# Thread: How exactly does the reflection of color off surfaces onto other surfaces work?

1. Registered User Level 1 Gladiator: Andabatae
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## How exactly does the reflection of color off surfaces onto other surfaces work?

We know that the color of surfaces can be reflected onto other surfaces, seemingly dependant (from my observation) on strength of light, closeness of surface to the other surface, and smoothness/roughness of surfaces.

-> Where a smoother surface would reflect more of its' color onto another surface, than a rougher surface, under the same light source/strength of light.
-> And a smoother surface would recieve more color off another surface, than a rougher surface, under the same light source/strength of light.

-> And the stronger the light hitting a surface, the more color would be reflected off it.

Then it seems to me:
-> The closer a surface to another surface, the more color would be reflected onto the other surface. (At least on rougher surfaces)

And that last point is also one of the reasons for my questioning.
So surfaces always have two properties regarding light, determining their look.
-> One property is: Which colors, which wavelengths of light it absorbs the most, and following from that, which colors it will reflect to the viewer, giving it its color.
-> The other is: How specularly reflective the surface is. Depending on surface roughness/smoothess. How much more mirror like the surface reflects light. Being more rough, having more diffusion and surface angles into different directions.

These two properties are, if I don't remember it wrong now, coming from a more microscopic level. Depending on the density of a surface, color is given from light entering a surface, being diffused and coming out again, while some spectrum of light gets lost inside. And if the surface is completely dense, we would have a mirror.

Please do correct me if I am wrong.

So back to my problem again. When we know, light comes in in one angle, and goes out in an angle to the opposite direction, which we know from the behaviour of light looking into a mirror, every specular reflection should be acting this way. Which means, the specular reflection on a surface changes, depending on which angle it is viewed from.

Now, I have observed exactly this in some tests I made with surface color being reflected onto another surface, especially in matte surfaces. Where I could explain the behaviour of the reflected color, with specular reflection.
For example, changing my viewing angle to a matte, colored toy cube, laying on a white piece of paper, and the reflected white from the paper on the cube adding up, and lessening respectively.
BUT in some cases, I was totally lost as to why the light was reflected the way it was.

Here are two cases, I couldn't explain:
We have blue curtains hanging closed in front of our windows. When viewed from the side, there is some space between the wall next to the windows and the curtains, so light can shine in from the sides.
I can see blue light is reflected from the curtains onto the white wall, depending on how much I close the curtain (move it closer to the wall, more and more). That got me wondering, how is this possible? When I explained to myself earlier, that the surface color reflection onto another surface can only be explained by specular reflection, otherwise the other colors would be absorbed. Because no matter how I changed my viewing angle, the blue light radius on the wall always remained the same. And especially, there shouldn't even be any blue reflected light from the angle that I was looking.
Is this explained by the roughness of the surface of the wall? Making the blue reflected light be reflected back into my viewing angle, no matter from where I was looking, because of it's diffused surface angles?
Which would basically mean, specular and diffused reflection are working together here, to give this effect?

I have matte cube again, or any other matte surface, in this case standing vertically in a 90 degree angle to the floor.
Now I slowly move some white paper on the floor closer and closer to the vertical surface. What happens, the closer the paper gets, the more white color is reflected onto the vertical surface.
Again I struggled to explain this to myself. If I had moved the paper away from and closer to a complete mirror, there would be no change in light strength reflected back to me, of course because a mirror reflects 100% incoming light. Now the only difference a non-mirror surface has, and as good as every surface has some degree of specular reflection, higher or lower dependetly, would be the roughness of its surface, also determining afore mentioned specular reflective strength of surface. Nevertheless, it would be reflecting specularly, just weaker, or stronger. So I can't completely understand, why the reflection of the color of another surface seems to get stronger/weaker, depending on how close it is to the other surface?
I mean, what seems to happen is, the rule of strength of light, the amount of light per area unit. As a piece of paper would reflect its' white light diffused, into many directions, the further away I move the paper from the other surface, the less light per area unit would hit the other surface. But then again, wouldn't that mean, the more I move the paper away from a real mirror, the darker it would become? Which is not what's happening. I think I am going crazy. Or is it, because the specular reflection of the paper on the matte surface it not so strong to begin with, so moving away, the paper becomes smaller, giving the illusion of less, but that can't be, because of what I just said before, with a real mirror. That shouldn't affect it. Or that in combination with the roughness of the surface recieving the white of the paper, diffusing its' reflection. It's a little much in my head right now.

To think, that in the beginning I thought reflected color is such a simple thing, because I just accepted a simple explanation I once got from a teacher. But when I started thinking of more diverse situations, that explanation seemed like basically just saying: "Light hits a surface and reflects color to another surface, if You look closely", lol. Without actually explaining how it worked.
There are so many things to consider, influencing it.

Thank You in advance, any help is greatly appreciated.

2. In the first scenario any object that has light falling on it becomes a source of light itself. You always see a blue reflection because there is always some light falling on the curtains otherwise you couldn't see them.

In the second scenario the mirror does increase in brightness but because you see it as a mirror image you are fooled by the image into thinking it doesn't reflect. To prove what I am saying take a mirror and set it up in a dark room ( a small bathroom will work)and stand outdside that room and shine a flashlight into the mirror at an angle you will see the mirror does reflect the light on objects in the room.
Last edited by dpaint; October 22nd, 2011 at 02:02 PM.

3. Registered User Level 4 Gladiator: Meridiani
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Tend to agree with Dpaint, you can observe similar situation in Cornell Box with single lightsource and colored walls reflecting their colors to sides of any object put into it. http://graphics.ucsd.edu/~henrik/pap.../ewr7_fig6.jpg

4. Registered User Level 2 Gladiator: Ordinarii
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dpaint is right.
I also think you are confused about Specular reflection.
Objects reflect light rays in both Specular and Diffuse reflections. Specualr meaning each ray is reflected at the same angle causing a mirror-like effect. Diffuse reflection on the other hand causes each ray to reflect at a different angle.

In the first case it is simple diffuse reflection. To check whether it is Specular or Diffuse you can simply point a flashlight at the object and if you see the flashlight's image on it it is specular, otherwise (if you just see random reflected light) it is mostly diffuse reflection.

5. You seem to be basing all of this on the assumption that specular/diffuse reflection depends on surface roughness/smoothness, which is often said, but completely incorrect. As Eryao syays, they are two different processes that usually occur simultaneously. Furthermore, only diffuse reflection colours the light; normally the specular reflection retains the colour of the light source. READ THIS:

http://www.huevaluechroma.com/021.php

6. Originally Posted by briggsy@ashtons
Furthermore, only diffuse reflection colours the light; normally the specular reflection retains the colour of the light source.
That's only true for non-metallic surfaces. Metals do color specular reflection; it's just that most metals are colored neutrally and the color change is minuscule.

7. Registered User Level 1 Gladiator: Andabatae
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## A little confused about replies

First of all, thank You for all the replies and interest, guys. =)

Okay, but either I am not understanding everyones answers in reply to my question, or maybe I over-complicated my question. lol

Like I said, I do understand there is usually always a diffuse and specular reflection happening.
Diffuse reflection of light coloring the reflected light, and specular reflection reflecting the light itself, without affecting it.

While specular reflection works, as the name suggests, like a mirror, with light coming in in one angle, and going out in a similar angle to the opposite direction.
Which is, why, depending on the angle an object with specular reflection is viewed from, the reflection of things on it moves/changes.

So when there basically are just these two processes that can possibly be going on with the way light interacts with a surface:
How can the light reflected from the curtain onto the wall, or the bright light reflection of a metal pen on my skin NOT change place, not move around when I change my viewing angle.

And more precisely, how can the light on the wall be blue (which means it must be specular reflection), if it is NOT a specular reflection. Because if it was, in theory I shouldn't be seeing any blue light on the wall from most angles.

Unless of course this was because the surface of the wall had too many different angles on a microscopic level. Which I thought later on could be a reason. I mean, had the wall in that case been a mirror, I wouldn't be seeing the curtain on it for sure. If the curtain is on the left side, the wall on the right side, and I am looking from the left side behind the curtain at the wall.

Also Briggs, Your website is fantastic. I've already discovered it before, but reading through it and compeltely understanding everything will probably still take me at least another year. It's exactly the technical knowledge about light and color an artist could possibly ever ask questions about. At least I feel that way, that there are a lot of questions answered, that I was wondering about before.
And wait, You are saying that surface smoothness does NOT influence or determine specular reflection strength? Please do explain that further.
Okay maybe strength is not the right word, since strength doesn't really decrease but the specular reflection will be spread over a broader area and seperate the whole of the parts of a specular reflection making it appear less focused/strong and more blurred because of increasing angle differencies on rougher surfaces.

Sometimes I do feel like I am getting caught up in a thinking corner, and can't see the whole anymore. It's sometimes hard to step out of it and start thinking anew and from a different perspective again.
Last edited by Shindoh; October 23rd, 2011 at 05:43 AM. Reason: Added another question.

8. Shindoh, you only need to read page I linked to, not the whole site.

If the surface is uneven the specular reflection becomes more or less "fuzzy", but this "fuzzy" specular reflection is still the colour of the light source (for most substances), and should not be confused with the true diffuse reflection, which takes on the colour of the object.

Originally Posted by Shindoh
And more precisely, how can the light on the wall be blue (which means it must be diffuse reflection), ... .
Corrected (assuming your curtains aren't metallic )

9. Registered User Level 1 Gladiator: Andabatae
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## I GOT IT

I GOT IT!! I　GOT IT!!! OH MY GOD!!

I understood it by myself now, by making schematic views of different situations. Comparing what exactly happens differently between a mirror and a rougher surface with specular reflection.
I seriously couldn't understand a lot of phenomena, for example why the reflection on anything not being a mirror, got more blurry, bigger in size, and connected to blurriness seemed to lose light intensity with greater distance.
Or that with the wall getting blue lights from the curtains, with specular reflection happening on the wall, without me looking from the right angle to actually see the curtain reflection in theory, but it still being there.
Or why a box object's vertical surface, standing on a white table, would become more white, or seem to recieve more white specular reflection, if I looked at it from a steeper angle from the top, as opposed to looking at it from the front.

Briggsy, haha, of course I meant reading through Your whole website and actually understanding most of the concepts would take me that long. Sometimes I also can't understand the way others explain something, so I try again and again, to understand it myself. But in the very least, having other peoples answers always stimulates me to think differently, maybe I didn't completely understand the explanation, but the parts I did understand make me tackle something from a new perspective again myself.

I guess I had some kind of mental block on this or something, like I mentioned before, getting caught up in a corner. I always kept comparing specular reflection to the way a mirror works. Which is of course true. I knew that more uneven surfaces would reflect clean, with objects being close, and blur with distance. But that's what I really couldn't explain in my mind. I thought why does an uneven surface change so much? If it's uneven, shouldn't the reflection just be more blurry, no matter the distance. Or why doesn't it get more blurry with distance in a mirror. I got crazy. Until I finally made a schematic view of different situations. I didn't do it before, because I kept thinking I knew what I was going to draw (very simple) and what was going to happen, which is why I didn't understand it. But upon actually drawing it, simplifying it down to a "certain few 5 rays" from any diffused light any object is reflecting, that are all in one angle and going into the mirror and to the viewer, and seeing what happened to those "certain few 5 rays" on an uneven surface, made me realize it. I thought "oh god!", were "those same 5 rays" in their same angle hitting a more uneven surface, they'd suddenly bounce into all kinds of directions. So how would I still see any reflection? Now I'd have some random rays from the diffused reflected light of the object, all coming off at different angles exactly right, to bounce back off the uneven surface in a specular reflection, and back into my eye from the angle I was looking from.
Since those rays going out from the object that is to be seen in specular reflection are now all in different angles (just right to reflect back from the uneven surface, and back into my eye in the angle necessary), the distance of their hitting points in relation to each other, on the specularly reflecting surface, will increase with the objects distance TO the uneven specularly reflecting surface IN QUESTION.
Which all helps to explain all the phenomena I saw and couldn't explain.

There we go, looking at my explanation and its' confusingly long sentences makes clear, why it was so hard to understand in the first place.

Anyway, thank You for bearing with me, everyone, You are great.

10. In diffuse reflection, a given ray tends to bounce off in a random direction, due to the irregularity of the surface it's bouncing off of.

In specular reflection, a given ray tends to bounce off in a predictable direction: angle of reflection equals angle of incidence, like a mirror.

(Note that these are both simplifications, but are good enough to explain most problems for painters. Also note that they are not mutually exclusive- both can actually happen at the same time for a given object.)

In the case of bringing the cube towards the piece of paper- when you start out with the cube far away, only a few rays are bouncing off of the paper at an angle such that they will hit the cube. These rays would be the ones that are bouncing off at something near to perpendicular to the plane of the paper. As the cube approaches the paper, more and more light rays off the paper will be at angle that will intersect with the cube. When the cube is just above or resting on the paper, even some rays that are bouncing off at nearly parallel to the paper will be hitting the bottom of the cube.

What you have in that situation is a large number of weak light rays hitting the cube. These add up to create a diffuse "glow". The further away from the paper, the weaker this glow is- because there are fewer rays that have a chance to hit something.

The curtains in your other example are displaying diffuse reflection onto the wall behind them. The reflection isn't moving for two reasons- because, as Briggsy said, you were conflating diffuse and specular reflection. The other is that even if your curtains were metallic, the resultant rays would still hit the same surfaces regardless of your viewpoint. What would change as you move is the reflection of the light source on the curtains themselves, If you shine a flashlight onto a mirror, you will find that the light bouncing off of the mirror will continue to hit the same spot as you move your viewpoint, but the reflection of the flashlight in the mirror will move around.

Of course, if your wall was glossy or reflective and your curtains metallic, you might end up with a reflection of a reflection of the light source, and even some diffuse reflection at the same time (whew!).

11. Registered User Level 1 Gladiator: Andabatae
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## I got it

Thank You everyone, for Your patience and help.

I already got it by myself a while ago and actually posted a lengthy reply going into more detail of my understanding and how I finally got it, but it seems it didn't get approved by the moderators?

That's already the second time. Why?

12. Originally Posted by Shindoh
Thank You everyone, for Your patience and help.

I already got it by myself a while ago and actually posted a lengthy reply going into more detail of my understanding and how I finally got it, but it seems it didn't get approved by the moderators?

That's already the second time. Why?
Because your post count is low, there are pretty sensitive spam filters at work. I'm not sure exactly what tripped them for that post, but I've approved it. Once you post just a few more times that shouldn't be an issue any more.