[Radiance-general] irradiance vs. radiance / luminance vs. illuminance

[Jack de Valpine]

Hi Nick,

In order to provide help on this one, it would be good to provide
information on the parameters being used as a starting point. So output
from rad -n <rif file> would be good. Another thing that might be
helpful is to get the material definition for the ground plane object.

-Jack

Nick Doylend wrote:
> Hi again,
>
> I've been puzzling over an extremely bright ground plane in one of my
> images.  I think I've tracked it down the the use of rpict's -i switch
> for calculating irradiance rather than radiance.  Can anyone help
> clarify whether I'm on the right track?
>
> If I want a luminance image, i.e. with levels corresponding to what one
> might actually see (and can compare cd/m2 values at different points) I
> omit the -i switch (and calculate radiance).  If I include the -i switch
> (and calculate irradiance), then I obtain illuminance (lm/m2 levels). 
> In both cases the radiometric units and turned into photometric units
> (using Radiance's 179lm/W conversion factor) by the image viewer
> software (I'm using the Ecotect RadianceIV viewer).
>
> Radiance is W/m2.sr while irradiance is W/m2.  Luminance is lm/m2.sr
> (cd/m2) while illuminance is lm/m2 (lux).  I think this means radiance
> and luminance are properties of a (solid angle) source while irradiance
> and luminance are properties of a (flat) surface?
>
> So my extremely bright ground plane is actually showing its lux level
> (the amount of light falling on it, rather than the amount of light it's
> reflecting).  If I want to illustrate the relative brightness of
> different surfaces I guess I need to go back and calculate luminance
> rather than illuminance.
>
> I did notice that if I clicked on the bit of 'ground' beyond the ground
> plane I get a much smaller value (in fact the same as the value in the
> luminance image - because I'm clicking on the ground as defined in the
> sky file - a glow source).  Why is this?  Does the concept of
> illuminance on a direct light source not make sense?
>
> Sorry for the long winded email equivalent of thinking out loud, and I
> appreciate that my understanding of photometry is pretty sketchy.
>
> Nick
>
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>   

-- 
# Jack de Valpine
# president
#
# visarc incorporated
# http://www.visarc.com
#
# channeling technology for superior design and construction

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