Translucency issue with LWF 0.255 method

[andstef]

Hello again,
I don't think many people here work with translucency, but right now I am in the middle of a project (rendering a lamp model for a contest) and I am trying to work as physically correct as possible to achieve correct results in illumination and of course photorealism.
So... my lamp is made out of tubes of translucent plastic with LED-s on the inside. The LED-s are just simple boxes with Vray Light material applied. When I render like this everything is fine, but when I turn off the LED-s (assign a different material, or turning the multiplier down from 10 to 0.1) the whole tube goes pitch-black. And this is strange, because the other objects in the room still are exposed correctly.
I didn't have this problem when I was working with an exponential color mapping and non LWF setup (target direct light instead of the vray sun, no gamma correction, and no darkening the materials ).
I'm using vray sp2. My setup is: Irradiance map- high , min 80, max 200, Light cache, Linear color mapping, Vray sun and sky at default values.
The translucent material's setup is as follows:
diffuse: r240, g240, b240
reflect: 151 gray
refract: 77 gray , use interpolation, affect shadows,
translucency: hard, back side color: r210, b212, g199, thickness 313,7 cm (whatever this means), scatter coef. 0.5, fwd/bck 0.5, light multiplier 3.
refract interpolation: min -1, max 0, interp. samples. 200 (for a smooth result)
So... does anyone have any idea how to fix this ?
Of course, right now I'm using a workaround, assigning a different material (no translucency) when I want the lights to go off. But what's happening isn't right, a material doesn't behave like this in reality, so...