# Candle lighting? rgb values....

Ive been setting up candles and modeling the light source as a sphere
(apparently accurate enough) but after researching through several of
peoples papers on using candles in Radiance i found sets of RGB values.

Beeswax 0.502 0.239 0.043
Tallow 0.759 0.240 0

But when i use these as my material then apply them to a sphere of size
approx 0.01 they dont appear bright at all, in fact radiance doesn't
appear to light at all with them, decimal values for rgb are way to low.
The values i need are up in the 100's but how can i find out a candles
watts/steradian/m2 RGB values? does anyone have any as reference or know
of any?

Cheers John

I guess what you have to do is to scale the rgb values you have by a factor
that depends on the expected light levels on the table (is there a table?)

when you consider light sources modelled with spheres:

A= apparent area of the sphere = PI * R^2 [m2]
F= luminous flux [lm]
I = luminous intensity= F/4PI [cd]
L=I/A [cd/m2]
R=L/179 [W/m2] = radiance = your R G B in the ligth definition

E=illuminance on a plane=I * cos3(theta) / h2

therefore you have to:
measure the light levels at a given point
from the illuminance you get the intensity
therefore after some number crunching you have the radiance....

hope it helps,
giulio

···

-----Original Message-----
Sutherland
Sent: 24 March 2004 14:01
Subject: [Radiance-general] Candle lighting? rgb values....

Ive been setting up candles and modeling the light source as a sphere
(apparently accurate enough) but after researching through several of
peoples papers on using candles in Radiance i found sets of RGB values.

Beeswax 0.502 0.239 0.043
Tallow 0.759 0.240 0

But when i use these as my material then apply them to a sphere of size
approx 0.01 they dont appear bright at all, in fact radiance doesn't appear
to light at all with them, decimal values for rgb are way to low. The values
i need are up in the 100's but how can i find out a candles
watts/steradian/m2 RGB values? does anyone have any as reference or know of
any?

Cheers John

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John Sutherland wrote:

Ive been setting up candles and modeling the light source as a sphere
(apparently accurate enough) but after researching through several of
peoples papers on using candles in Radiance i found sets of RGB values.

Beeswax 0.502 0.239 0.043
Tallow 0.759 0.240 0

But when i use these as my material then apply them to a sphere of size
approx 0.01 they dont appear bright at all, in fact radiance doesn't
appear to light at all with them, decimal values for rgb are way to low.
The values i need are up in the 100's but how can i find out a candles
watts/steradian/m2 RGB values? does anyone have any as reference or know
of any?

A typical wax candle emits about 1 cd of light, that's how the
candela unit got its name. If you take a sphere with a diameter
of 1 cm, then you get an emitting surface of 0.000314 m2.
The luminance is therefore 1/0.000314 = 3183.10 cd/m2.
In Radiance terms this is 3183.1/179 = 17.78 W/sr*m2.

The (weighted) average of your RGB values should be roughly the
same as that. The values you have above only specify the color
proportions, which you need to scale accordingly.

-schorsch

···

--
Georg Mischler -- simulations developer -- schorsch at schorsch com
+schorsch.com+ -- lighting design tools -- http://www.schorsch.com/