Predicting Pixel Defect Rates Based on Image Sensor Parameters

Abstract

Experimental measurements have shown that image sensors are continuously subject to the development of in-field permanent defects in the form of hot pixels. Based on measurements of defect rates in 23 DSLRs, 4 point and shoot cameras, and 11 cell phone cameras, we show that the rate of these defects depends on the technology (APS or CCD) and on design parameters the like of imager area, pixel size, and gain (ISO). Increasing the image sensitivity (ISO) (from 400 up to 25,600 ISO range) causes the defects to be more noticeable, with some going into saturation, and at the same time increases the number of defects. Partially stuck hot pixels, which have an offset independent of exposure time, make up more than 40% of the defects and are particularly affected by ISO changes. Comparing different sensor sizes has shown that if the pixel size is nearly constant, the defect rate scales linearly with sensor area, suggesting a measurement metric of defects/year/sq mm. Plotting this rate for different pixel and sensor sizes (from 7.5 down to 1.5 microns) shows that defect rates grow rapidly as the pixel size shrinks. Curve fitting shows an empirical power law with defect rate proportional to the pixel size to the power of-2.5. However, separating the pixel types shows that CCDs scale more slowly, with a power of-2, which translates into the pixel area. CMOS sensors, on the other hand, scale more rapidly with the pixel size to the power of-3.3. The result is that for 6-7 micron pixels the CCD defect rate is ~2.5 greater than the CMOS, but for 2 micron pixels the defect rates are both much higher and about equal. This paper presents a formula for predicting the expected rate of defect development for a given set of sensor parameters. This formula can be used by sensor designers when determining the imager parameters, taking into account the length of time the imager is expected to be in service.