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Defect pixels

Defect pixel is a generic term used to designate a pixel that does not behave like it should, but it is not a non-functioning or dead pixel. This short paper will list and explain the types of defects.

Dead pixels do not respond to light at all and they don't provide any information. They can be replaced using some interpolation but the lost information won't be retrieved. Dead pixels can be black, white (or let's say the maximum output value) or any intermediate value (also called stuck pixels).

Dead pixels are usually caused by open or shorts in the pixel circuitry, resulting in pixels that do not have reset or readout or supply or that are shorted to one of these signals. if the shorts occurs with the readout bus, it may result in a dead column.

Hot pixels respond do light normally but suffer from excessive dark current and can saturate at reasonable exposures even in the dark. Such pixels can only be used at short exposure time or at low operating temperatures.

Excessive dark current can be related to any of the causes of dark current including peripheral, surface, transfer gate leakage or others. It is usually caused by lattice defect or impurities or a not well lattice matched growth of the oxides.

RTS (Random Telegraph Signal) pixels respond to light and provide once in a while a sequence of correct values but they can randomly jump up and down with a well defined offset. Such pixels typically take two or three offset values but can have many more offset values. The time that the pixel spends in each offset state can be characterized but as the jumps are random in nature, they are very difficult to correct. However, advanced algorithms can fully retrieve the information from such pixels.

RTS is more common in very small pixels with very small technology feature sizes. As the number of atoms forming an electronic device decreases and the distance between devices decreases, a single impurity can produce a significant variation in the device's performance. The quantum state of the impurity may change from time to time and provide the jumpy properties changes.

Wide variance noise pixels have in average the right response to light but their noise is much larger than for the other pixels. The information can be retrieved using time averages.

Blinking pixels can be either dead blinking if they jump randomly between two dead states or blinking operating if they jump between the right value and a dead state.

Clipping pixels behave normally up to a certain value (resp. from a certain value) where they will clip. They are only usable below (resp. above) their clipping value. The pixels that start at a too high value sometimes only have excessive offset.

The criticity of a defect pixel also depends on its clustering with other defect pixels. An isolated defect is usually less of a problem because it can be detected analyzing its neighborhood and either interpolated or corrected. Clustering defects can cause more problems because the detection or correction filters may not work.

Special shapes of clusters are treated separately. For example dead rows or columns are related to readout circuitry defects and usually render the device unusable. Larger round clusters are related to surface contamination, non-movable dirt or surface damage.

As some applications are more tolerant to defect pixels than others, the EMVA1288 standard has chosen not to report a number of defect pixels but only statistics about the behavior of pixels. Each user has to decide what is acceptable for his application. The Aphesa EMVA1288 test equipment goes beyond the standard and provides additional information related to the form of defects and their clustering.

The number of defect pixels can also evolve over time by ageing. Cosmic rays are known to be a cause of additional defect pixels over time but usual semiconductor ageing, electromigration, scratches or other causes can increase the number of defect pixels. A very well cause are non-movable particles on the pixel array that will act as light shields or lenses.

 
 

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Aphesa develops custom cameras and custom electronics including FPGA code and embedded software. We also provide EMVA1288 test equipment and test services as well as consulting and training in machine vision and imaging technologies. Aphesa works in several markets including industrial, medical, oil&gas and security.