VRayLight units

[Gijs]

Re: VRayLight units

Tom, I think this line is very important:

First, a few assumptions:

(*) VRay always works with Generic units internally. VRayLights also do that and assume that 1 generic unit is equal to 1 meter. If this is not so, the multiplier of the light will need to be adjusted, if you want the light to have specific predefined intensity.

what it basically means is that Vray is asuming that your lightpatch is not 2" x 2" but 2m x 2m. Therefore Vray will adjust the power as if it was a 2x2 m patch that emits the total amount of light. Since it is not 2x2m, your light will become very dim. 150W has to be multiplied by (1/0.0254)^2. This means that the resulting multiplier has to be about 150 x 1550 which is about 232500. Since a 150W lightbulb produces only about 10% light, this means that 23250 in your case equals a 150W light bulb.

[jujubee]

* Removed cause I didn't know what I was talking about...

[Da_elf]

actually i have 15watt bulbs that shine with the intencity of a 60watt bulb so i kinda have to agree that watts isnt really the best term to use is it?

[jujubee]

*removed again

[Tom B]

Or use Newtons, the unit of force. Then it is energy per square metre which ties in with how it works now.

[Gijs]

So technically speaking then his light is not far off from what it should be value-wise... I think this is wrong. It's bright enough but not illuminating the room enough (visually speaking.)

Well, there are a few more things to consider. First of all it depends on the rgb of the material he is using. Secondly it depends on the exposure. Maybe Vray's standard exposure is based on more light entering the camera. I am getting quite fair results with this technique.

actually i have 15watt bulbs that shine with the intencity of a 60watt bulb so i kinda have to agree that watts isnt really the best term to use is it?

I think it does make sense, because everybody knows (not just we as CG-geeks) what the amount of light a 25W bulb or a 100W bulb produces. So it is a good reference. You only have to be aware of the fact that a X Watt incandecent bulb has an efficiency of about 10% and fluorecent tubes have a better efficency.

Anyhow, I think for interior lighting with Vray lights, it is much easier to use this technique, the only thing you have to do once is make a table of multipliers and you're ready to go.

[dlparisi]

Re: VRayLight units

Vray is asuming that your lightpatch is not 2" x 2" but 2m x 2m. Therefore Vray will adjust the power as if it was a 2x2 m patch that emits the total amount of light. Since it is not 2x2m, your light will become very dim. 150W has to be multiplied by (1/0.0254)^2. This means that the resulting multiplier has to be about 150 x 1550 which is about 232500. Since a 150W lightbulb produces only about 10% light, this means that 23250 in your case equals a 150W light bulb.

Gijs,
That's it. Although Vlado said that checking "normalize intensity" specifies the power in watts, it does scale it to meters just as you describe. If I scale the scene correctly and set generic and working units to meters everything behaves pretty much as expected. According to http://en.wikipedia.org/wiki/Incandescent_light_bulb, a 100W bulb has an efficency of 2.6 so I set the mulitpler to 2.6 and got what looked like a 100W bulb illuminating the room. When I scaled everything to feet/inches AND multiplied the multiplier by 1550 (i.e set to 4030) it looked exactly the same!

So what should I have my generic units set at ? Meters? Inches? What does everyone use?

Thanks much. This will make it a little easier to light scenes realistically now.

David

[jujubee]

REmoved again to save people time.

[Da_elf]

well its all good for people studying light physics. however i do visualisation so at times i really couldnt care less what the units represent. i just want to get the settings so it looks good. not so that it looks real. because when i do interior photography im not going to let a 60 watt bulb on the wall light my room for me. im going to light it so it looks good with my own softbox flashes

[dlparisi]

well its all good for people studying light physics. however i do visualisation so at times i really couldnt care less what the units represent. i just want to get the settings so it looks good. not so that it looks real.

Since the VrayLights don't reference any ies file it's an approximation anyways. Sometimes I do need to combine a specific ies file (perhaps a wall washer) with generic Vraylights and knowing an approximate value to set the vraylight to is valuable, if just as a starting point.

[_jaro]

This is getting me pretty confused, and I would really like to understand it clearly.
Vlado said that "Normalize Intensity" defines the light intensity as the total power in wats of the light source. So if I create 15 cm * 15 cm light source, and I set the multiplier to 90 I should have 90W light source that is equal to for example the 90W bulb? ..... but when I create simple scene with floor and ceiling, put my 15*15 light source with mult. 90 an Norml. Intens. on, I'm not getting proper results, for having 90 W light source in the room. It's too dark.
Am I getting something wrong? I guess yes, but I don't know what.

[Gijs]

if your units are cm, multiply that value by 100 x 100 x e, where e is the efficiency of the lightsource. Other than that, it will depend on your exposure and gamma if you get your scene bright enough.

[_jaro]

Gijs, what you are saing is that if the efficiency of 100 W bulb is 2.6%, the equal multipier value ( for cm ) will be 26 000?

[Gijs]

100 x 100 x 0.026 = 260

[_jaro]

It's getting much more clear to me now.
And the part of my problem ( that was confusing me a lot ) was wrong SCALE of my testing scene , sometimes you miss the basic point .