Take the radiance we all know and love...
Radiance is defined as the power emitted per unit area of the source into unit solid angle. The product of area and solid angle is called the geometric extent, which is at best conserved in any optical system.
Radiance can be decreased by filters or diffusers, but never increased by any optical system.
It is for this reason that radiance is used to account for light perceived by the eye through its photometric equivalent, luminance.
..and add some biophysical considerations
In evaluating hazards to the retina, the irradiance of the retinal image should be considered. With increasing exposure time this image is spread across the retina due to saccades and eye movements.
What is of interest is not the radiance of the source but the radiance in the area of the retina irradiated.
Radiance measurement conditions
In traditional radiometry, the radiance or luminance measurement is made with an optic that permits selecting the measurement field of view, and thereby the area of the source measured.
In all cases, the luminous area should be uniform and must extend beyond the field of view.
Photobiological Safety Solutions:
- IDR300-PSL Photobiological Safety Spectroradiometer
- ISR300-PSL Luminaire Blue Light Hazard Spectroradiometer
Spatially averaged radiance measurement conditions
On one hand we have a relationship between exposure time and angular subtense of the retinal image, on the other risk groups defined by permissible exposure time.
Combining the two, we have measurement conditions defined by risk group. Where the source is smaller than the field of view, a spatially averaged radiance will result.
The bottom line
For RG1 with time basis of 10,000s, measurements of spatially averaged radiance should be made in an 11mrad field of view.
Measurement at 200mm will encompass a circular area of 2.2mm diameter, no matter how large the luminaire to be measured.