# Light pipe

Hi Giulio,

I am interested in your response regarding the light pipe problem of
Stefano. There are several correlations of light depreciation with lightpipe
depth, and of course the light pipe reflectance is key to this. Most light
pipes are 97 - 99 per cent specular reflective. Could you please elaborate
how one would implement the glow material and brightfunc relative to a given
relation and light pipe reflectance?

Regards,

Anthony

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the simplest way to proceed is to create an equation that links a
coordinate to the radiance of the surface in form of a multiplier (hence
the equation is between [0,1]).
you may use a cal file or directly express it into the brightfunc
definition.

As an example:
Supposing the relation between light output and distance is this:
(you may use the proper equation here)

y = a*x*x + b
and
y= 0 when x = 1000m
y = 1 when x = 0

it would be:

y = -0.000001*x*x+1

that becomes an 'if' sentence to avoid negative values...
y = if(-0.000001*x*x+1, -0.000001*x*x+1,0)

Putting all together I would define a light material this way:

void light max
0
0
3 100 100 100

max brightfunc light
2 'if(-0.000001*Px*Px+1,-0.000001*Px*Px+1,0)' .
0
0

light cylinder pipe
0
0
7
0 0 0
2000 0 0
.125

void plastic white
0
0
5 1 1 1 0 0

white polygon base
0
0
12
0 -10 -3
0 10 -3
2000 10 -3
2000 -10 -3

if it is ok you should see half cylinder black and the other half with a
gradient. Remember to use really low -ds 0.01.
Ciao,
Giulio

NOTE: Unfortunately the uv trick doesn't work here, since you need light
and you cannot nest it into instances (and meshes).
Hopefully your light pipe follows a simple direction like Px, Py or
Pz... otherwise you need to change the equation a little (you may also
find another equation...).

Thanks Marin for the photos and data from your experiments, this is most
interesting; I am carrying out experiments on hollow mirror lightpipes, I
note yours was a square duct.

I measured the intensity distribution of one emitting patch of a lightpipe
in 1988. The pipe used the 3M film, but had a square cross section and
glowing sides.

Is it a solid light pipe or hollow, is it prismatic standard transparent?

See the intensity distribution and the luminance distribution along the full
lightpipe length at

www.personal.psu.edu/mum13/lightpipe

These are interesting results. Have you published (Thesis, conference,
papers?)

The intensity distribution was measured approximately at the center of the
length of the pipe.

Thanks again for this interesting overview of your work.

the simplest way to proceed is to create an equation that links a coordinate
to the radiance of the surface in form of a multiplier (hence the equation
is between [0,1]).
you may use a cal file or directly express it into the brightfunc
definition.

Guilio, thanks for the example, I'll try implement it for my cylindrical
light pipe.

Regards,

Anthony

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Anthony, the results were never published. I was a grad. student back then and this was an independent study. It is a hollow prismatic standard lightpipe. I have a cylindrical lightpipe here and believe the results should be quite similar.

Martin

···

-----Original Message-----
From: [email protected] on behalf of Anthony J. Farrell
Sent: Fri 6/3/2005 6:28 AM
To: [email protected]
Cc:
Subject: *** SPAM *** [Radiance-general] RE: Light pipe

Thanks Marin for the photos and data from your experiments, this is most
interesting; I am carrying out experiments on hollow mirror lightpipes, I
note yours was a square duct.

I measured the intensity distribution of one emitting patch of a lightpipe
in 1988. The pipe used the 3M film, but had a square cross section and
glowing sides.

Is it a solid light pipe or hollow, is it prismatic standard transparent?

See the intensity distribution and the luminance distribution along the full
lightpipe length at

www.personal.psu.edu/mum13/lightpipe

These are interesting results. Have you published (Thesis, conference,
papers?)

The intensity distribution was measured approximately at the center of the
length of the pipe.

Thanks again for this interesting overview of your work.

the simplest way to proceed is to create an equation that links a coordinate
to the radiance of the surface in form of a multiplier (hence the equation
is between [0,1]).
you may use a cal file or directly express it into the brightfunc
definition.

Guilio, thanks for the example, I'll try implement it for my cylindrical
light pipe.

Regards,

Anthony

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