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.

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.

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.

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.

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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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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