if you have a digital camera, you can do a simple field test without expensive light meters. just point your cameras lens to the directions your n900s top side is aligned when used.

then just press shutter button halfway and look at aperture and shutter time (f4 1/500 for example) when moving a camera little bit. specially when there is dark outside and lamps are used, simple 5 degree movement can more than double the amount of light.

just tested with my dslr. when facing a lamp(read: screen parallel to roof) : shutter time 1/5000. 5 deg turn and it changed to 1/1600 so amount of light changes 5000/1600 = 3,125 times.