The luminous intensity indicates how bright the image passed by the binoculars is. The rule here is: the higher, the brighter. The light intensity is determined by the size of the front lens part. With a 10x42 binoculars, the second number 42 is for the light intensity. The front lens part of the viewer is 42 mm in size. The larger the diameter, the more light can fall into the viewer. You can calculate the luminous intensity calculates by taking the squad of the exit pupil.
Luminous intensity = Exit pupil2
However, the final result of the amount of light that comes out of viewer depends mostly on the quality of the used lenses and coatings. The number does not say anything about the ultimate light intensity of binoculars.