Don't understand how lights interact each other

[DANIELE_CASAROLA]

Hi!
Please take a look at this simple scene:

http://www.ossosso.com/scambio/shoot_10.JPG

There's just a product in the middle (a lamp), a plane back and 3 lights: two of them are planar lights and the third is a series of little sphere lights inside the lamp.

This is the render with just planar lights on:
http://www.ossosso.com/scambio/shoot_11.JPG

This is the render with just inside sphere lights on:
http://www.ossosso.com/scambio/shoot_12.JPG

As you can see the little sphere lights doesn't affect the outside part of the product geometry, 'cause it is black, and this is how it should be.

This is the render with all lights swicth on:
http://www.ossosso.com/scambio/shoot_13.jpg

As you can see now the outside part of the geometry is brighter then shoot_11 and I really dont' understand why... the sphere lights dont' bounce over there, so why when I swicth on the geometry is brighter?
Unfortunatley the client is quite sensible to the material look, and he likes to have a render with both sphere lights on and off, but the material shouldnt' change his brightness when I switch on sphere lights.

Thank you very much for any kind of help

[alexyork]

Just a stab, but are the interior sphere lights somehow interacting with the plane lights to increase the overall brightness?

Simplest solution if you are short of time would be to do two renders and comp in post for max control.

[Shimakaze]

As Alex said, I would just render it out with VrayLight Select elements and composit in Photoshop.
As for why it happens, I can't tell without looking at the scene, but your GI quality is very low, and the lack of precision could very well influence the result. If you look closely at the render with just the lamp on, there is some bleeding on the wires it hangs from. I would try increasing the quality, or switch to brute force to see if that helps. If so you can optimize render settings from there. You might also want to enable "check sample visibility" in irradiance map settings

[svetlozar.draganov]

From what I can see the inside sphere lights also affects the outside part of the geometry because of the GI, yes the effect is very subtle since overall color is very dark but once the outside lights are turned on it becomes much more visible.