[Radiance-general] re: trans material with sunless sky

[Phillip Greenup]

>Another question.  Pcond on a luminance image mimics what
>the eyes sees.  Pcond on an illuminance image mimics ... ?
>
>judy

can somebody tell me what is the purpose of illuminance images?  am i 
correct in saying that these are created by using the -i option in rpict/rview?

quoting from the Radiance Reference manual, 'This only affects the final 
result, substituting a Lambertian surface and multiplying the radiance by 
pi'.  so the numbers you end up with in these images are simply that, the 
reflected radiance multiplied by pi.  what does this represent?  it does 
not represent the irradiance falling on the surface.  for any 
non-Lambertian surface, it does not represent the irradiance reflected off 
the surface either.

so, for Lambertian reflectors, the results are reflected irradiances, which 
are the incident irradiances multiplied by the reflectance of the 
surface.  in a visualisation such as this, reflected irradiances are of 
very little interest.  radiance and incident irradiance are actually much 
more useful.

too often have i seen Radiance images presented overlaid with 
iso-illuminance contours.  according again to the Radiance Reference 
manual, under falsecolor, 'If the -i option of rpict was used to produce 
the image, then the appropriate label (for a falsecolor image) would be 
"Lux"'.  this means that the illuminance values displayed in these images 
are some manifestation of reflected illuminance.  from a photometric point 
of view, this is essentially meaningless.

if this is presented to a client to show them the illuminance distribution 
within their space, it really tells them very little.  iso-illuminance 
contours are meant to represent the illuminance falling onto an imaginary 
sensor at that location.  the -i option of rpict does not calculate 
this.  the only real way of presenting illuminance information is on a 
plane, real or imaginary, horizontal, vertical or tilted, not overlaid on a 
visualisation.

i understand that people have a better understanding of illuminance, and so 
to present things in this manner makes it easier for clients to 
understand.  but they should not be led to believe that these contours 
actually represent illuminances within their room which are to be compared 
with specifications, standards, etc.

can anybody correct me here?  i don't mean to ruffle any feathers, but i 
think Radiance is generally used by people with a good knowledge of light, 
and so it should be used accordingly.

Phil Greenup