Optics: How would you determine wavelength?

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Discussion Overview

The discussion centers around the determination of wavelength in relation to how different colored objects reflect light, particularly focusing on a project involving light sources and sensors. Participants explore the implications of using specific wavelengths of light on various colored objects and how this affects perceived color.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant describes a scenario where blue light (450nm) illuminates a red object, questioning how the wavelength would differ if the object were white or if the light source were a white LED.
  • Another participant argues that a red object would appear black under blue light due to its absorption of non-red wavelengths, while a white object would reflect the blue light, suggesting predictability in outcomes based on object color.
  • A third participant states that if the object is a common reflector, the reflected light retains the same wavelength as the incoming light, emphasizing that the reflection coefficient varies by color. They express skepticism about determining an object's color based solely on reflectivity at one wavelength, noting that color perception is dependent on the entire spectrum of reflectivity.
  • A later reply indicates a shift in understanding, suggesting the participant will reconsider their approach based on the discussion.

Areas of Agreement / Disagreement

Participants express differing views on how color perception and wavelength reflection interact, with no consensus reached on the best approach to determine wavelength or color in the given scenarios.

Contextual Notes

Participants highlight the complexity of color perception, noting that reflectivity at a single wavelength may not provide a complete understanding of an object's color. The discussion reflects various assumptions about light interaction with colored objects.

Aaron Van Rossum
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If there is an light source that is emitting light with the wl of 450nm (blue) and it illuminated a low reflective red object. How would the wavelength be different then if the object was a white object or the emitted light is from a white led. How would we determine this. I am working on a project that shoots light at a certain wavelength at an object and determine its color through a sensor.
 
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For the first situation (blue light on red object), if there is only blue light on the red object, the object should appear black and not reflect anything, this is because the object would simply absorb all wavelengths of light which are not red. If the object is white, then I think you should be able to predict that. Similarly, you should be able to predict what would happen with a white led.
 
If you object is just a common reflector (no fluorescence), what you send is what you get. The reflected light has the same wavelength as the incoming light.
For the red object the reflection coefficient of blue light is much lower than that of red light. This is why we call it red.
But the wavelength is the same. I don't see how you could determine the color of an object by just measuring reflectivity at one single wavelength. Our notion "color of the object" depends on the whole spectrum of the reflectivity. And even if you know that, you can still have an object that looks blue to you and green to your wife (for example). Even though the spectral distribution of reflectivity does not change.
 
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Ah ok now I understand. I’m going to have to take a different approach to this. Thanks!
 

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