A movie is only as good as its screen play. The screen play has to give the reader an accurate representation of what will eventually be going on during the finished film. A screen play must also be written in such a way as to convince actors, financiers and producers that your movie is worth making in the first place. Putting specific lighting directions into a screen play is a simple process that can be done in just a few seconds.
Write your lighting directions into the stage directions of your script. Stages directions are the blocks of text that set the scene. These bits of text describe what the characters are doing, what props are in the room and other things of that nature. This is the appropriate location for lighting directions.
Don't over-complicate things. Having too many stage directions in a script can be boring to readers, so never use more words than you have to. If you want the scene to be dimly lit, write something simple such as, "The room is dimly lit." Don't write 200 words about the specifics of how and why the room is dimly lit--that isn't your job, it's the director's.
Never write what you can't see. Everything in a script has to translate to the finished film. If it doesn't, then it's wasted space in the script. So if your scene is dimly lit because it symbolizes a character's mood or something of that nature, you can't actually write the reason or its symbolism. All you can write is, "The room is dimly lit."
Never write unnatural lighting directions. Don't write lighting directions into a script just for the sake of having them. Consider what's going on in your scene--if you want it to be dimly lit, it has to be so because "a small lamp is the only lighting in the room" or due to a similar reason.