Modeling an oil lamp source

radiance-general
1

Hi list,

this is the first time I am using colored light sources, and I am sure that I make some mistakes. So for your amusement and in the hope someone enjoys to show me the worst of my misunderstandings, I post how I try to model a light source, which is to represent an oil lamp.

I estimate a flame with 2cm diameter, 10 lumen,1900 K black body temperature.

I derive rgb 1.00 0.23 0.00 from the color temperature. This is only for the chromacity, and to be honest I did not do the CIE transformations but looked up in a table for 1900K.

I add a line to lamp.tab reading

/oil lamp/ .54 .41 .95

so that I can use lampcolor and reuse my guesses later.

No I run lampcolor, for the lamp type I enter my freshly defined oild lamp, I use meters as length unit, to model the flame I use a sphere geometry, and I give my estimated 0.01m radius for the flame (resulting in a luminaire sphere of 2cm diameter). My guessed total luminous flux is 10 lumens, leading to:

Lamp color (RGB) 28.93 8.62 0.0

Does this make sense? Am I totally wrong? I would apply a light modifier created with these values to a 1cm radius sphere now.

Thanks, CU, Lars.

2

Hi Lars,

This looks about right. The value for blue is zero because it's actually outside the Radiance color gamut. If you don't want to use a table next time, the conversion from color temperature to CIE (x,y) is provided in src/cal/cal/blackbody.cal.

-Greg

P.S. I'm still waiting on your scene description to sort out the other problem with the long rpict start-up time.

···

From: "Lars O. Grobe" <[email protected]>
Date: April 22, 2008 2:00:49 PM PDT

Hi list,

this is the first time I am using colored light sources, and I am sure that I make some mistakes. So for your amusement and in the hope someone enjoys to show me the worst of my misunderstandings, I post how I try to model a light source, which is to represent an oil lamp.

I estimate a flame with 2cm diameter, 10 lumen,1900 K black body temperature.

I derive rgb 1.00 0.23 0.00 from the color temperature. This is only for the chromacity, and to be honest I did not do the CIE transformations but looked up in a table for 1900K.

I add a line to lamp.tab reading

/oil lamp/ .54 .41 .95

so that I can use lampcolor and reuse my guesses later.

No I run lampcolor, for the lamp type I enter my freshly defined oild lamp, I use meters as length unit, to model the flame I use a sphere geometry, and I give my estimated 0.01m radius for the flame (resulting in a luminaire sphere of 2cm diameter). My guessed total luminous flux is 10 lumens, leading to:

Lamp color (RGB) 28.93 8.62 0.0

Does this make sense? Am I totally wrong? I would apply a light modifier created with these values to a 1cm radius sphere now.

Thanks, CU, Lars.

3

Lamp color (RGB) 28.93 8.62 0.0

Now I got completely confused. I am modeling my source in feet, but scale it later, before rendering, to metres. Now, I get the radiance derived from the total luminous flux of a source (10 lm) and its surface (from its diameter 0.02m). The result in units of radiance is per /sr m2, so if I apply the light modifier to a sphere and scale it, the total luminous flux would increase, right? So scaling light sources with xform is a stupid mistake? Do I get it right? I scale more or less everything in my model, and usually xform cares about everything, such as textures and transformations, but I guess in this case it does not work.

Thanks, Lars.

4

Hi Lars,

Luckily, scaling a light source does not cause problems, since the radiance of a surface is the same no matter what ruler you use to measure its diameter...

In other words, changing world coordinate units doesn't affect the lighting calculation values.

-Greg

···

From: "Lars O. Grobe" <[email protected]>
Date: April 23, 2008 5:42:18 AM PDT

Lamp color (RGB) 28.93 8.62 0.0

Now I got completely confused. I am modeling my source in feet, but scale it later, before rendering, to metres. Now, I get the radiance derived from the total luminous flux of a source (10 lm) and its surface (from its diameter 0.02m). The result in units of radiance is per /sr m2, so if I apply the light modifier to a sphere and scale it, the total luminous flux would increase, right? So scaling light sources with xform is a stupid mistake? Do I get it right? I scale more or less everything in my model, and usually xform cares about everything, such as textures and transformations, but I guess in this case it does not work.

Thanks, Lars.

5

Hi Greg,

I had a look at blackbody.cal can you guide us as to how the kelvin parameter is passed to this cal function into a usable rgb value?

Color temperature is very useful stuff for designers (especially retail) but so poorly represented in most renderers. Any help there would be appreciated

-steve
S&J LTD

···

----------------------------------------

Subject: Re: [Radiance-general] Modeling an oil lamp source
Date: Tue, 22 Apr 2008 15:03:33 -0700
To: [email protected]
From: [email protected]

Hi Lars,

This looks about right. The value for blue is zero because it's
actually outside the Radiance color gamut. If you don't want to use
a table next time, the conversion from color temperature to CIE (x,y)
is provided in src/cal/cal/blackbody.cal.

-Greg

P.S. I'm still waiting on your scene description to sort out the
other problem with the long rpict start-up time.

From: "Lars O. Grobe"
Date: April 22, 2008 2:00:49 PM PDT

Hi list,

this is the first time I am using colored light sources, and I am
sure that I make some mistakes. So for your amusement and in the
hope someone enjoys to show me the worst of my misunderstandings, I
post how I try to model a light source, which is to represent an
oil lamp.

I estimate a flame with 2cm diameter, 10 lumen,1900 K black body
temperature.

I derive rgb 1.00 0.23 0.00 from the color temperature. This is
only for the chromacity, and to be honest I did not do the CIE
transformations but looked up in a table for 1900K.

I add a line to lamp.tab reading

/oil lamp/ .54 .41 .95

so that I can use lampcolor and reuse my guesses later.

No I run lampcolor, for the lamp type I enter my freshly defined
oild lamp, I use meters as length unit, to model the flame I use a
sphere geometry, and I give my estimated 0.01m radius for the flame
(resulting in a luminaire sphere of 2cm diameter). My guessed total
luminous flux is 10 lumens, leading to:

Lamp color (RGB) 28.93 8.62 0.0

Does this make sense? Am I totally wrong? I would apply a light
modifier created with these values to a 1cm radius sphere now.

Thanks, CU, Lars.

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6

Hi Steve,

The file "blackbody.cal" just defines a couple of functions, cct_x(t) and cct_y(t). You can use these in icalc or rcalc, like so:

% icalc blackbody.cal
cct_x(2200)
$1=0.5056338
cct_y(2200)
$2=0.4152074

% echo 2200 | rcalc -f blackbody.cal -e '$1=cct_x($1);$2=cct_y($1)'
0.5056338 0.4152074

If you want to go from CIE (x,y) to Radiance RGB, use in combination with xyz_rgb.cal like so:

% echo 2200 100 | rcalc -f blackbody.cal -f xyz_rgb.cal \
  -e 'xi=cct_x($1);yi=cct_y($1)' -e 'Yi=$2;Xi=Yi*yi/xi;Zi=Yi*(1-xi-yi)/yi' \
  -e '$1=R(Xi,Yi,Zi);$2=G(Xi,Yi,Zi);$3=B(Xi,Yi,Zi)'

In this case, the second input field (100) is the Y value.

I hope this helps.
-Greg

···

From: steve michel <[email protected]>
Date: April 23, 2008 4:41:08 PM PDT

Hi Greg,

I had a look at blackbody.cal can you guide us as to how the kelvin parameter is passed to this cal function into a usable rgb value?

Color temperature is very useful stuff for designers (especially retail) but so poorly represented in most renderers. Any help there would be appreciated

-steve
S&J LTD

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