CCD Blemishes Explained, including Hot Pixels, Black Pixels and Traps
All scientific cameras to some degree exhibit blemishes which impairs the faithful reproduction of the light signal. The primary source of the blemishes is the sensor itself and here are some of the blemishes that occur:
Black pixels are regions of the sensor typically pixels or small clusters of pixels which have significantly lower response than their neighbors (less than 75% the response of the average pixel). They are typically formed due to contamination on the sensor surface or embedded in the sensor. The effect of Black pixels can be removed by taking a flat fielding reference or by post processing interpolation to mitigate their effects. Black pixels are rarely a major issue unless they extend to many pixels.
Hot pixels have a much higher dark current than their neighbors (50 times higher than specification). They typically have a different temperature response than the bulk of the sensor and so can appear to differing amounts at different temperatures. They are usually due to contamination embedded in the sensor. The effect of Hot pixels, unless they are particularly large, can usually be removed by taking a background
A combination of blemishes may adversely affect a column. A column defect may be due to some of the following:
- A combination of more than 30 black pixels or hot pixels - Hot Column: a column which has a dark current greater than 2 times specification - Black Column: a column which has saturation less than 90% of the average column - A trap: see next section.
Traps are peculiar to CCDs, they usually only occur in a single pixel and they can be caused by contaminants getting into the CCD during the production process or by the effects of radiation on the CCD structure. They act by becoming temporary holders of charge. As charge is shifted though a trap, the trap holds onto a portion of the charge (the trap size), while the trap is filled subsequent charge transfers are unaffected. The charge in the trap slowly dissipates until it is refilled by new charge created by illumination or by new charge being shifted through the pixel.
Traps can be any size even down to a single electron charge trap but they are usually only noticeable when their size is greater than 200 electrons. Traps are identified by analyzing their impact on an illuminated CCD at a mixture of high and low light levels. The dynamic nature of the traps is difficult to model and therefore they are difficult to compensate for. Severe traps can be overcome by providing an initial light illumination to fill the traps before the proper exposure is required but this adversely affects the signal to noise.
Andor grades our standard cameras with the following definitions. Within the active image area which is defined as central area ignoring the 2.5% of the pixels around the edge of the sensor the following blemishes are allowed. Some large area or specialist sensors have their own definition as agreed by the sensor's manufacturer and their grading is defined in their specification sheets.