INSIGHTS
Historically, CCD (Charged Coupled Device) sensors have existed much longer than CMOS sensors, that is to say, for more than 40 years. Due to constant improvement and optimization over the years, CCD sensors today stand for excellent image quality. In 2009, the American scientists Willard Boyle and George E. Smith were awarded the Nobel Prize for Physics for the invention of the CCD sensor. Originally developed in 1969 for the storage of data, the potential of the Charge Coupled Device as a light sensitive apparatus was soon realized. By 1975, the first sensors with a resolution sufficient for television cameras appeared. However, it took more than 10 years before the process technology was mature enough to begin production of CMOS (Complementary Metal Oxide Semiconductor) sensors. In the mid-nineties, the first commercially successful CMOS sensors appeared on the market.
CMOS and CCD – Small Differences Along the Way from Light to a Signal
Date: 2011/09/15
Source: Dr. Joachim Linkemann
It can become too bright for a sensor
If especially bright light is present, users must sometimes struggle with other effects. When a CCD pixel is overexposed to light, the pixels can generate an excess of electrons that can migrate into neighboring pixels. In this situation, very bright image structures seem to extend into the darker structures, an effect known as “blooming”. CMOS sensors don't see the migration effect and are not prone to blooming like that.
After a CCD sensor has been exposed to very bright light, the charge transportation process can cause bright stripes to appear in the image. The bright structures within the image appear as white smeared lines. This effect is known “smearing” and can be seen, for example, in images acquired at night when a car approaches the camera with its headlights on. As with blooming, CMOS sensors have an advantage compared to CCD sensors because they do not exhibit smearing.
Small bad habits and how they are moderated
For technical and cost reasons, quite a large number of commercially available cameras with CMOS sensors implement a rolling shutter. With this image acquisition method, all of the lines in a sensor are not read out of the sensor at a single instant in time. Instead, there is a time lag between the start of readout for each line in the frame.
In cameras with a global shutter, exposure is started for all pixels at the same time and is stopped for all pixels at the same time. In cameras with a rolling shutter, exposure is started sequentially line by line and is ended sequentially line by line. With rolling shutter technology artifacts can occur, particularly with rapid moving objects (unwanted, for instance, in intelligent transportation systems). These days, other CMOS sensor technical inadequacies can easily be compensated for within the camera. This includes non-uniformities that appear during the conversion of charges to gray values (ADC inequality) or that result from variations in the electrical properties of the numerous individual readout structures used in a CMOS sensor. Defective pixels can be corrected by interpolation in the camera. In addition to correcting the inadequacies already mentioned for CMOS sensors, the electronics in today's cameras can also correct for the irregularities exhibited by CCD sensors.
The agony of choice
CMOS sensors have a shorter history than CCD sensors, but they are now technically mature and even represent the best choice for some applications. Even novelties such as backside illuminated sensor technology (BSI), which was first introduced for CCD sensors, are currently entering the CMOS world. With this technology, the photosensitive area in each pixel is increased because no electronics are obscuring the pixel area.
But when you must choose the optimum solution of a specific task, many of the aspects mentioned in this article will come into play. In the end, the constraints and the details of your application should determine which sensor technology represents the best alternative.