How do I control the blurriness of light through frosted glass? I'm trying to create soft lighting through non-bumpy frosted glass. This is my attempt and it's not really working the way I want it to as reflections have no blurriness and not the kind of blurriness I'm going for:



All relevant information is in the screenshot. 

I have used a cube as a light source inside the structure. There is a plane (thick) in front of and behind the cube light.

Changing the roughness to max doesn't get me the required result either. Also, at higher IOR values, the cube is only about halfway visible then just cuts of from this perspective. The shape of the light on the wall behind is a hard circle which makes no sense to me.

Another problem is, the reflections do not have the blurriness of the glass. I think that has more to do with the material of the floor which is a simple glossy and diffuse shader mix with Fresnel as input to the mix shader.

Here are some images of the material I'm going for, taken from Stanley Kubrick's films :



Another still from Space Odyssey.



I would like to be able to control the softness of the light behind the source by changing parameters of the glass in front of it.

This is my first post on stackexchange and only 3 days using blender so I'm pretty new to this.
Use a mix of glass and translucent shaders. To control how blurry the light source behind the glass is, play with the factor values on the mix and the roughness of the glass shader.

Here's an example of an emitter object enclosed on a glass cube. The glass walls  have to have some thickness (you can use a solidfy modifier) so that it is not a solid glass cube.



An other factor that will have a great impact in the light effects you are after, is the color transform used in the color management settings. Being able to use a bright light source and a much larger dynamic range for the image, will help you have a more realistic result. Experiment using the filmic blender colour tranforms.



(Click on the images to enlarge)

Read also:

Render with a wider dynamic range in cycles to produce photorealistic looking images