You probably want to look into subsurface scattering for the skin. Jimenez's screen space shader is the popular one. However, I'm concerned about how sharp your light-shadow transition is independent of the material being rendered. That does not look correct to me. You should check out what's going on there before doing anything else. FWIW, a cheap starting point with skin is to simply shut off specular entirely. This is a good sanity check.

You probably want to look into subsurface scattering for the skin. Jimenez's screen space shader is the popular one. However, I'm concerned about how sharp your light-shadow transition is independent of the material being rendered. That does not look correct to me. You should check out what's going on there before doing anything else. FWIW, a cheap starting point with skin is to simply shut off specular entirely. This is a good sanity check.

I think it looks right for a gamma-correct Lambertian diffuse falloff. Linear-space Lambertian is actually pretty harsh, especially compared to doing it in gamma-space. Obviously it looks totally wrong for skin, since the SSS will "soften" the falloff and avoid that "rocky" look that you would get otherwise.

So yeah, there's the screen-space stuff which has its pros and cons. At work we use Pre-Integrated Skin Shading (also has an article in GPU Pro 2), which has a different set of pros and cons. For some background reading, I would also suggest reading the skin rendering chapter from GPU Gems 3. The actual techniques it describes aren't really attractive for real-time rendering (texture-space diffusion is too expensive and doesn't scale well, and GGX is a cheaper approximation than multiple Beckmann specular lobes), but it's still a great article.

@Promit Yes, the falloff is very harsh, and without specular too (it stay the same). Do you know another diffuse equation i can use to have something softer ?

	float ndotl = dot(N, L1);
	//compute half lambert term
	ndotl = (ndotl * 0.5) + 0.5;
	ndotl *= ndotl;
	ndotl = clamp(ndotl, 0.0, 1.0);

So this method only affect diffuse calculations right ?

Yes. Current sub-surface scattering techniques only address diffuse reflectance, so you can leave specular alone.

Will sub-surface scattering necessarily help the falloff though?

Absolutely. Take a look at this image from Eric Penner's slides:

[attachment=22314:SSS.PNG]

Notice how the falloff becomes much softer as curvature increases.