dark room, darkroom

Dark room or dark room ( two words ) means an indoor space with no light, or with no lights on at night--
The treatment for measles used to include keeping the patient in a dark room.
How can you read in a dark room? Turn on a light.

Darkroom or darkroom ( one word ) means the room a photographer uses to develop film negatives, which would be spoiled by light--
A darkroom may be lit by one dim red or green light bulb.
Don't open the darkroom door, or you may ruin the film.

Now that you know that, you can say--
"An ordinary dimly lit or dark room is okay for loading a film camera, but you'll need a darkroom to develop the film."