Optics: How would you determine wavelength?

[Aaron Van Rossum]

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.

 

[lekh2003]

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.

 

[nasu]

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.

 

[Aaron Van Rossum]

Ah ok now I understand. I’m going to have to take a different approach to this. Thanks!