I'm having a very irritating bug with my N900: While I use it the screen constantly dims and brightens, and dims again, and...

The rate at which it does this varies, and it has nothing to do with me touching the screen, using the keys, or whatever.

I had the Simple Brightness Applet installed, so I thought maybe that was what caused the problem, but now I've uninstalled it, and I still have the problem. I can't remember if this occurred before I installed it or not, but I think it did.

This is extremely annoying. Does anyone know if this is a software bug or a hardware issue? I couldn't find any talk about this her on the forums.

Do you use a screen protector of some kind? If so maybe the sensor is tricked all the time by reflections, assuming it covers the sensor.

I've experienced this before. I always thought it was something to do with the proximity sensor as i always notice it dims or lightens up when i cover it for a while, thought it was maybe a feature like the n95 and the backlight on the keyboard (when it gets dark, the keyboard would light up, and vice versa), then again... those are two different sensors.

Happens to me occasionaly but I know it`s me when I hold the device in a way when my left hand partly covers the sensor area - the screen would dim correctly. You dont have to completely cover the sensors, if it`s in the shadow it will dim.

A few possibilities come to mind:

(1) The device has an ambient light sensor at the top left corner on the front of the device (when holding in landscape mode), which is used to adjust the brightness of the screen automatically to some extent. Are you sure you are not covering the sensor when using the device?
Also as someone already pointed out above, it's also possible that screen protectors or strategically placed dirt / thumbprints might cause problems.

(2) It is possible to set the backlight timeout to a very short period (10 seconds) in the device settings. Check that this is not the case for you:
Settings application -> Screen -> Backlight timeout
(I'm not sure of the exact text on those items as my device is set to another language)

(3) Does the problem occur always, or only in specific locations or under specific conditions? Alternating light levels would cause the device to try to constantly adjust to the ambient light level by brightening and dimming the screen.
Also, some artificial light sources can mess up the light level detection on some light sensors, I'm not sure if the N900's sensor is affected by this. Most electric light sources don't actually provide constant light, but a rapidly blinking one, too rapidly to be detected by human eye but well detectable by a sensor.

(4) Have you tried rebooting the device?

(5) Have you tried reflashing the device? (If you try it, remember to take a backup first!)

(6) It's also possible your device might be faulty. If nothing above helps, consider replacing it.

Lots of people have reported this problem. It's the ambient light sensor, which responds even to a shadow from a hand holding the device on the left. There's a time delay of a few seconds before the display brightness adjusts.

You can see this for yourself. When you notice the display dimming, take your hand away or rotate the N900 towards a bright window. Within five seconds the display will return to its previous brightness.

There are two workarounds. The first one is to cultivate a habit of holding the device differently. The second workaround is to set the screen brightness to full (under "Settings" "Display"). At full brightness the screen does not vary in response to the ambient light sensor.

Regards,
Roger