Avian’s Blog

Every time I go to sleep in my room and turn off the lights I get the impression of seeing little points of light on the ceiling. They look a bit like stars on the night's sky, but are a lot fainter.

At first I thought it was only my eyes adapting to the dark. But once I turned off the light when I was still standing and the lights remained on the ceiling, not floating in front as a typical residual image on the retina. I decided to investigate this further.

First I had to make sure this wasn't some kind of an optical illusion. I took my digital camera, put it on my bed facing upwards and set it to maximum sensitivity (ISO 800) and longest exposure time (4s). I then took two pictures: one with the protective cap on the lens and one without the cap. This way I could then remove any camera artifacts by subtracting the first image from the second. Following is the resulting image:

There is still a lot of noise in the image, but you can clearly see some green dots. These dots are larger than one pixel across so I guess they can't be caused by CCD sensor noise.

So if this is not an illusion, what is it? I can't see any significant features on the ceiling when the light is turned on. It could be light from a room above coming through the floor. This would be possible if the ceiling would be wooden and had cracks in it, but as far as I can see, the building is concrete and also all rooms have thick carpets on the floor.

Another possibility would be that the ceiling finish contains pieces of some phosphorescent material. I believe this is more likely, since I don't remember seeing these dots when I wake in the middle of the night when the light has been switched off a couple of hours. However in that case it is weird that the dots only appear above the bed. I looked for them on other parts of the ceiling but haven't seen any.