comparing Vray and Arnold and exposure

[sharktacos]

Doing a simple comparison of the same scene in Maya with a geo plane lit by a directional light with a Lambert material, I'm noticing that the Vray render is significantly brighter than the Arnold render. If I increase the exposure of the Arnold by 1.6514 stops, they are equivalent. Exposure set to 1.6514 is the same as multiplying by pi. So it appears that the Vray render is Pi brighter than an Arnold render.

In contrast, if I use an HDR image with a dome light to do IBL lighting, I get the exact same result in Vray and Arnold. It's just the lights that give a different result.
I'm wondering why this is, and which one is right? That is, I imagine it should be possible to have an equivalency/parallel between IBL lighting and the lights in Maya. I'm just not sure how to do that exactly. Anyone know?

[vlado]

V-Ray is calibrated so that a white rectangle light with intensity 1.0 produces appears exactly as white (1.0, 1.0, 1.0) in the final render (I mean the directly visible light surface itself) and matches the lighting that would be produced by a self-illuminated rectangle with the same white color and intensity through GI. This is logical and predictable and matches the behavior of the dome light. I don't know what Arnold is calibrated to.

Best regards,
Vlado

[sharktacos]

Area Lights This hold true for a rect light. If I compare a Vray rect light, a mesh light plane, and a Maya area light, I get the same results. The same is true in Arnold, provided their "normalize" function is disabled.

Sphere Lights If I do the same with a Vray sphere light, sphere mesh, and a Maya point light, the sphere light and mesh give an identical result, but the point light is quite a bit darker with quadratic decay on than the others. The point light intensity needs to be set to around 10,000 to match them. Changing the radius on a point light does not appear to have any impact on its brightness, it only affects the shadow softness.

QUESTION: what number does the intensity on a Maya point light need to be set to to match the intensity of 1 on a Vray Sphere light? Is it indeed exactly 10,000? (I was just eye-balling it, so if there is an exact formula that would be good to know)

Directional Light: If I shine a directional light straight down onto a plane geo with a Lambert material with 100% diffuse reflectivity (color: white, diffuse amount: 1.0) I get a pixel value on the geo of pure white in Vray (1.0) and I get a pixel value on the geo of 0.318 in Arnold. I believe Arnold is thus giving 1/Pi which is, as I understand it, the formula for Lambertian reflectance. Vray appears to be multiplying this internally by Pi resulting in a pixel value of 1.0, which is presumably done because it is more intuitive. Am I right?