[Radiance-general] Basic daylighting and artificial lighting test(ambient value problem)

[Chris Yates]

Thomas wrote:
"Also I assume that ESP-r models walls as abstract 'interfaces' with certain
properties. These get exported to Radiance as single polygons.
The light leaks you see in your image are effects of interpolation, where
Radiance incorrectly calculates the brighness of the wall based on the
outside illuminance. To avoid this you can switch interpolation off (big
impact on rendering time, option '-aa 0'). More efficient would be to build
the model with 'real' walls, so that your interior 'walls' are boxed inside
the exterior 'walls'. That needs to be supported by your exporter, though."

Hi Thomas, what if a small diameter radiance "cylinder" could be drawn
around the edges with the light leaks? Like a kind of "silicone sealant"! As
you know, I'm no coder but I'm guessing it could be python scripted from
Sketchup. There'd be a small increase in rendering time. If the cylinder is
small enough it won't show. Or, if the appropriate size and colour are
selected you could achieve "cartoon edges"!

Cheers

Chris

-----Original Message-----
From: radiance-general-bounces at radiance-online.org
[mailto:radiance-general-bounces at radiance-online.org] On Behalf Of Thomas
Bleicher
Sent: 12 July 2009 09:37
To: Radiance general discussion
Subject: Re: [Radiance-general] Basic daylighting and artificial lighting
test(ambient value problem)

Hi Nalin.

Welcome to the wonderfully complex world of Radiance.

My answers are below ...

On Sun, Jul 12, 2009 at 8:33 AM, nalin swaroop<nalinswaroop at gmail.com>
wrote:
> Hello all
>
> I am new to radiance and am trying to learn its various features I 
> have modeled a room 16*6*3 in ESP-r and then exported the model to 
> radiance now I am trying to simulate this for two times of a day one 
> at 1200 noon(clear sky) and other at 20:00(night)

I'm not very familiar with ESP-r and how it models scenes and converts to
Radiance. You should check that 20:00h is actually dark for the day you
selected. Depending on your location and time of the year the sun can still
be visible.

Also I assume that ESP-r models walls as abstract 'interfaces' with certain
properties. These get exported to Radiance as single polygons.
The light leaks you see in your image are effects of interpolation, where
Radiance incorrectly calculates the brighness of the wall based on the
outside illuminance. To avoid this you can switch interpolation off (big
impact on rendering time, option '-aa 0'). More efficient would be to build
the model with 'real' walls, so that your interior 'walls' are boxed inside
the exterior 'walls'. That needs to be supported by your exporter, though.

> what I want to ask is what should be my basic rendering settings to 
> get a decent enough output and what should I set the ambient 
> value(-av) to for the night case should it be zero?

The ambient value is necessary to compensate for the loss of illuminance
that Radiance cuts off when it limits the number of bounces.
Basically every further bounce would still contribute to the illuminance but
if you stop following the ray after N bounces you neglect this small amount.
Setting the '-av' value to the average of the room will then add this
amount. You could say, with the right '-av' value you get a more accurate
result than without.

Finding the right '-av' value is an art in itself. For outside scenes you
get a value in the comments of the output of the gensky program.
Run gensky in a terminal ('command prompt') with your date and time and look
for the comment line '# Ground ambient level:'.

For indoor scene (without artificial light) you can use this value and
multiply it with a reasonable daylight factor for the room.

If you add aritificial light you can calculate an approximate '-av'
from your target horizontal illuminance (in Watts/m^2) divided by Pi. Say
you want 200lux (0.29W/m^2) then your -av would be 0.29/3.14 = 0.092.

> I want to check the illuminance value for the room during the day and 
> using artificial lights during the night set the illuminance level of 
> the room to say 200 lux but the problem is if I give some value to the 
> ambient value(-av) it adds up in the lumen value of only artificial 
> light and gives higher output so for both daylighting and artificial 
> lighting at every part in the room I am getting high lumen values, so, 
> is this correct?should ambient value ever be used for realistic 
> results? also for the daylight analysis what should be the ambient 
> value and

I think I answerd all the above. I might have missed a point here but that's
due to your lack of interpunctuation. Capitalisation is power!

> also does the illumination value depend on the no. of bounces if yes 
> then how would I know which no. of bounces to give for correct 
> results.

That's another thing where you need experience and you have to define how
accurate you want your renderings to be. Your scene (based on the image) is
empty so most light hits the floor and walls directly from the luminaires
(assuming there is no contribution from the outside). You have to make a
guess about the number of significant bounces. In this situation I'd say you
are reasonably accurate after 3 or 4 bounces.

To verify your estimate you can calculate a single point with rtrace and
increase the number of bounces until your results do not vary significantly.

Also don't expect Radiance to be unrealistically accurate. I always got
frustrated by Lighting Designers who believe that Radiance can give you the
lux values or daylight factors to the second decimal place.
Even your average light meter has an error of 3%. They have to learn how to
deal with good estimates.


Cheers,
Thomas

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