Hi guys.
I have a directional and a hemispheric light with inverse directions. The hemispheric light has specular set to 0 and diffuse set to the color of the directional light. The directional light has a shadow map.
When I activate both lights, I see specular reflections in shadowed areas and I don’t get why. Because the hemispheric light shouldn’t contribute to specular (I set its specular component to 0) and the directional light uses a shadow map. When I disable the directional light, the specular reflections disappear altogether (like expected). When I disable the hemispheric light, the specular in shadowed areas disappears (like expected). But both together create this imo unexpected effect, seemingly some interaction of the two light sources.
Has anybody had this issue and knows how to resolve it? Any help greatly appreciated! 
This is expected if you’re using PBR materials: there is no specular light color for PBR materials, as it isn’t physics-based (the specular color defined in the light structure isn’t taken into account in the lighting calculation).
If you are using standard materials, the specular light color should be taken into account: if that is the case, could you create a PG so we can investigate this further?
@Evgeni_Popov Thanks for your response! Ah, my understanding of PBR was incomplete there. I want to continue using PBR.
Then how do I avoid the “specular” looking highlights on the surface created by the hemispheric light? Is there another light value I can set to 0? Can I mask out the “specular” looking part by using a shadow map or sth for the hemispheric light as well?
There isn’t a per-light value you can set to disable only the hemispheric light’s specular contribution when using PBR. In PBR, light.specular is ignored for direct lighting, so the hemispheric light can still produce a specular-looking BRDF highlight using its light color.
Also, HemisphericLight can’t cast shadows, so it can’t be masked with a shadow map.
The usual options are material-level: increase roughness, reduce direct specular (material.specularIntensity = 0 for PBRMaterial, if acceptable), or for OpenPBR lower the specular weight. But these affect the material, not just the hemispheric light. If you need “diffuse-only fill light but keep directional specular,” you’d need to avoid HemisphericLight for that object, fake the fill with ambient/emissive/custom shader logic, or use a non-PBR/custom material path.
Note that I wasn’t able to reproduce a specular highlight in the shadows of this PG: