[Radiance-general] RE: 60Watt bulb luminaries data

[John Mardaljevic]

Anthony,

Wait a moment whilst the spots before my eyes subside.  Ah, that's better.

> I am currently carrying out basic validation techniques of the Radiance
> software using a black box and standard 60Watt bulb therein.
> 
> I cannot locate a standard incandescent 60W bulb (just bare hung from
> ceiling, no lamp shade) on the desktop radiance program.
> 
> Can anyone help me by way of an add on file for a standard light bulb (even
> if not 60W!) or refer me to a suitable IESNA file?

The results you get are likely to depend on the size of the box
and the type of lamp, not to mention a few other factors.

You could model the bulb as a luminous sphere.  But you'd have to know
what the luminance of the bulb was.  Also, a sphere light source is not 
subdivided, i.e. it is sampled by a single ray (p511 RwR).  Which may
cause problems if the box is small compared to the bulb.  A 'frosted'
or pearl bulb may have a largely isotropic luminous output, provided you
don't see too much of the (luminous) shank where it connects to the
socket.  Any measured output distribution (say, IENSA) would, I guess,
be sensitive to the type of lamp.  And it would assume a 'point source'
distribution, even though you would model it as, say, a disc or polygon.

Squinting at my desk lamp through a pinched-finger aperture (hence the
spots) revealed that the uniform luminance of the pearl bulb was an
illusion - much of the output of the bulb is from around the filament.

In sort, I suspect that any comparison between measurement and predictions
for your bulb & box will say much about the precision of the photometry, 
maybe a little bit about the limitations of single ray light source sampling,
and not much at all about the intrinsic accuracy of Radiance for realistic
architectural scenarios (electric or daylighting).  The direct light source
calculation for small angular light sources (e.g. 'spherical' bulb from
several feet away with no occlusion) is pretty much a 'no brainer' and 
Radiance will give a result that is near as dammit to the exact solution.
When you get close up to a non-ideal bulb however, then it's a matter of
getting the photometry right. 

-John

-----------------------------------------------
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