<p>(Transferred from comment)</p>
<p>Fresnel reflection is: Light bounces of the surface. Most dielectrics reflect light independent of wavelength on the surface.</p>
<p>Diffuse reflection models light going into a material and then coming out again, possibly at a different location and with a &quot;random&quot; orientation (a phenomenon we call subsurface scattering). This involves refraction, so light does not bounce off the surface. At this point, it absorbs some of the light and that is dependent on wavelength. Therefore, when it goes out, there appears to be a (different) color.</p>
<p>Refraction itself does not change color (in this case), but absorption does.</p>
<blockquote>
<p>Can I understand what you mean as that, Lambertian model actually an approximation of the process that light goes into a material, then absorbed and scattered by particles, some of light comes out again but with different direction?</p>
</blockquote>
<p>Yes, that is pretty much what the lambertian (or in general diffuse reflection models) do. You will be hard pressed to find a material that follows the lambertian model exactly, most real world materials behave a bit differently. However, in terms of rendering, especially in games, a lot of people find lambertian to look good enough (or other models to not add a benefit that justfies their extra costs). They will put more work in the specular BRDF then, e.g. with the GGX / Trowbridge Reitz model.</p>
<p>There is a deeper discussion on the Lambertian model in some material about extending the diffuse model, see for example: <a href="https://ubm-twvideo01.s3.amazonaws.com/o1/vault/gdc2017/Presentations/Hammon_Earl_PBR_Diffuse_Lighting.pdf" rel="nofollow noreferrer">PBR Diffuse Lighting</a> by Earl Hammon.</p>

CC BY-SA 4.0
2024-10-08T15:42:13.860Z
2024-10-08T15:42:13.860Z
Tare
<p>Recently I read <a href="https://cseweb.ucsd.edu/classes/sp17/cse168-a/CSE168_03_Fresnel.pdf" rel="nofollow noreferrer">CSE168_03_Fresnel</a>. In this pdf, when it comes to Fresnel Metals, it says</p>
<blockquote>
<p>Another important property of metals is that this reflectance is wavelength dependent. This causes the reflections to be tinted by the color of the metal (this is particularly noticeable with colored metals like copper and gold)</p>
</blockquote>
<p>Yes, because many metals' refraction index <span class="math-container">$\eta$</span> is wavelength dependent. So reflected light should have sort of color(which it calls colored reflections). But following statements confused me(which it calls uncolored reflections):</p>
<blockquote>
<p>This contrasts the behavior of dielectrics like colored
plastic, which don’t actually tint the reflected color off
the surface (the visible color comes from scattering and
absorption below the actual surface)</p>
</blockquote>
<p>For a long time, I think these colored plastic should have some diffuse materials which absorb light with specific wavelength and cause it colorful. If the material of plastic obeys fresnel law and its refraction index does not vary with wavelength, where its color comes from? what does <code>the visible color comes from scattering and absorption below the actual surface</code> means?</p>

CC BY-SA 4.0
2024-10-08T13:39:24.617Z
Not sure I understand the question correctly. Fresnel reflection is: Light bounces of the surface. Most dielectrics reflect light independent of wavelength on the surface. Diffuse reflection models light going into a material and then coming out again, possibly at a different location and with a "random" orientation. This involves refraction, so light does not bounce off the surface. At this point, it absorbs some of the light and that is dependent on wavelength. Therefore, when it goes out, there appears to be a (different) color. Refraction itself does not change color, but absorption does.
Tare
CC BY-SA 4.0
2024-10-08T13:48:29.200Z
Just to add to what Tare posted, Fresnel is all about surface reflections aka specular light. Some light, when it hits the surface, will bounce directly off the surface and not do any subsurface scattering.  In this case dielectric materials have very little interaction with the light so the outgoing reflected light color tends to be very close to the incoming light color.
pmw1234
CC BY-SA 4.0
2024-10-08T14:37:05.803Z
@Tare, when I say diffuse materials, I have something like Lambertian model in mind. Can I understand what you mean as that, Lambertian model actually an approximation of the process that light goes into a material, then absorbed and scattered by particles, some of light comes out again but with different direction?
ckf104
CC BY-SA 4.0
2024-10-08T15:42:34.660Z
I'll put into an answer. But basically: yes, lambert is just a model :)
Tare
<color><pbr><reflection><fresnel>
Colored vs. Uncolored Reflections
ckf104
# Colored vs. Uncolored Reflections

(Asked by: username_0 on 2024-10-08)

Recently I read [CSE168\_03\_Fresnel](https://cseweb.ucsd.edu/classes/sp17/cse168-a/CSE168_03_Fresnel.pdf). In this pdf, when it comes to Fresnel Metals, it says

> Another important property of metals is that this reflectance is wavelength dependent. This causes the reflections to be tinted by the color of the metal (this is particularly noticeable with colored metals like copper and gold)

Yes, because many metals' refraction index $\eta$ is wavelength dependent. So reflected light should have sort of color(which it calls colored reflections). But following statements confused me(which it calls uncolored reflections):

> This contrasts the behavior of dielectrics like colored
> plastic, which don’t actually tint the reflected color off
> the surface (the visible color comes from scattering and
> absorption below the actual surface)

For a long time, I think these colored plastic should have some diffuse materials which absorb light with specific wavelength and cause it colorful. If the material of plastic obeys fresnel law and its refraction index does not vary with wavelength, where its color comes from? what does `the visible color comes from scattering and absorption below the actual surface` means?

**Tags:** color, pbr, reflection, fresnel

> **Comments:**
> **username_1**: Not sure I understand the question correctly. Fresnel reflection is: Light bounces of the surface. Most dielectrics reflect light independent of wavelength on the surface. Diffuse reflection models light going into a material and then coming out again, possibly at a different location and with a "random" orientation. This involves refraction, so light does not bounce off the surface. At this point, it absorbs some of the light and that is dependent on wavelength. Therefore, when it goes out, there appears to be a (different) color. Refraction itself does not change color, but absorption does.  
> **username_2**: Just to add to what username_1 posted, Fresnel is all about surface reflections aka specular light. Some light, when it hits the surface, will bounce directly off the surface and not do any subsurface scattering.  In this case dielectric materials have very little interaction with the light so the outgoing reflected light color tends to be very close to the incoming light color.  
> **username_0**: @username_1, when I say diffuse materials, I have something like Lambertian model in mind. Can I understand what you mean as that, Lambertian model actually an approximation of the process that light goes into a material, then absorbed and scattered by particles, some of light comes out again but with different direction?  
> **username_1**: I'll put into an answer. But basically: yes, lambert is just a model :)  

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## 1 answers

### Answer by username_1 on 2024-10-08. Score: 1 | Accepted answer

(Transferred from comment)

Fresnel reflection is: Light bounces of the surface. Most dielectrics reflect light independent of wavelength on the surface.

Diffuse reflection models light going into a material and then coming out again, possibly at a different location and with a "random" orientation (a phenomenon we call subsurface scattering). This involves refraction, so light does not bounce off the surface. At this point, it absorbs some of the light and that is dependent on wavelength. Therefore, when it goes out, there appears to be a (different) color.

Refraction itself does not change color (in this case), but absorption does.

> Can I understand what you mean as that, Lambertian model actually an approximation of the process that light goes into a material, then absorbed and scattered by particles, some of light comes out again but with different direction?

Yes, that is pretty much what the lambertian (or in general diffuse reflection models) do. You will be hard pressed to find a material that follows the lambertian model exactly, most real world materials behave a bit differently. However, in terms of rendering, especially in games, a lot of people find lambertian to look good enough (or other models to not add a benefit that justfies their extra costs). They will put more work in the specular BRDF then, e.g. with the GGX / Trowbridge Reitz model.

There is a deeper discussion on the Lambertian model in some material about extending the diffuse model, see for example: [PBR Diffuse Lighting](https://ubm-twvideo01.s3.amazonaws.com/o1/vault/gdc2017/Presentations/Hammon_Earl_PBR_Diffuse_Lighting.pdf) by Earl Hammon.


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