement, calibration, and the implications of color perception in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests normalizing each channel and computing wavelengths based on ratios of the different channels, referencing how color vision typically operates.
  • Another participant expresses uncertainty about the "mapping function" and mentions the complexity of using Fourier transforms, while also considering a matrix calibration approach involving sensor outputs and spectral response.
  • A question is raised about the operational definition of "accurate spectral response" and whether the goal is to create a high-precision spectrograph or a colorimeter, noting that these are distinct but related objectives.
  • One participant proposes using a least squares fit of the reference curve as a potential method for calibration.
  • Concerns are raised about the limitations of the 6-channel sensor in resolving individual wavelengths due to the nature of tristimulus color vision and the potential for metamers.
  • Another participant discusses the relationship between the sensor's six fundamental "camera-colors" and human color perception, highlighting the complexities of intermapping these systems and the mathematics involved in integrating sensor sensitivity over the spectrum.

Areas of Agreement / Disagreement

Participants express various viewpoints and methodologies without reaching a consensus. There are competing ideas regarding the best approach to calibrate the sensor and the implications of color perception on the accuracy of spectral response.

Contextual Notes

Participants note the limitations of the equipment and the complexity of the mathematical approaches discussed, as well as the need for clarity on the specific goals of the project.

AxisCat
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Hi All,

This is a project I started a couple of years ago then got stuck or bored and stopped working on it. I am thinking about picking back up where I left off. I am using a AMS AS7262 color sensor with the following overlapping channels: 450, 500, 550, 570, 600 and 650 nm, each with 40nm FWHM.

I used a Digikröm CM110 Monochromator and stepped through the spectrum in 1nm increments saving the output from the sensor to a file. My light source was a standard 100 watt halogen household bulb. This is what I measured:
chart.jpg

The continuous dark blue curve is from a photodiode I am trying to use as a reference. I understand this is really hard stuff to do with the limited equipment I have available. Is it even possible to recreate a fairly accurate spectral response using just these 6 discrete channels? The manufacturer provides some information that eludes to it being possible.

Anyone have any thoughts?

Thanks,
Axis
 
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AxisCat said:
Hi All,

This is a project I started a couple of years ago then got stuck or bored and stopped working on it. I am thinking about picking back up where I left off. I am using a AMS AS7262 color sensor with the following overlapping channels: 450, 500, 550, 570, 600 and 650 nm, each with 40nm FWHM.

I used a Digikröm CM110 Monochromator and stepped through the spectrum in 1nm increments saving the output from the sensor to a file. My light source was a standard 100 watt halogen household bulb. This is what I measured:
View attachment 336147
The continuous dark blue curve is from a photodiode I am trying to use as a reference. I understand this is really hard stuff to do with the limited equipment I have available. Is it even possible to recreate a fairly accurate spectral response using just these 6 discrete channels? The manufacturer provides some information that eludes to it being possible.

Anyone have any thoughts?

Thanks,
Axis

Off the top of my head, I suggest that you first normalize each channel and then assign/compute wavelengths according to ratios of the different channels- that's generally how color vision works.
 
I appreciate the reply. The "mapping function" is the part I am stuck on. Someone suggested Fourier transforms because the curves looked like filters and the math can be reversed. That looked really complicated so I haven't researched it any further. Another person's thought was using a matrix to calibrate. The input matrix would be 6x1 (sensor output x 6-channels), the correction matrix would be 6x300 (700nm - 400nm), and the output matrix would be a 1x300 representing the spectral response from 400-700nm. I really don't know which direction to take with this.
 
What does

AxisCat said:
accurate spectral response

mean to you operationally? What does accuracy mean in this context? If you desire spectral response you want to use a very high precision spectrograph.....that is what they do. Or are you trying to make specific colorimeter? I have designed each. They are different things although obviously not disjoint.
So you need to specify exactly what you wish to do.
 
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Why don't you just do a least squares fit of the reference curve?
 
AxisCat said:
Anyone have any thoughts?
This sounds interesting.
Bearing in mind how our tristimulus colour vision can give a matching colour for many combinations of source spectra (metamers), does this six channel sensor not have the same potential problem? I can see how it will get a correct wavelength for monochromatic light but for light with multiple wavelengths, how can it resolve the individual wavelengths? Albeit with better accuracy.
What would be your application for this sensor?
 
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AxisCat said:
I appreciate the reply.
So you have a camera that sees six different fundamentall "camera-colors" as defined by each of the six types of sensors.
Your eye sees three fundamental "eye-colors" for similar reasons. It constructs any color by recognizing the admixture of the three sensors.
Clearly one can create correspondences between these two systems, but there will be serious uniqueness questions for any intermapping. The physics just involves integrating each sensor sensitivity over the (object+illuminant) spectrum. This is the mathematics of color filters.
 
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