The human has formed this image from memory, but because this memory is swarming about, chemically speaking, with all the other circumstances of the encounter, it merges in the mind of the human. Like scenes that the filmmakers in Hollywood make, where a person on top of a wave may in fact be standing in a room, superimposed on a wave, the human encountering a ghost superimposes what the human used to look like. To the human, this has all been received as one impression. The majority of time, during 3rd and 4th Density, an entity will find itself incarnated. Upon death, the entity leaves its physical body, and this can also occur slightly before death too.
Disincarnate entities are in light form.
The reason some such entities appear brighter in light form than others is related to their spiritual maturity. The older and wiser, the brighter. They gain in spiritual bulk, one might say. The entity that feels an issue is unresolved will hang about, desiring to influence proceedings. This is much displayed in your media in ghost stories of one kind or another.