Over the past several years, high-resolution cameras, based on charge-coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensors, have replaced traditional cameras based on films, video tubes, and photomultipliers. While specifically designed CCDs for microscopic applications have been available for several years, the production of microscope camera systems based on CMOS have also recently emerged.
While CMOS sensors were initially thought to be inferior to the CCD sensors as a result of certain performance issues, technological advancements in the 1990s provided CMOS sensors with several advantages, some of which include smaller pixel sizes, reduced noise, better image processing algorithms, and larger image arrays.
The Working Principle of CCD & CMOS Image Sensors
The technology behind both CCD and CMOS sensors is based on the photoelectric effect, which occurs when incident photons interact with a semiconductor material, such as silicon, to promote electrons from valance band to conduction band. The electrons released during this process are proportional to the photon flux density, which includes both the wavelength and intensity of the incident light on the surface of a photodiode. While the electron signal during the illumination is converted into a voltage in CMOS sensors, the signal in CCD sensors is transferred to a metering register. This voltage or charge is then passed through a digital-to-analog converter to form the digital image.
As both of these sensors function by accumulating electrons transferred to the conduction band and not the color of the incident light, the CCD and CMOS sensors are innately monochromatic; meaning the image obtained would be black and white in nature. However, the color of the image can be obtained by passing the incident light either through a series of red, green and blue filters or with transparent miniature polymeric thin-film filters that are deposited in a mosaic pattern over the pixel array.
Performance Differences Between CCD & CMOS Image Sensors
CCD image sensors consume significantly more power than CMOS sensors due to the requirement of more than five supply voltages at varying clock speeds. On the other hand, CMOS sensors have lower power consumption because they only require a single-voltage power supply.
Unlike CCD sensors, CMOS sensors also allow for the incorporation of a number of processing and control functions which can be directly incorporated into the integration circuit of the sensor. Therefore, CMOS sensors can have several other features, of which include timing logic, exposure control, white balance, gain adjustment, shuttering, analog-to-digital conversion and other image processing algorithms, in addition to the primary task of photon collection.
CMOS sensors are very versatile for their ability to capture pictures at very high frame rates between 30 – 60 frames per second (fps). This allows time-lapse sequences and videos to be obtained in real time with software interfaces.
Disadvantages of CMOS Sensors
One of the biggest disadvantages associated with CMOS sensors is the high degree of noise observed while examining the images produced by these devices. However, recent advances in sensor technology have made it possible for better integration of signal processing circuit with the image array, thereby allowing noise levels to be significantly reduced.
In optical microscopes, the light from a light source is condensed by a condenser and incident upon the specimen, which is then transmitted into the objectives. The transmitted light is focused by a projection lens onto a sensor surface made up of semiconductor materials. The sensor then processes this information and converts it into a visual image. The image quality obtained by optical microscopes depends on the efficiency of the sensor to capture and process the electron signal to visual information. Advances in CMOS technology have led to vast improvements in their potential applications for this purpose. It is now common to find the CMOS sensors in a variety of imaging devices such as scanners, security cameras, cameras on computing devices, such as laptops and personal computers, as well as within microscopes.