Pixel time series trend: What are the causes? I have taken an video of the following static scene for 10 mins (3 frames/Sec); totally 1800 frames for 10 mins. 



I also show here the pixel time series, that is, a set of values of a particular pixel say (1,1) for 1800 frames..



I could see a linear trend in the series. For some pixels the trend is increasing and for some the trend is decreasing. Due to trend, though the scene is static, there appears to be a small movement in the object. It is possible to de-trend it. 

My Question is: what are the causes of this trend? I use a standard camera fixed on a immovable plane and I illuminate with light in order to photograph it.
You are very likely to observe trends in either your camera or your light source. 

Sources from the camera can include:


thermal effects inside the sensor (basically, the more you use it, the more current will traverse it in various parts and it will get warmer)
drift in the ADC of your sensor (probably linked to the aforementioned).


Also, light sources can also drift with time: thermal effects will influence light bulbs.

You can try to mitigate these effects by:


changing your light source (I would assume LED lamps are more stable, but this needs to be checked)
waiting long enough before starting the measurements in order to avoid transitory phenomenon
carefully playing with the settings of your camera. Typically, I would fix manually all the exposure-related settings (exposure time, gain, framerate...) so that the autocontrols do not make vary the exposure values when an object enters the scene. However, this will prevent the camera from tracking natural scene trends.
simply fitting a linear regression to your signal in order to remove the trend while processing.