Rtrace and Rcontrib


Hello ,

I am a new user of Radiance. I basically use Radiance implemented in other software such as RadianceIES in iES_VE. I am trying to understand the daylight analysis carried out using rtrace and rcontrib since these two are provided in RadianceIES for illuminance analysis.
However, I am confused about what parameters are used by these two tools.
Do they use direct sun, diffuse sun, direct sky, diffuce sky?
What type of skies do they use? CIE skies or Daylight Coefficients?
I read that rcontrib uses DCs (correct me if I am wrong). Does it use data from weather files?

Finally I would appreciate it if anyone could suggest any books or papers to understand more about daylight calculations used for static analysis or CBDM.
Also, are there any webinars or online courses presenting Daylight simulation - calculation techniques?

Thank you for your time!



I think you are kind of confused. RTRACE and RCONTRIB are Radiance programs
that run ray-tracing processes. As such, they both work with Radiance
scenes description, which can or not contain diffuse sky, sun, direct sun,
etc. The preferred method for generating a sky/sun is to use the GENDAYLIT
and GENSKY programs.

The difference between both programs, at least from my experience, is that
RTRACE calculates the luminance/illuminance over a set of points; while
RCONTRIB calculates the contribution from different materials on the scene.
For instance, in a scene with a sky and sun, RTRACE will consider both of
them to calculate the illuminance over a desk. RCONTRIB, on the contrary,
allows you to calculate the contribution from each of them separately.

The most common use for RCONTRIB (I think) is to calculate the Daylight
Coefficients. That is, the contribution of different parts (bins, patches?)
of the sky on the illuminance/luminance of a point. The way to do this is
to provide RCONTRIB with a scene and a .cal file that contains information
about the subdivision of the sky (i.e. reinhart, tregenza, or other). This
process is currently more easily handled by RFLUXMTX program.

After calculating the DC matrix, you can use the programs GENDAYMTX or
GENSKYVEC to generate the brightness of each sky patch, multiply it by the
DC matrix, and then add all together to calculate the illuminance.

However, DC matrix method has been modified several times in order to
create the 3-phase method, to add direct sun contribution, etc. You can
find information about this in the tutorials in

Does this make any sense?




Thank you @German_Molina !
All the information was very helpful!

I have been trying to make a validation study using rtrace and rcontrib and seeing that rcontrib was a lot closer to the on-site data, I needed to understand how these two tools work for daylight simulation.


Hi Maria,
There’s quite a bit on validation, sky models and daylight coefficients (now, referred to as CBDM) here:
PS. “quite a bit” is perhaps something of an understatement…

John Mardaljevic PhD FSLL FIBPSA
Professor of Building Daylight Modelling
School of Architecture, Building and Civil Engineering
Loughborough University
LE11 3TU, UK

Tel: +44 1509 222630 (direct)



Personal daylighting website:


Thank you @John_Mardaljevic!
I found your PhD thesis a year ago and it has been really helpful to understand Radiance.

From your professional experience do you believe that new lighting designers and researchers should learn using Radiance as command lines? Or it would be ok to use Radiance implemented in other software such as Desktop Radiance, RadianceIES, DIVA etc.

I personally found it a lot easier to use Radiance in other software since it is more user friendly and I found it a bit hard to follow the tutorials for Radiance in command-lines and understand exactly what those numbers are used for. However, some times it is difficult to figure out what type of simulations they use such as rtrace, rcontrib etc.


I’m probably the last person who should answer that question since I have next to no experience with Radiance-embedded tools. Perhaps others could chime in?