Q+A CENTER

Finally, if we examine the complex issue of image manipulation, the CMOS device provides a number of advantages relating to cropping, pixel integration, in-pixel ADC and other possibilities which places it ahead of the CCD in pure potential usage.

Is theonly relevant number in choosing a camera the pixel count?

The way cameras are sold today it may seem that way, but pixel count is not the only way that the image sensor affects the final image. Pixel count is the number of pixels that the image sensor has; what is equally important is the size of the pixels, their distribution on the chip, the RGB filtering system and the frame per second, the low-lux performance, power consumption and, of course, the price of the camera. The camera manufacturers know very well that pixel count is a very small part of the overall image quality, for most cases the average buyer would be perfectly happy with a 1.3 Megapixel camera.

What are the technical restrictions that limit a CMOS digital camera'simage quality, and how are the designers getting around them?

The main things that restrict the sensor's quality can be categorized into two parts. The first relates to signal strength. This include the Well Capacity or the number of electrons that can be held in the photodiode; the Quantum Efficiency, or how efficient the diode is in converting the photons into electrons; and the Effective Light Sensing Area, which is directly proportional to the amount of light that can be detected. The second category relates to the noise strength. This includes the Dynamic Noise Floor or the base background noise generated by the circuitry, heat, leakage and imperfections across the diodes; the Fixed Pattern Noise generated by the mismatch between the pixels and the ADCs; and Crosstalk generated from unintended signals coming from neighboring devices and circuits. To improve the signal, the designer needs to maximize the photodiode area by improving the layout and selecting the best micro lens (the small lenses atop each diode) coverage. To reduce noise, he must isolate sensitive areas from the rest of the chip while the foundry has to find better materials and recipes to reduce leakage and imperfections. The foundry can optimize the doping to increase the quantum efficiency.