Virtual Light Meter

[bhchllc]

I was wondering if there is a technique for determining what the specifc lumens value is at a certain point in a scene ... like a virtual light meter. I'd like to have a more absolute method for determining specific lumens at a certain point in space to aid in lighting design that is independant of camera exposures and such.

[vlado]

Is it lumens exactly that you need or some other quantity? To get the lumens at a certain point in space, you need to sum all the light that comes to that point from all directions, which is somewhat complicated (but it can be done, if you render a spherical 360 degree camera from that point and sum the brightness of all pixels in the resulting rendering, divide by the image resolution and multiply by 4pi, with linear color mapping).

Other quantities like illuminance at a certain point and in a certain direction can be calculated by measuring the brightness of a small white patch in the scene.

Best regards,
Vlado

[bhchllc]

(but it can be done, if you render a spherical 360 degree camera from that point and sum the brightness of all pixels in the resulting rendering, divide by the image resolution and multiply by 4pi, with linear color mapping).

whoa. nice. Seems like a plugin waiting to happen. I guess illuminance data is really more what I am after because my primary concern is how much light is striking particular surfaces.

As far as illumance on a surface, how would you go about converting float values to intensity in a more usefull way? Most of the time, interior light levels are discussed in terms of foot candles and I guess what I am after is a scientific way of knowing how many footcandles are hitting the surfaces of my scene. For example, 50-75 footcandles is the recomended light levels for a kitchen... Im trying to figure out a way to know where I am at (obviously using photometrics and sun/sky) relative to that.

What would be great if there was a way of rendering a scene and have the ouput RGB value color coded based on foot candles but still respect the base materials for GI calculations. You would specify an output range and the scene would render as a gradiant from blue to red or whatever showing how much light was hitting these surface. It would be a great design aid for lighting design of spaces.

These high-end GI renderers are so sophisticated with regard to their internal calculations, it seems like such a small step to give the artist/designer access to some of this internal data in a much more usefull way.

[vlado]

Well, you can use the VRayRawTotalLighting render element, for the time being. With some careful set up, it can be made to produce exactly what you need, although in linear scale and not in false colors. If you are interested, I can give more exact details.

Best regards,
Vlado

[bhchllc]

Thank you Vlado. I will look into it and get a bit more familiar with it before I come back and ask any specific questions.

... never ceases to amaze me, this software.

[bhchllc]

Vlado,
I know you are busy now with the release of SP1 (thanks) but when you get a chance to elaborate a little more on how to do this, I would very much appreciate it.

Anyone else, feel free to contribute as well

I just purchased a light meter so I can take reading when we shoot HDRI's to record exactly what the footcandles and/or lux is at the time of shooting and I am trying to figure a way to calibrate them so when used in Vray, they are putting out the same amount of "light" relative to the photometrics we use. It is all part of the same process we discussed above to be able to gather more emperical light data from a scene.

Thank you.

[OZRAY]

This thread is so old but now in 2012 I have request from client that want that precision of footcandles in there 3D. No one have really found a way to create a kind of light meter in VRay scene that give you footcandles. Or my client already have spec of the supplyer about all the installed light ex: 7ft-Candles at 3ft from light source or 45ft-candles at 3.5ft from light source...

I'm sure I can have that precision in VRay but need a hint how to start...

Regards,

[vlado]

Can you not convert the output from the VRayLightMeter helper to foot-candles? Also, the 3ds Max photometric lights allow you to specify that kind of stuff.

Best regards,
Vlado

[IVOR_IP]

You don't really need a lightmeter for that.
That's just a spec for light source intensity.

Figure out what the the foot-candle at 1' and then change that to lumens(which Vraylights have) You might have to specify an IES file for proper distribution.

-----------
Footcandles = candelas / Distance^2

and one candela is a lumen within a unit sphere.
-------------------------------