Hello IanK,
The source units of lumens are the total luminous flux emitted by the source. they are the photometric units corresponding to watts in radiometric units. The measurement of luminance in angle space is in lumens per square median per steradian. This is somewhat analogous to the perceived brightness of the extended source as seen from the detector. They are related -- if you double the source lumens you will see the detector values double. But they are not measuring the same thing. The detector is plotting luminance in angle space over a field in angle space. The peak luminance value listed is the maximum value over that field. You can also see listed the total power in lumens. That is the total luminous flux falling on the detector, which should be less than the sum of the sources - in this case one source.
Hello IanK,
The source units of lumens are the total luminous flux emitted by the source. they are the photometric units corresponding to watts in radiometric units. The measurement of luminance in angle space is in lumens per square median per steradian. This is somewhat analogous to the perceived brightness of the extended source as seen from the detector. They are related -- if you double the source lumens you will see the detector values double. But they are not measuring the same thing. The detector is plotting luminance in angle space over a field in angle space. The peak luminance value listed is the maximum value over that field. You can also see listed the total power in lumens. That is the total luminous flux falling on the detector, which should be less than the sum of the sources - in this case one source.
Hi David,
Thank you for responding to my question.
The detector is plotting luminance in angle space from what the detector is receiving from the source.
What I do not understand is why adding a lens in front of the Lambertian source would cause peak illuminance to go higher since luminance lumens per square meter per steradian. Is this an issue with pixel size?
My main goal is to be able to collect luminance values of an optical setup at some distance away; something similar to using a luminance meter, at some arbitrary distance away from a source. Can OpticStudio’s NSC model this?
Hi all,
I just wanted to comment on this thread, as I think there are a few other considerations going on when OpticStudio reports luminance/radiance that may also explain the results you’re seeing.
When OpticStudio generates Luminance (Angle Space) results, it will start with Luminous Intensity data, and then scale data per-pixel based on the area of the detector times a cosine factor with the angle represented at the center of any given pixel. In other words, the solid angle information is broken down per-pixel. For the same size, resolution, and Max/Min Angle Range values for a Detector Rectangle, this angular pixel mapping would remain the same, but then you would get different results if your far-field distribution has changed. Since it looks like the lens “narrows” the angular distribution of rays after the lens, you are putting more energy in a smaller angular range, resulting in a higher peak luminous intensity value, thus getting a higher peak Luminance (Angle Space) result.
This stands in contrast to the treatment of radiance where I think you’d need to know the changing solid angle of the illumination itself after the lens in order to observe the conservation of radiance you’re expecting. Again, OpticStudio will just post-process either the illuminance or luminous intensity data for Luminance depending on whether you select Position or Angle Space. You can also find more discussion on this in our Help Files at “The Analyze Tab (non-sequential ui mode) > Detectors Group > Detector Viewer ”.
Let us know if you have any more questions here -- thanks!
