About Image comparision of images taken from the webcam

Question

Hi,
My requirement is i connect a webcam to the system i will take photos using the webcam in the room for every fixed interval of time (like 1 hour or 2 hours)
and i will compare the present image with previous image,  By doing so i have to find out that wheather light in the room is on or off
Means  i take on photo when the light is on and i take another photo when light is off and i compare both photos then result should come "IMAGES ARE DIFFERENT"
IF i take two photos when light is on then result should have to come as "IMAGES ARE SAME" BUT    Its not working
i have taken two photos by keeping same paper in front off webcam with out any change those two photos are seems to be  100% similar and same but when i compared those two images pixel by pixel the result was "IMAGES ARE DIFFERENT"

WHY LIKE THIS IS THERE ANY OTHER WAY TO COMPARE THOSE TWO IMAGES THAT COMPARISION MUST BE ON MAJOR CHANGES LIKE LIGHT IN THE ROOM NOT ON VERY MUNUTE CHANGES