VRayLight using mesh and "sizeless" units too dim

[olly]

When I set a vraylight to use a mesh and set the units to lumens the resulting output is way below a vraylight set to sphere and the same lumens value.

In fact a mesh light seems to get dimmer as the size/face count increases.

This is something that has bothered me for a while, but keep forgetting to post about it.

Anyone else experiencing this?

[hristo.dimitrov]

I could not reproduce this issue in our environment.
Can you please send us a scene that contains the issue to support@chaosgroup.com and tell us which versions of 3ds Max and V-Ray you have the issue?

[olly]

Thank you for taking a look - I have sent the files as requested under ticket number 158-259-325.

[asen.atanasov]

Hi,

we took a look at your scene and everything looks right. The total emitted light from a light source, regardless of light source area, is the luminous flux measured in lumens. The emitted light per unit area is called illuminance and it is measured in lux = lm/m^2. This means that if we have two lights with different areas and equal emission in lumens, the light with the larger area will have smaller illuminance (emitted light per unit area).

First of all, if you have V-Ray Sphere Light and V-Ray Mesh Light with a sphere mesh of the same size and they are both set to emit light of equal lumens, then they render the same.

Then, in your scene you have one V-Ray Mesh Light with a mesh consisting of five identical spheres. This mesh light is set to a total of 3200 lm, meaning that each of the five spheres emits 3200/5=640 lm. Then you compare this with 5 V-Ray Sphere lights, each of which is set to 3200 lm. Hence, the total emission of these five lights is 5*3200=16000 lm. To achieve the same result with one V-Ray Mesh Light you need to set its flux to 16000 lm.

Best regards,
Asen

[olly]

Thank you for looking at this, but it has left me confused.

I thought the purpose of Luminous power (lm) and Radiant power (W) was that when this setting is used, the intensity of the light does not depend on its size?

[olly]

Taken from the help:

Luminous power (lm) ? Total emitted visible light power measured in lumens. When this setting is used, the intensity of the light does not depend on its size.
Radiant power (W) ? Total emitted visible light power measured in watts. When using this setting, the intensity of the light does not depend on its size.

So if using either of these unit types "the intensity of light does not depend on its size" why does their area affect how bright they are??

It has been a long week, but I am still baffled by this - someone please put me out of my misery!

[LarsSonparsson]

I think that you are thinking lights brightness differently.
I mean, you are talking about how bright the light object is on screen. Manual means that light intensity to the scene does not change if you scale the light.
Like if you have spherical light 1m and 10cm. Setting both intensity to be 100w. Now both lights will light your scene with same brightness. But, of course the 1m light shape looks dimmer, because that same amount of light is emitted from larger area.