Relative brightness is a calculation of how bright the image should be when viewed through binoculars. It is presented as a square value of the exit pupil. 10x50 binoculars have an exit pupil value of 5.0 (dividing lens diameter with magnification). Square of 5.0 gives us a value of relative brightness which is 25.0. As the relative brightness value increases, we have a brighter image. On the opposite the lower the value, the darker the image.
In the past, relative brightness was an important value in determining the brightness of the optics. The manufacturers were using the same kind of technology and materials of the lenses, therefore, the optics were comparable. Nowadays, they use different types of lenses and modern coatings so the relative brightness has lost its meaning because the brightness of the optics depends more on the quality of the coatings than on the relative brightness.