The standard level of light in a room is usually 1 if it is being lit by a torch or some such. Outdoors it may be 1 or in some places higher depending on the time of day. Further sources of light and darkness will add or subtract from this ambient figure. A torch may shed 1 light a lamp may shed 2. The maximum levels to move either way seem to be around 50, such as the almost impenetrable darkness shed by the NPC:Huorns in Area:Fangorn Forest.
The number are hidden by the game and need not be of concern. The only fact you will usually have to concern yourself with is if you see a "Dark Room" and you haven't been been effected by Blindness then you need to add light to be able to see normally. Or to find some way to apply a sufficient level of Infravision to you character.
Usefull commands to see who or what in the room is shining light or shedding darkness.
- appraise light