[Radiance-general] Luminance meter

[John Mardaljevic]

Raquel,

We bought an LS100 a year ago.  At the time of choosing, there didn't  
seem to be much in the way of competition, but that may have  
changed.  Regarding your intended use, there are a whole raft of  
confounding factors that can (and invariably do) make comparison of  
measured and modelled values in an urban setting highly problematic.   
I addressed some of these issues in a recent LR&T paper:

Verification of program accuracy for illuminance modelling:  
assumptions, methodology and an examination of conflicting findings
Lighting Research and Technology, Volume 36, Number 3, 1 September  
2004, pp. 217-242.

Abstract:
This paper examines the role of assumptions commonly made in  
validation studies for lighting simulation programs and quantifies  
the sensitivity of results to uncertainties in key model parameters  
such as sky conditions and surface reflectivity. Actually occurring  
overcast skies are often taken to approximate the CIE standard  
overcast sky for the purpose of comparing predictions against  
measurements in real buildings. The validity of this assumption is  
tested against measurements of the sky luminance distribution for  
real skies. Illuminance predictions are particularly sensitive to the  
assigned reflectance of surfaces when the direct component of  
illumination is small. A number of confounding factors that can lead  
to imprecise estimates of surface reflectivity for building facades  
are identified and a methodology to minimize their effect is  
proposed. This study reveals that commonly made assumptions with  
respect to sky conditions and moderate imprecision in model  
parameters can lead to erroneous assessments of program accuracy. The  
degree to which existing validation findings can be extrapolated to  
very different application scenarios is discussed in the context of  
reported conflicting assessments of program accuracy.

Although concerned mainly with illuminance predictions, much of the  
discussion applies also to luminance.  Contact me directly if you  
require a copy of this paper and don't have access to the journal.

-John

Radiance 2006 Scientific Workshop, 13-14 (Wed-Thu) September, De  
Montfort University, Leicester, UK.
http://www.iesd.dmu.ac.uk/~jm/radiance2006work.html

-----------------------------------------------
Dr. John Mardaljevic
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

jm at dmu.ac.uk
http://www.iesd.dmu.ac.uk/~jm