[Radiance-general] Re: Comparison of values measured by sensors and calculated with Radiance (Greg Ward)

[Minki Sung]

Dear Greg,

Thank you for your advice and I just tried as you said but actually the
results were not improved as following link.

*http://iga.iis.u-tokyo.ac.jp/xe/?document_srl=346*

That might be so because the lamp is actually UV lamp but as you know, there
is almost no difference between flourescent and UV lamps besides with or
without phosphor coating on the surface of lamp. Nevertheless,
I thought the absence of phosphor coating could cause some differences on
the diffuse of UV including the Fresnel effects. I'm finding some references
on the diffuse of UV lamps.
The reflectances were referred to a japanese paper which had done
experiements with UV and the UV output of lamp was referred to the
manufacturer's specification but the UV intensities at 1 m distance is
almost the same.

 Additionally, I'm checking the UV sensor but as it also follows cosine law
as illuminance sensors do, there might be no problem with UV sensor.

Sung

Message: 4
Date: Wed, 1 Apr 2009 11:35:14 -0700
From: Greg Ward <gward at lmi.net>
Subject: Re: [Radiance-general] Comparison of values measured by
       sensors and     calculated with Radiance
To: Radiance general discussion <radiance-general at radiance-online.org>
Message-ID: <937118BE-1267-44D0-85BF-B84958EEF758 at lmi.net>
Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed

Hi Sung,

Try adding the following parameters to your rtrace command:

       -dj .7 -dt 0

How sure are you of your surface reflectances?  Did you actually
measure the lamp output, or are you assuming the manufacturer's lumen
specification is correct?  How far off are your values?

Since you notice the most errors near to the lamp and at low angles,
it's possible that the Fresnel effects are important.  The emission
of the fluorescent phosphor may be considered diffuse, but the actual
output of the lamp will be a function of angle due to Fresnel
reflection of the glass bulb.  You can account for this
(approximately) by applying the "winxmit.cal" function included with
Radiance, like so:

void brightfunc Fresnel_effect
2 winxmit winxmit.cal
0
0

Fresnel_effect light lamp_light
0
0
3 X X X

lamp_light cylinder fluorescent_tube
(etc...)

I think you get the idea.

-Greg

> From: Minki Sung <mkii at iis.u-tokyo.ac.jp>
> Date: April 1, 2009 11:27:29 AM PDT
>
> Dear all,
>
> A bared cylinder lamp (with 60cm length and 3.2cm diameter) is
> installed on the wall and illuminance values were measured at some
> points and to some directions as shown in the linked image.
> Unfortunately, the illuminance values calculated with Radiance are
> different from measured values, especially when the sensor is near
> the lamp or  the incident angles are high (K~O).
>
> Link image
>
> What can be supposed as the cause of the discrepancy?
>
> rtrace command was as follow.
> cat pts.pts | rtrace -I -oov -ar 128 -ad 512 -as 256 -ab 2 -ds 0.02
> test.oct > test.dat
>
> and all the walls, floor and ceiling was covered with non-glossy
> black paper. (reflectance 0.05)
>
> Sung.
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