[Radiance-general] bightdata upside down?

[Georg Mischler]

pillo wrote:

> i found a strange thing about  luminaire simulation:
> i tried light+brightdata to map on a sphere the luminance desired (evaluated
> by ies2rad) , it works, i see the light on the walls and the light emitting
> ball...
> but the STRANGE thing is that where the luminous flux is expected to be hi
> (lower side of the ball), the luminance is low...
> if you look at the scene you see floor and low portion of wall that receive
> light BUT the ball is dark (upper luminance >lower luminance)
> this is not correct.
> i ll appreciate any help to  solve this strange problem


Surfaces with light output distribution data applied can indeed
look very confusing in Radiance, because they may get a surprisingly
dark look when viewed from outside their main emission direction.
You need to differentiate here between the luminous intensity of
your ball in a specific direction, and the luminance of a specific
half of that ball. What really counts is the direction from which
you look at each specific point.

Did you look at the ball from exactly below? I'm sure that you'd
see it glaringly bright from there. Since you see the lower half
of the ball darker than the upper half, I suspect that your
eye point is located approximately within the lower cutoff area
of the luminaire (in the border area of the output cone).

You're looking at the lower half of the ball in a flat angle.
Since the luminaire emits little light in horizontal direction,
you see it darker under that angle. But when looking at the
upper half, your view angle is steeper, which means that more
light is emitted in your direction from there.

To better understand the effect, just assume that the distribution
data aren't fixed in the center of the ball, but at each point
you're looking at individually. If you imagine the ball composed
of very small patches (like a disco mirror ball), where each patch
has the same distribution attached, then the result you see might
make more sense.


-schorsch

-- 
Georg Mischler  --  simulations developer  --  schorsch at schorsch.com
+schorsch.com+  --  lighting design tools  --  http://www.schorsch.com/