Relating digital imager defect rates to pixel size, sensor area and ISO

Image sensors measurements show a continuous development of in-field permanent hot-pixel defects over time. We have accumulated experimental data on cameras with sensors ranging in size from large area (>;300 mm²) DSLRs (Digital Single-Lens Reflex cameras), medium sized (~40 mm²) Point and Shoot, and small (20 mm²) cell phone cameras. Our results show that the rate of defects depends on the technology (APS vs. CCD), and on design parameters such as imager area, pixel size (from 1.5 to 7 μm), and sensitivity (from ISO100 to 6400). Previous measurements showed that increased image sensitivity (ISO) enhances the defect appearance. Comparing different sensor sizes with similar pixel sizes showed that defect rates scale linearly with sensor area, suggesting the metric we call defect density measured by number of sensor defects/year/mm². In an attempt to model this defect density as a function of both pixel size and ISO, we discovered a very good fit of the empirical densities to a power law in both the pixel size and the ISO, with slightly different parameters for CCD and APS (CMOS) sensors. Including the ISO in the equation allows us to predict the expected defect development rate for a wide set of sensor parameters. The power laws that we obtained show that defect densities are lower for CMOS pixels when pixel sizes are large (near 7 μm), but become higher than for CCDs at about 2 microns even for modest ISOs (near 400).