Dear light guru of Concept art, Reflective light question

# Thread: Dear light guru of Concept art, Reflective light question

1. ## Dear light guru of Concept art, Reflective light question

Hey again all, I have another question about light. There is one thing that is confusing me and it would be nice if someone could explain what i am missing.

Diffuse reflection absorbs all other spectrum of light and reflects the colour of the object, ( red box reflects red light waves but absorbs the other )

but ive been trying to work out why reflected light is coloured on objects that should in theory absorb that spectrum.

I went outside with a orange piece of card and held it up to a blue towel, and the towel had orange reflected light, but if this light is diffuse shouldn't the light be absorbed. So my question is what light is it that is reflecting back to my eye. If it is specular shouldn't it move with the angle of my view.

I have read through HueValueChroma which was very VERY helpful, and to my knowledge this is called subjective mixing because it is subtracting different spectrum's of light but im am surely missing something as i cant figure out why the light is coloured

in my example i have red light on the green box, This is something i see commonly in paintings with coloured reflections under chins reflecting from the persons shirt and even see this in life. But i don't understand why. I would much rather understand why this happens then just painting it because it happens

I hope what i am trying to ask is clear
and thank you for taking the time to read though this

2. I think you're overcomplicating it a bit. The reflected light from a colored object/surface is mainly made up of that wavelength of light (red in your example). That is why our eye perceives it as red - that is the wavelength. So "reddish" light is what is bounced. From that point it depends on all the usual variables - local color and surface qualities of the object that the reddish reflected light then illuminates.

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As Jeff said, you see colour because of the absence (or lower quantity of) some wavelenghts.
Most objects do not absorb light on an all or nothing basis - If they did, a blue towel would never appear any other colour, and if you hit it with a coloured light lacking the blue wavelenghts, it would simply appear black by not reflecting any light at all (since all light would be absorbed).
Thats not what happens, instead you get a muddy colour because most of the light is absorbed, but not all.

Check google images for "spectrophotometry + absorbance spectrum" - This is essentially a mapping of the amount of light absorbed by different materials. Notice how there are a lot of wavelengths that are only partially absorbed, and that hardly any materials absorb ALL of a specific wavelength.

Depending on the dye used to colour the towel, most other wavelenghts would be reduced when reflected by the towel, but give it a high dose of mainly orange light and some of the orange will still be reflected.

Also note that a specular reflection does absorb a slight bit of light, just not as much as the diffuse reflection. A coloured mirror is a good example of this.

Hope this makes sense.

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6. Originally Posted by JeffX99
I think you're overcomplicating it a bit. The reflected light from a colored object/surface is mainly made up of that wavelength of light (red in your example). That is why our eye perceives it as red - that is the wavelength. So "reddish" light is what is bounced. From that point it depends on all the usual variables - local color and surface qualities of the object that the reddish reflected light then illuminates.
Jeff thank you, Once again it appears i had over complicated things in my head for no reason haha

Originally Posted by AndreM
As Jeff said, you see colour because of the absence (or lower quantity of) some wavelenghts.
Most objects do not absorb light on an all or nothing basis - If they did, a blue towel would never appear any other colour, and if you hit it with a coloured light lacking the blue wavelenghts, it would simply appear black by not reflecting any light at all (since all light would be absorbed).
Thats not what happens, instead you get a muddy colour because most of the light is absorbed, but not all.

Check google images for "spectrophotometry + absorbance spectrum" - This is essentially a mapping of the amount of light absorbed by different materials. Notice how there are a lot of wavelengths that are only partially absorbed, and that hardly any materials absorb ALL of a specific wavelength.

Depending on the dye used to colour the towel, most other wavelenghts would be reduced when reflected by the towel, but give it a high dose of mainly orange light and some of the orange will still be reflected.

Also note that a specular reflection does absorb a slight bit of light, just not as much as the diffuse reflection. A coloured mirror is a good example of this.

Hope this makes sense.
Andre thank you, I had assumed that it was a all or nothing mechanic but your explanation cleared it up 100% thank you

-Jerry

7. JerryActric

AndreM is exactly right, but it's an understandable mistake because a lot of books get this wrong. I recently put up some graphs showing the range of wavelengths reflected by various coloured paints on a new page that you may have missed:

http://www.huevaluechroma.com/045.php

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