> Hi, I'm trying to render a surface I exported from a VTK visualisation software,
> but there are some strange lights under the surface
>

Looking at your image, it realy look like radiosity artefacts.

Things that can help:
Increasing the count value and add the second parameter. Set that second 
parameter to about 10 to 20 times your count value. It set a pool of 
sampling directions that are used in rotation.
Try to use a smaller value for minimum_reuse. Don't go overboard, 
something like 0.001 should be enough. Default at 0.0015.
Use a smaller pretrace_end. You can go as low as 0.00125 for a large image.
Reducing error_bound may help, as reducing low_error_factor from the 0.5 
default value. Note that the value for that parameter are larger than 
that in rad_def.inc as they are based on the older default of 0.8 from 
version 3.5.


It don't look like your area light have problems.
You can increase the sampling a lot if you use the adaptive option.
using adaptive 0 turn it ON and start sampling using only the 4 corners 
points. adaptive 1 start sampling with an array of 3*3, adaptive 2 start 
at 5*5 and adaptive 3 at 9*9. Using adaptive, you can use arrays of 
65*65 with almost no slowdown over a 9*9 one.

If you want to use the orient option, you also must use a square array 
and use the circular option.



Alain

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