[Radiance-general] RE: gendaylit

Thu, 13 Jun 2002 11:37:41 -0400

I did what you suggested, Raphael. 

Again, the weather station data for Pittsbugh tell me that the diffuse horizontal component is 41 W/m^2, and the "beam" component measured towards the sun is 401 W/m^2 

I was shortcutting the luminous efficacy of these values by dividing both values by 2, assuming that approximately 50% are in the visible range. That was a mistake. 

gendaylit's man page generates a CIE clear sky as follows:
gendaylit -ang 60 0 -W 840 135

These irradiance data are integrated over the full spectrum, not just the visible range. Therefore:
gendaylit 4 13 7 -a 40.3 -o 79.9 -m 75 -W 401 41 
This gives me horizontal irradiance values of 41 for the sun and 38 for the sky, which is ok. 

The important difference between gendaylit and gensky is the input of irradiance data. If those data given to gensky -B -R are integrated over the full spectrum, not just the visible range, interior daylight levels will double! Gensky wants W/m^2 in the visible range only, whereas gendaylit uses the actual irradiance data from weather stations, which are approx. twice as high. 

Martin

	-----Original Message----- 
	From: Raphaël Compagnon [mailto:[email protected]] 
	Sent: Thu 6/13/2002 7:37 AM 
	To: [email protected] 
	Cc: Martin Moeck 
	Subject: Re: gendaylit

	Martin,

	It seems you have inverted the parameters providing direct beam irradiance
	and diffuse global irradiance.
	So instead of calling:  gendaylit 4 13 7 -a 40.3 -o 79.4 -m 75 -W 20.5 200.5
	do this:

	gendaylit 4 13 7 -a 40.3 -o 79.4 -m 75 -W 200.5 20.5

	My experience with gendaylit is that it is a rather reliable program. Its
	validation is documented in gendaylit author's master thesis.

	Raphaël Compagnon