[Radiance-general] Units

[Greg Ward]

Hi Alejandro,

> From: Alejandro Troccoli <atroccol at cs.columbia.edu>
> Date: July 15, 2004 4:56:17 PM PDT
>
> I am in the process of implementing Preetham's "A Practical Analytic 
> Model for Daylight" into Radiance. I have some questions regarding 
> Radiance units, which I understand are radiometric units.  Here are 
> some questions I came up with. My biggest confusion relates to 
> luminous efficacy, and when to apply it.
>
> 1) If I have a spectral distribution of the sun in wavelength 
> increments from 380 to 780, and apply cieresp.cal to obtain the XYZ 
> CIE values, what units is Y in?

It depends on what the spectral distribution represented.  Oftentimes, 
these are only relative values, and you can't expect to get meaningful 
absolute units as a result.  What you will have is a valid color, where 
CIE (x,y) chromaticity is computed by:

	x = X/(X + Y + Z)
	y = Y/(X + Y + Z)

Note that the absolute values drop out of the above formulae.

If on the other had, your units are watts/whatever, your result from 
cieresp.cal will be lumens/whatever.  If you had radiance in 
watts/steradian/meter^2 on input, your Y output will be 
lumens/steradian/meter^2, or "nits."

> When I convert to RGB using something like xyz_rgb.cal, should I 
> divide by the luminous efficacy 179 or not? Reading Chapter 5 of 
> Rendering with Radiance (pages 305-307), it seems I should just take 
> the RGB values and use them.

Yes, you should divide by 179 to get back into RGB radiance color 
values.  The pages 305-307 deal with reflectance, which is unitless and 
therefore has no conversion factor.

> 2) If I have data in CIE XYZ, with Y in cd/m^2. Should I convert to 
> RGB and divide by 179?

Again, yes.

> 3) When using skygen, what units is the solar intensity in? What units 
> does skyfunc return?

Do you mean "gensky?"  If so, the solar radiance is in units of 
watts/sr/m^2 using the standard 179 value for luminous efficacy.  
Unless you know better, it is recommended to simply use this value, so 
it drops out of the calculation when you return to photometric units 
during the analysis phase at the end.

I hope this helps.
-Greg