One of the biggest problem you may encounter with an unbiased render engine is as simple as letting the sun shine through your glass windows, which is something you really might need to do more than often.
So why is this a problem for unbiased engines? It's fairly simple and complex at the same time. When a light ray enters a dielectric medium as you probably already know, it does not keep its original trajectory, it is deviated by an amount depending on the Fresnel (Nd) value. This is the refraction phenomena, which simulate the fact that when light enters a medium, its speed very slightly decrease resulting in a change of direction. And that is precisely that direction change that is making it very hard for an unbiased engine to make a very distant and small light source to be sampled efficiently on the camera's film, because the light ray direction is unpredictable.
Of course you always have the possibility to enable Metropolis sampling in the engine's properties. That will work and converges but quite slowly. Plus light rays that go through dielectrics become caustics, which will make the inside of your scene take even longer to light up. The only solution so far is to find a way to make the sun light ignores the refraction of the glass.
So, let's start by the beginning and take a look at a render where the sun is shining on a glass coffee table:
The render as it will come out by default.
As you can see, the sun is not passing through the coffee table's glass. Despite most unaware people wouldn't notice, a trained eye will.
The first step to allow sun to go through the glass, will be to lower the material's opacity. The amount of opacity will depend on how clear you want the glass to be, for this perfectly clear glass case study, I decided to use 10%:
Material's opacity tuned down to 10%.
This lets 90% of the light pass through the glass, while leaving 10% for us to improve the surface shading, because if you go too low, the object will simply disappear from the render! Here is the result of 10% opacity:
The coffee table glass with a 10% opacity.
As you can see the sun now shines nicely through the glass, but we have lost a lot of reflection intensity on the top. To recover as much as we can, we will have to make some changes to the material. For example we can use the Coating feature to boost the reflection intensity as much as we can:
By enabling Coating and pushing it to its maximum, we recover our reflections partially.
This will give the following result:
The Coating has significantly improved the reflection intensity.
This result is not bad in itself. It's not perfect of course and it's kind of a hack to be able to quickly render sun through dielectrics.
Duplicating layers to increase reflections.