6.4.1 General attributes

Iray tab in the Render Settigns Window when Iray Photoreal is selected
Max. Path Length
Limits the maximum number of bounces of light paths which contribute to the rendering result. Decreasing this value cuts off indirect lighting contribution but speeds up render time. Increasing the value adds physical accuracy but slows down render time.
Caustic Sampler
Improves the quality of caustic but takes longer to calculate.
Architectural Sampler
The Architectural Sampler can be used to improve the convergence speed of difficult scenes. A common scene type that profits from this specialized sampler is indoor architectural visualization, especially if it is mostly illuminated by indirect lighting.
One specific example would be a room that is illuminated by light sources placed in neighboring rooms or by outdoor lighting (such as the Sun and Sky model) shining through a small window. As the sampler introduces additional overhead and also only works well when a lot of iterations need to be spent on finalizing the picture it should be avoided for rather simple scenes, or in general mostly directly lit scenes such as outdoor or design visualization.
Nominal Luminance
The nominal luminance is a hint to Iray on what is considered a "reasonable" luminance level when viewing the scene. This luminance level is used internally to tune the firefly filter and error estimate. When the nominal luminance value is set to 0, Iray will estimate the nominal luminance value from the tonemapper
For visualization, a reasonable nominal luminance would be the luminance value of a white color that maps to half the maximum brightness of the intended display device. For quantitative architectural daylight simulations a reasonable nominal luminance could be the luminance of white paper under average day light.
FireFly Filter
Controls a built-in filter to reduce bright spots that may occur under some difficult lighting conditions. Such bright undesired pixels are often called “fireflies.” Consistency is not affected by this filter: fireflies are automatically detected and masked out in the early rendering stage when variance is high but eventually these missing contributions are faded back in over time. Thus, the results at a low sample count may appear slightly darker than the final, converged image. The filter works best in combination with the built-in tonemappers. If a tonemapper is not enabled, the filter estimates can be too pessimistic for some scenes, resulting in some fireflies to still appear.
Effect of different values for Nominal Luminance