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Its about spot light lamps in combination with Cycles Glossy and Glas Shaders. In this post i only make use of unchanged shaders "Glossy BSDF" and "Glass BSDF" from cycles.

The blend file is here: http://pasteall.org/blend/index.php?id=47499

And what i am trying to do is a simple Cycles setup using a camera, a flat cube and a spot light:

enter image description here

  • The Spotlight has Strength 300 with a spot shape size of 20 degree
  • The cube material uses Glass BSDF (Beckmann) with roughness 0.1 and IOR 1.45
  • The camera looks straight downwards
  • All items are aligned on the Z axis

This is the render result with 128 samples after about 15 seconds:

enter image description here

Now i want to get rid of the fireflies. I followed the recommendation to use more samples. I increased the number of samples from 128 to 1024 which created this image: enter image description here

The downside here is that now the image renders in 2 minutes and actually the spot is not so much "spotty" but more "cloudy"

In the next experiment i increased the spot Strength from 300 to 1000 and now the flies are back: enter image description here

Someone told me that increasing the lamp size will fix it. So i increased the lamp size from 0.2 to 1.0 and the flies are gone but the spot is again cloudy.

enter image description here

Ok, lets make a break here. Actually i am not even interested in the reflection on the glas surface (for now). I rather want to get the spot light passing the glass and travel further down to another flat cube that uses a glossy material (a mirror actually).

I change the setup a bit as follows:

enter image description here

So i now expect to see the reflection of the light spot on the lower plane, and indeed the spot is there, on the left side i see the reflection from the glas and on the right side i see the reflection from the lower plane (the mirror:

enter image description here

but ... fireflies are back :( Now i need some help out of this :) thanks for any hint!

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Try using the denoise feature in 2.79.

Using the default denoise settings with your file at 1024 samples (and 100% resolution):

enter image description here

A different option without denoising is to modify the default glass shader (see :How to illuminate the darkness inside glass objects in Cycles?).

(1024 samples at 100% resolution)

enter image description here

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  • $\begingroup$ Thanks for all the information! this helped me to get much better results. I actually ended up with doing a combination of settings in the blender render panel, using the denoiser and creating a Glas material that works equally well for spotlight lamps and for area lamps and supports fresnel also in a convenient way :) Actually i merged a few proposals for glass materials in a way that suits my needs in this project. I now only need 128 samples to get good results. $\endgroup$ – Gaia Clary Aug 18 '17 at 16:46
  • $\begingroup$ @GaiaClary please write an answer detailing what you did. Other users with similar questions might find it useful. $\endgroup$ – cegaton Aug 18 '17 at 16:49

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