I just recently purchased Reality and I’m struggling to learn it. However, I did find this that may help you.
“Why does LuxRender leave bright spots in my image? <other render engine> doesn’t do this
By default, LuxRender uses a metropolis-hastings sampler. It is, in a sense, attracted to bright objects. As a result, it has an occasional habit of leaving clumps of “fireflies” or bright pixels in the image. These will dissapear over time as the image gathers more samples. The metropolis sampler is extremely helpful in finding difficult light paths, such as caustics.
If you still find this behavior undesireable, you can use the lowdiscrepancy sampler instead. This sampler will distribute samples in a quasi-random pattern across the image, which should keep the noise even as it clears up. This can be useful for animation frames, where consistency of noise is more important than overall clarity.”
This came from a FAQ section in the help section of the program.