Vray light mtl renders black artifact edge

[gangle]

I am having an issue with Vray light material creating artifacts.

When I assign light material to, say a light fixture in an interior, The render, no matter how it's configured leaves a black, jaggy artifact border at the light material's edge! What's causing this?

It's a maddening problem that I know has a simple fix or adjustment, but I can't find it!

Thanks for any help. -Gary

[instinct]

Do you use any of the Sharpening Image filters ? Sharpening imagefilters have negative parts in there impulse response and hence cause negative values when applied to values above one. so if your LightMtl is over 1 and you use a sharpening filter you get black edges.
To fix it either turn un subpixelmapping and clamping in the colormapping settings or get rid of the sharpening ImageFilter.

Regards,
Thorsten

[kopar]

Also, if you're clamping your render (or rendering to an 8-bit image format as those 'must' be clamped to become 8 bit), you'll get outlines around areas of very high contrast. Not clamping your render will increase your render times, but hopefully your post software does a better job clamping extreme values than Max/Vray.