[Radiance-general] pmap distribution

[Erico Rosa]

Hi there,

I would like to measure the illumination from a vertical pipe bringing
daylight into a building. I have two situations, a clear sky with 30 and
60 degrees solar angles - 75,000 and 40,000 Lux reaching the top of the
tube - and would like to measure the transmittance of the pipe at its
bottom, using materials with 95, 96, 97, 98 and 99% reflectance. Using
manual calculations I can fairly expect something between 52-88% and
80-96% transmittance for the 30-degrees and 60-degrees sky,
respectively. I initially used the metal material which has given me
very low numbers. When I switched to the mirror material with a faceted
tube (polygons) replacing the initial round tube, the numbers started to
match (or at least sort of). 

For measurements, I divided the ring in the bottom of the tube in
approximately 200 measuring points and retrieved numbers that vary from
5,000 to 150,000 Lux for the first tube and 9,000 to 400,000 Lux for the
later. Dividing the individual values for the number of measuring points
give me exact numbers for the "30-deg" tube, and numbers 20% higher (but
accurate when reduced) for the "60-deg" one. Being a newbie at this, I
am quite puzzled with the photon distribution and if the measurements
are accurate.  Is it the bandwidth that is producing numbers that far
low and high? Is it bias that is distributing the photon like this? 

I am testing with an 80 degrees solar altitude clear sky and the results
are 100% higher that I expected (but again, accurate when reduced).

My parameters are:

mkpmap -appb <file> 1000000 50 5000 - apc <file> 1000000 -ds 0.001 -dp
32768
rtrace -I+ -ab 1 -ad 32768 -aa 0.001 -ar 32768 -app <file> -apcb 50 5000

Erico Naves Rosa
Doctor of Design Candidate
Harvard Design School