Radiance

Hi All,

I have taken images of same scene from two different camera at same time and same day. How to find the radiance scale factor between the two cameras.

I will be grateful if somebody can explain me mathematical relation to compute the radiance of a point in the scene.
if the pixel value of the point and cameras ISO value, F-stop, Shutter Speed are known

Best Regards,
Brajesh Lal

I'm not sure what you mean by the "radiance scale factor". Are you trying to compute the cameras' response curves? As for your second question, you won't be able to calculate radiance from a single exposure, even if all the details about that exposure are known. The process you are describing requires a high dynamic range (HDR) image, which is comprised of multiple exposures. The typical minimum is three exposures separated by 2 f-stops each, but ideally the longest exposure should have zero black pixels and the shortest have no white pixels. This ideal is only necessary to capture scenes of extreme dynamic range, however.

Radiance-online.org also hosts an hdr mailing list which is definitely worth subscribing to:

http://www.radiance-online.org/mailman/listinfo/hdri

Robert Guglielmetti IES, LEED AP
Building Energy Efficiency Engineer
National Renewable Energy Laboratory (NREL)
1617 Cole Blvd, MS-5202
Golden, CO 80401
[email protected]
303.275.4319

···

From: [email protected] [mailto:[email protected]] On Behalf Of Brajesh Lal
Sent: Wednesday, July 01, 2009 7:55 AM
To: [email protected]
Subject: [Radiance-general] Radiance

Hi All,

I have taken images of same scene from two different camera at same time and same day. How to find the radiance scale factor between the two cameras.

I will be grateful if somebody can explain me mathematical relation to compute the radiance of a point in the scene.
if the pixel value of the point and cameras ISO value, F-stop, Shutter Speed are known

Best Regards,
Brajesh Lal

Hello Brajesh,

I second Rob's motion that you check out the HDRI mailing list. Archives can be found at radiance-online. I also recommend you download and try out Photosphere if you have a Mac available to you, as it does exactly what you want to do.

The following might also be of some use:

  http://radsite.lbl.gov/radiance/refer/Notes/filmspeed.html

Best,
-Greg

···

From: "Guglielmetti, Robert" <[email protected]>
Date: July 1, 2009 8:11:18 AM PDT
I’m not sure what you mean by the “radiance scale factor”. Are you trying to compute the cameras’ response curves? As for your second question, you won’t be able to calculate radiance from a single exposure, even if all the details about that exposure are known. The process you are describing requires a high dynamic range (HDR) image, which is comprised of multiple exposures. The typical minimum is three exposures separated by 2 f-stops each, but ideally the longest exposure should have zero black pixels and the shortest have no white pixels. This ideal is only necessary to capture scenes of extreme dynamic range, however.

Radiance-online.org also hosts an hdr mailing list which is definitely worth subscribing to:

http://www.radiance-online.org/mailman/listinfo/hdri

Robert Guglielmetti IES, LEED AP

From: [email protected] [mailto:[email protected]] On Behalf Of Brajesh Lal
Sent: Wednesday, July 01, 2009 7:55 AM

Hi All,

I have taken images of same scene from two different camera at same time and same day. How to find the radiance scale factor between the two cameras.

I will be grateful if somebody can explain me mathematical relation to compute the radiance of a point in the scene.

if the pixel value of the point and cameras ISO value, F-stop, Shutter Speed are known

Best Regards,

Brajesh Lal

Hi all,

I am really sorry for writing on the same point again but really in-need for clear and complete answer, if possible.
Here, in my institution, people are in doubt if simple daylight calculation tools like Ecotect (both standalone Ecotect as well as Ecotect using Radiance engine methods) are efficient/enough for characterising different complex fenestration systems (CFS) and most importantly differentiate them for purpose of classification and rating?
Many are saying there's no need to go through Radiance to get almost the same thing that can be achieved using other simple tools (like Ecotect). For me, as a beginner with some readings and research, I found that Ecotect is still not efficient tool to differentiate different CFS (especially if they are very close in geometry, etc.) so we need something that's more robust like Radiance to do the Job; does that sound right?

Your reply is highly appreciated.

Regards
Ikrima

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Ikrima,

I am going to try to build the puzzle of CFS, Phases, BSDF and
calculations.... at least the way I understand it.

*CFS* are those systems that, via interreflection or other light transport
phenomena, redirect light (or solar radiation). Thus, in order to get a
reliable result you will have to consider all the phenomena involved.
Then, *common
simple performance indexes*, such as the miss-used Shading Coefficient and
the Aperture Percentage, *always loose a lot of information, trying to
reduce all the complex behavior of a CFS to one single number*. We all know
that venetian blinds are more "transparent" from certain viewing directions
than from others, but these performance indexes do not tell you that.

Now... Radiance can certainly perform calculations of spaces with CFS using
its "common" Ray-tracing. However, this may be slow for some purposes (i.e.
annual simulations and climate-based daylight modelling), and *this is why
2, 3 and 5 phase methods have been developed*. The *BSDF* representation, I
would say, goes in the same direction... It allow summarizing all the
bounces, reflections, refractions, etc. that occure withing the CFS in a
single matrix or tensor.* By using BSDFs*, Radiance itself and other tools
(i.e. EnergyPlus) can treat CFS as blackboxes, avoiding all the opcits
within the system. A BSDF that uses the Klems Full representation has
21,045 numbers (instead of one, such as the Shading Coefficient).

Being said all that, I would not trust a calculation method unless it can
actually deal with the optics of a CFS that is drawn and/or it can use BSDF
(or similar) information.

Lets remember that a perforated screen, a venetian blind, a light diffusing
device can all have a Shading Coefficient of 50%, but all of them will
behave very differently. I made some presentations about this on my
previous work (we sold complex Shading Devices), trying to promote the use
of BSDF in EnergyPlus calculations... the differences (in solar heat gains)
were more than considerable.

I hope that someone else gives us his/her perspective on this topic...
there are a lot of concepts that I might be misunderstanding.

Best!

···

2015-06-18 10:50 GMT-03:00 Ikrima Amaireh <[email protected]>:

Hi all,

I am really sorry for writing on the same point again but really in-need
for clear and complete answer, if possible.
Here, in my institution, people are in doubt if simple daylight
calculation tools like Ecotect (both standalone Ecotect as well as Ecotect
using Radiance engine methods) are efficient/enough for characterising
different complex fenestration systems (CFS) and most importantly
differentiate them for purpose of classification and rating?
Many are saying there's no need to go through Radiance to get almost the
same thing that can be achieved using other simple tools (like Ecotect).
For me, as a beginner with some readings and research, I found that Ecotect
is still not efficient tool to differentiate different CFS (especially if
they are very close in geometry, etc.) so we need something that's more
robust like Radiance to do the Job; does that sound right?

Your reply is highly appreciated.

Regards
Ikrima

This message and any attachment are intended solely for the addressee
and may contain confidential information. If you have received this
message in error, please send it back to me, and immediately delete it.

Please do not use, copy or disclose the information contained in this
message or in any attachment. Any views or opinions expressed by the
author of this email do not necessarily reflect the views of the
University of Nottingham.

This message has been checked for viruses but the contents of an
attachment may still contain software viruses which could damage your
computer system, you are advised to perform your own checks. Email
communications with the University of Nottingham may be monitored as
permitted by UK legislation.

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