Optics 101

CMOS Image Sensors in Optical Microscopy

Many types of imaging systems and sensors can be coupled with optical microscopy. One of the most common imaging sensors in recent years has been charged coupled devices (CCD). Despite being produced decades ago, the use of complementary metal oxide semiconductor (CMOS) image sensors in optical microscopy was often overlooked. In this article, we look at how CMOS image sensors have started to be used in some optical microscopes.

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It was thought that CMOS imaging systems were unsuitable for optical microscopes, which paved the way for the widespread use of CCD image sensors. However, in this time, CMOS image sensors have advanced and now yield small pixel sizes, a low noise, highly capable imaging algorithms, larger imaging arrays, a low energy consumption and a single-voltage power supply; and have since come into some favour to combat the high voltage requirement of CCD arrays, which is often manifested through multiple voltage power supplies. It should be noted that the term “CMOS” refers to the process by which the image sensor is made and is not a specific imaging technology.

CMOS Image Sensors

CMOS imaging sensors, much like their CCD counterparts, image samples under an optical microscope using the photoelectric effect. The photoelectric effect is the emission of electrons, or other charge carriers, after a sample has been subjected to photons of light. In optical microscopy, the image sensor contains a series of semiconducting photodiodes, where each photodiode acts as a pixel. Light is gathered by the objective, focused using a projection lens, where it travels through the sample and onto the photodiode. The emitted electrons are collected within a potential well of the photodiode until the light illumination ceases. From this point, the electrons are converted into a voltage which helps to construct the image pixel by pixel.

Because images created by CMOS image sensors (and CCD sensors) are a function of the number of electrons that accumulate at the photodiode, and not the color of light, they inherently provide images which are black and white in nature. However, it is possible to create color images with CMOS image sensors by passing the incident light through red, green, and blue filters.

Structure of The Image Sensor

In a conventional CMOS image sensor, the photodiode array is located within the central region of the device and is often surrounded by the sequentially arranged colour filters to provide color images, with the potential well located below. To concentrate the inbound photons to the photodiode array, a positive meniscus microlens is often employed on top of the device which directs the path of the photons towards the photodiode array. Other components within the image sensor that aid in the overall conversion from photon, to electron accumulation, to image generation include analog signal processing circuitry, row and column bus transistors, a reset transistor, and an amplifier transistor.

Each pixel in a CMOS image sensor contains the photodiode alongside a series of transistors that help to convert the accumulated electron charge into a measurable voltage output, as well as aid in resetting the photodiode and in the transfer of the voltage to a vertical/row column bus. The resulting image generated through this approach enables the pixels to be read and identified through simple x,y coordinates.

Advantages of CMOS Image Sensors

One of the main advantages of CMOS image sensors over their CCD counterparts is their ability to incorporate multiple processing and control functions which extend beyond the collection of photons from the illuminating light. Some of the possible functions include timing logic, exposure control, analog-to-digital conversion, shuttering, white balance, gain adjustment, and initial image processing algorithms, but to perform all these functions requires the photodiode array to act in a manner akin to a random-access memory (RAM) cell rather than a conventional photodiode array. Another key advantage of a CMOS image sensor over a CCD array is that they have a built-in amplifier within every pixel, which enables more photons to be detected and electrons to be generated– this is known as active pixel sensor (APS) technology.

Even though CMOS image sensors are now being used a lot more within optical microscopes to image the sample, there are many different types which ultimately depend on the type of microscope and samples being imaged. Each manufacturer uses a slightly different type of image sensor system, so it is not a “one size fits all” imaging approach, which often varies with respect to the number of pixels and frame rates. However, with research today requiring the use of many different types of imaging modes, this will be of no surprise to many.

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