"For we are strangers before thee, and sojourners, as were all our fathers: our days on the earth are as a shadow, and there is none abiding."
Although Godliness has the property of light, our mortal physical state is designed to protect the unprepared from the power of the light within us.* Thus, our righteousness is described not as the full light which is our birthright, but as the shadow--because the existence of every shadow necessitates a casting light.
When bodies are transfigured or made celestial, their structures will be changed in such a way as to allow the natural light to shine forth out of them. The transfigured, the translated, and those resurrected as angels glow not because their essence is changed, but because the structures that obscure our inherent light are removed.
*Just as we would be overwhelmed by standing in the presence of God in our sins, we would be overwhelmed if we stood in the true presence of any human being. This is why love sometimes aches even in simple moments of family togetherness: every so often, when I watch my wife and daughter, a piece of their souls shine through more than usual and I am both overwhelmed and physically moved by it.