While most of my articles to date have focused on newly available technologies and how they are being implemented, occasionally it is entertaining to look down the road and see how next-generation tech might impact the products available in our industry.
Enter “quantum dots,” a technology that certainly sounds futuristic – if nothing else; however, quantum dots are more than a flashy name, in fact, a few short years from now, they might be the reigning technology used to produce the highest quality surveillance camera images.
Image Sensor Technologies
Before we dig into what a quantum dot actually is, let’s take a look at a brief history of the prevailing image sensor technologies that are used in our industry.
For those unfamiliar, an image sensor is the component in a camera that is used to detect visible light and convert it to an electronic signal that can be used for transmission or display. Light falls on an array of pixels within the sensor in varying intensity. Within that pixel, a semiconductor material absorbs light (photons) and releases electrons in proportion to the intensity of the light. The electrons are converted to a voltage which is then processed by the camera circuitry. The imager itself is color blind, so a filter in front of the imager allows it to assign a color tone to each pixel. To date, two primary technologies have been used to facilitate this exchange: CCD and CMOS.
Charge Coupled Devices (CCD): The CCD imager has been around since the 1970s and was the primary imaging component in surveillance cameras for much of the industry’s history. The CCD functions through what is known as a global shutter, meaning the imager starts and stops the light exposure for all pixels simultaneously. The amount of light is transferred to a storage array where it is read out one row at a time through a single output amplifier and analog-to-digital converter. While CCDs were historically known for a wide dynamic range (ability to handle both bright and dark scene elements) and low noise, all A-D conversion occurs outside of the image sensor. This means more external circuitry, power consumption, and heat, as well as limitations on refresh rate.
Complementary Metal-Oxide Semiconductor (CMOS): While CMOS technology has been in existence for as long as CCD (longer, by some accounts), it offers several advantages over CCD and is the prevailing technology of the day. In contrast to CCD, CMOS sensors have typically used a rolling shutter, starting and stopping one pixel row at a time (though some global shutter CMOS sensors are used today). The biggest difference, however, is in the conversion from analog to digital signals. CMOS sensors perform this operation within the image sensor itself at the pixel level, allowing for a much higher frame rate while using far fewer external electronic components.
Historically, there were many compromises in image quality from CMOS sensors, such as dynamic range. Without an overly technical explanation, these limitations come from how the electrons are able to move around in a CCD sensor vs. its CMOS counterpart. Over time, however, the CMOS technology evolved to overcome these issues, resulting in the general demise of CCD technology.
Quantum Dots (QD)
With the history lesson complete, we look forward to the next generation technology: quantum dots. Quantum dots (QD) are nanometer-sized hunks of semiconductor material. While CCD and CMOS both use silicon as the semiconductor sensor which detects light in the 300 to 1000nm range (the visible spectrum), QDs use a variety of specialized materials based on application.
QD are similar to traditional imagers, in that photons strike the semiconductor material and electrons are released. The difference is that the quantum dot is tunable, meaning that simple adjustments in its material or size can result in the ability to absorb light almost anywhere in the visible or infrared spectrum.
This has enormous implications for security. While infrared cameras are common in security, they require a vastly different imager manufacturing process. This process involves the use of different semiconductor materials that are adept at absorbing infrared light. Pixel arrays using these materials are separate from the CMOS circuits, and they must be bonded detector-to-circuit at every pixel. This is a costly process.
Quantum dots, however, can be simply sized differently to absorb infrared light, meaning their production is effectively the same cost as quantum dots sized for visible light. The removed bonding requirement means smaller pixel sizes, smaller optics, and with it the promise of new infrared image sensor applications at a lower cost.
Speaking of cost, another advantage of QDs lies in how they are applied to image sensors. QDs absorb light much more efficiently than silicon, which means the light absorption layer in QDs image sensors can be significantly thinner – so thin, in fact, that the QDs can be suspended in solution and simply printed on top of the circuitry. Thinner sensors mean more compact cameras. This thin layer of QDs excels in both high and low brightness levels, meaning that it inherently has a wide dynamic range.
Quantum dots are being widely used today in the reverse process to create light. Quantum dots are electroluminescent (emit light) when given a charge, with the light wavelength also determined by the size and material of the dot. This technology is commercially known as QLED and is available in a number of high-end consumer brand televisions.
QLEDs offer several advantages over OLEDs in that they have purer colors, last longer, and can be manufactured more inexpensively. They can also be printed over almost any substrate, including flexible and rollable displays.
The main barrier to adoption today in today’s camera image sensors is stability. Quantum dots oxidize in air, which creates inconsistencies in the produced image. To eliminate this issue, they must somehow be sealed in a type of polymer that does not impact the way electrons move around the sensor.
Other issues exist in the manufacturing process with producing consistently sized and formed QDs. Solutions to these issues are being actively researched by scientists. The amount of attention being paid to these solutions may mean that the days of CMOS technology are likely numbered, and our industry is due for another quantum leap forward in image technology.
Brian Coulombe ([email protected]) is Principal and Director of Operations at Ross & Baruzzini | DVS. Connect with him on Linkedin at www.linkedin.com/in/brian-coulombe or Twitter, @DVS_RB.
