View Full Version : Science - Light Questions.
08-17-2010, 08:30 AM
This is sort of a generic science question, not app-specific. Science Nerds, prepare yourself...
I'm trying to find a table of light sources by apparent brightness. I'm trying to use a standardized scale for Emissive/Glow/Self illumination Maps based on apparent brightness. So, say you're painting emissive maps for the sun, a flashlight, a streetlight, a car's headlight, a window.... all of those have fixed values on a scale of 0-1 based on some unit of measurement.
#1 - I can't find the right form of measurement; am I looking for Lumens, Lux, Candelas, what?
#2 - I'm hopefully looking for a list of what all of those would come out as. Like, is a headlight 40,000 Lumens, versus the sun is 100,000? A grid or table or list of these values would be super awesome.
I looked around a bit and got lost in Lux, Lumens, Candelas, and confusing equations. I can do math between them if I know what measurement it is I'm really looking for, but a lot of the definitions of light are somewhat vague and interrelated, so I'm not really sure which I'm actually after. Any help would be appreciated. Thanks in advance!
08-17-2010, 08:42 AM
I wish I could give you the first class of Zap Anderson 3ds max mental ray course on FXPHD..it answers like everything. Ill watch it again and put a summary here or maybe r_fletch_r will beat me to the punch.
08-17-2010, 08:44 AM
Ooh, good times. Yeah, a summary would be nice... I'll understand it once it's explained, I'm sure, I'm just digging around through wikis and I have no frame of reference to get started. Once I know what I'm looking for, I'll want to go back and figure out what X, Y, Z lights all represent on that scale.
Yeah, any summary would be awesome. Thanks!
08-17-2010, 08:57 AM
Hi, light measurement can be a little confusing because Photometry, Radiometry and common language overlap to some extent. In example Brightness is a perceptual concept tied to the perceiver.
The Photometric unit you're looking for is Luminance [cd/m^2], where cd are candela, measure of the Luminous Intensity and m^2 are the apparent surface in the Luminous Intensity direction, you usually got it by multiplying the surface area by the cosine of the angle between the normal to surface and the viewing direction. Keep in mind that Luminous Intensity is expressed by a vector, the sum of all vectors describing Luminous Intensity is the Photometric Web.
Luminance [cd/m^2] = Luminous Intensity / Apparent Surface
Luminous Intensity [cd] = Luminous Flux [lm] / Solid Angle of emission [omega]
Give the internet a go starting from these elements, and you should be able to figure out what you need.
Is this what your looking for? http://www.3drender.com/glossary/colortemp.jpg
08-17-2010, 09:05 AM
Ark - sort of, not quite. I want the brightness, not the color temp. That's a good chart though. What I want would have similar things (Quartz Lights, Tungsten, Sun, etc), but with candella ratings, etc.
Sync - I was thinking Candelas and Luminance sounded similar; glad to hear they're related. The problem I have is that the sun is apparently 400 Billion Candelas, based on size. So it'd technically be emitting 40 Million times more light than a 10,000 Candelas streetlight. However, at our position on earth, it doesn't appear 40M times brighter; would you calculate the sun's total Luminence based on it's total size, or would there be a size based on it's apparent size in the sky?
08-17-2010, 09:25 AM
Talking about emitted candelas is quite misleading as it is a vector not a scalar unit. The energy amount emitted by a luminous surface is the Lumen, which expresses the Luminous Flux. You could obtain the Luminous Flux from a perfectly diffusing surface (Lambertian) by dividing its maximum Luminous Intensity, along the normal to the surface by PI.
The eye can adapt to see different Luminance magnitude orders, so the sun looks really almost bright as a candle at night, but is a wonderful ability of human eye. And if you want to calculate sun Luminance I think you should use its actual dimensions, like when you get the apparent surface by the cosine you don't take into account perspective, distance and so on.
08-17-2010, 09:49 AM
I think I understand what you're saying, Sync, as far as using the Candelas divided by some factor that represents how much if it is pointed in your direction. I have no way of figuring out what percent of that light is, which points at you.
Part of why I want a fixed value is because we're doing light mapping, so the Sun/Candle effect you're talking about would be accounted for if the values of the sun and candles are calculated correctly. It's also why I don't want to just guess the values, because while a Candle may not appear very bright at daytime, it'd appear bright at night if it was the only light source in a dark room. Knowing that actual values will help determine the rest of the calculations.
08-17-2010, 04:52 PM
There might be some useful info for you at http://www.erco.com/products/downloa...ex_dltut_1.php (http://www.erco.com/products/download/index/index_3620/en/en_index_dltut_1.php). Be sure to check out the Printed Media section, there are a bunch of PDFs that go into great detail about architectural lighting, especially this one (http://www.erco.com/products/download/others/downloaddat_3992/en/en_downloaddat_dltut_1.php?aktion=_startseite&sprache=en&dir=30_media/25_guide_pdf).
I hope that helps.
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