Innovation in photodiode sensitivity underpinned by AI integration, device miniaturization, and energy-efficient designs delivers sharper forecasts and a more sustainable future.
MARIE FREEBODY, CONTRIBUTING EDITOR
As the world grapples with escalating environmental
challenges, demand
has never been greater
for durable and high-
performance monitoring technologies. As a result of the number of different environmental applications that require advanced detection, as well as the volume of necessary deployments for these solutions, opportunities are surging for systems that offer improved levels of precision and adaptability.
Photonics innovations are already
essential to many remote sensing applications. Optical sensing technology thrives in environments that are remote, hazardous, and extreme, enabling improved outcomes in emergency situations and challenging settings. Modalities ranging from lidar to opto- and photoacoustic sensing to thermal imaging are among the many techniques that are well established in the realm of optical remote sensing.
An airborne drone with a hyperspectral and lidar payload maps an algal bloom with phenotyping (false-color overlay). The airborne system worksalongside a ‘ground truth’ mechanism (boat) offering direct sampling via a Raman imaging flow cytometer. Courtesy of Headwall Photonics.
The sophistication of today’s solutions begins at the component level and extends to full systems. Photodiodes enable precise and rapid data collection across diverse scales, from the microscopic to expansive terrains. Sensing incoming light from UV-IR LEDs or lasers, commercial photodiodes support the lidars and spectrometers that play a critical role in many advanced detecting, monitoring, and forecasting systems. AI integration into these systems is anticipated, as are continued improvements in sensitivity
and spectral range as well as device miniaturization.
At the same time, the technology gains that are central to today’s ever-improving solutions are only part of a broader trend, insofar as it relates to current needs in environmental sensing and detection. “As recognition that industrial and civic activities are the causes of environmental problems becomes established, the purpose of sensing may shift from understanding environmental issues to preventing the occurrence of environmental problems, evaluating pollution improvement (purification), and quantifying the environmental contributions of businesses,” said Jake Li, marketing research manager at Hamamatsu.
“Additionally, the increase in the production of biofuels and the manufacturing of new industrial products, such as bioplastics, may create new sensing needs.”
From assessing flood damage and monitoring air quality, to forecasting solar energy and optimizing agricultural conservation practices, optical remote sensing is revolutionizing environmental
management. As the sensing and monitoring technologies that enable these and many other applications continue to improve, the distinct advantages that
they deliver are finding utility across existing and new-to-emerge sectors. Notable advantages include noncontact and nondestructive measurement as well as the capability to collect vast data sets in real time.
The result is faster, better-informed decision-making in both situation assessment and prevention.
Urban pollution monitoring
Especially in large metropolitan areas, motor vehicles remain a significant contributor to urban ozone and particulate pollution. This problem persists despite the introduction of periodic emissions inspection programs and tightening air quality standards aimed at protecting human health and the environment. In the U.S., the Federal Highway Administration (FHWA) Spring 2024 long-term forecasts of national vehicle miles traveled (VMT) show total VMT increasing (at an average annual rate of 0.5%) between 2019 and 2050.
Vehicle inspection and emissions
testing firm Opus Inspection Inc. has
developed technology that aims to help with ensuring that standards are not circumvented on emission tests. The company’s remote sensing solution uses fast-response photodiode arrays to
measure vehicle emissions from the roadside as motorists drive by. The device provides real-time roadside inspection that can be used to monitor full motor vehicle fleets.
“In Denver, remote sensing devices are used to complement the local periodic inspection programs by screening the cleanest emitters for exemption from the program,” said Niranjan Vescio, director of remote sensing at Opus Inspection.
“In other programs, they identify the highest emitters for urgent reinspection and repair.”
Portable spectroscopy systems are commonly used to identify concentrations of so-called criteria pollutants — such
as ozone, nitrogen dioxide, and particulate matter, all of which are notorious
for their adverse health effects and
damage to crops and ecosystems. These systems operate efficiently without requiring physical sampling, which makes them ideal for dynamic, real-time monitoring.
Opus Inspection Inc.’s remote sensing systems enable roadside emissions inspection in real time using fast-response photodiode arrays to measure vehicle emissions. Courtesy of Opus Inspection.
Many such systems are equipped with photodiodes. Developments in manufacturing processes and, further upstream, materials science, have enabled photodiodes to withstand extreme temperatures, high radiation levels, and mechani-
cal stress. The resulting components capture subtle changes in light intensity and wavelengths that are vital for detecting trace pollutants and capturing minute changes in the environment.
Particulate matter is a common thread between general emissions monitoring and other environmental applications, such as assessing water quality. Particulate monitoring typically involves the use of light-scattering methods: Light from a source (a laser or LED emitter) is scattered and then measured using a photodiode array, typically silicon. The demand for more efficient, accurate, and reliable gas monitors has increased the demand for deep-UVC LEDs and silicon carbide (SiC) detectors for UV gas sensors as well as MIR and SWIR LEDs and indium
gallium arsenide (InGaAs) or lead selenide (PbSe) photodiodes for IR gas monitors.
LED and detector manufacturers, including Marktech Optoelectronics, are pioneering active emitter-detector technologies that integrate multiple-wavelength LEDs and photodiodes into a single compact package. These multi-chip devices enable water quality instruments to simultaneously detect various analytes in water samples and several gases in continuous emission monitors. By streamlining functionality, innovations such as these enhance detection capabilities and significantly reduce the complexity and cost of environmental monitoring.
“SWIR and MIR LEDs coupled with compound semiconductor detectors such as InGaAs and PbSe photodiodes are newer optical sensing technologies used to monitor carbon dioxide and methane
releases and levels in the atmosphere,” said Mark Campito, CEO of Marktech. “These newer solid-state or semiconductor-based optical detectors and LED emitters are replacing older thermal emitters and pyroelectric infrared detectors.”
Methane, a major air pollutant and greenhouse gas, is often released due to equipment malfunctions in industrial facilities, or pipeline leaks. In 2022, more than 1000 methane leaks were identified worldwide. According to Li, gas detection through optical absorption by IR or UV light is 3× more sensitive than traditional technologies for the detection of methane and also enables leaks to be pinpointed more quickly and easily. This means that maintenance can be conducted swiftly upon the detection of a leak.
Emissions monitoring at every scale
Though the emission of an array of greenhouse gases can be assessed locally at chemical plants, assessing overall levels in the atmosphere requires global monitoring, which can be achieved using satellite-based optical sensor systems. Real-time lidar systems are integrated with global navigation satellite systems and an inertial navigation system. Photodiodes play an essential role in these systems, too, converting reflected light signals into an electrical signal with high sensitivity and fast response speed.
Their function in such systems, however, overcomes a challenge that stems from the fact that lidar systems operate
under background noise created by sunlight (and other light sources). Even though light filters largely compensate for these factors, a low dark current level and high selectivity are essential for the photodiode to reduce the effect of noise.
For example, measuring air quality in a parking lot with two different lidar systems produces different point clouds depending on atmospheric conditions. In rain, snow, fog, or in a dusty parking lot, a system operating at 905 nm may provide a completely different picture than that of a system operating at 1550 nm. On the other hand, the same 1550-nm lidar survey carried out at an altitude of 120 m in the remaining areas results in a slightly better picture than that of a 905-nm system used at an altitude of 50 m.
Comparison of Photodiode Materials:Silicon and Indium Gallium Arsenide
Courtesy of Inertial Labs.
“Modern lidars often work with several laser pulses simultaneously, which requires detecting multiple signals with high accuracy. Photodiodes with a matrix structure of photodiode arrays can simultaneously analyze many signals,” said Anton Barabashov, vice president of business development at Inertial Labs, a designer, integrator, and manufacturer of inertial measurement units and other sensing systems.
“Thanks to this, we can scan large areas, and the detail of objects increases.”
Soil and Earth monitoring
Soil emits significant levels of greenhouse gases including carbon dioxide, nitrous oxide, and methane — each of which contributes to climate change. Optical sensing technologies such as NIR spectroscopy and back-thinned CCD
(BT-CCD) image sensors track these emissions and support precision agriculture by improving crop health and soil condition assessments. Additionally, satellite-based sensors and lidar are used to detect pollutants, mining tailings, and plastics on land and in oceans. InGaAs photodiodes have enhanced SWIR imaging for these applications, enabling the identification of hazardous mining by-products as well as potential resources within old tailings, such as lithium or other valuable elements.
“Carbon emissions from the soil in farming primarily occur when soil
organic matter decomposes due to microbial activity,” Li said. According to Li, several technologies have been developed to help reduce these emissions. This has allowed farmers to gain additional revenue through selling carbon credits, he said.
Single (left) and multiple-element silicon photodiodes, plus assorted linear arrays and quadrant photodiodes (right). The complex 65-element silicon photodiode (center) can be mounted on an aircraft wingtip to detect laser pulses bounced off ice particles to monitor particle size distribution. Courtesy of Opus Inspection.
Hyperspectral technologies are bridging the critical gap between satellite-based observations and local sampling in Earth monitoring. Advanced visible-NIR and SWIR imaging systems deliver high-resolution, actionable data by capturing detailed spectral signatures at spatial resolutions far greater than satellites can achieve. This makes them indispensable for analyzing soil health as well as applications such as detecting algal blooms and monitoring methane emissions. Operating from aerial or ground-based platforms, these systems provide precise environmental insights at scales and resolutions unattainable by satellites or traditional sampling alone.
“With hyperspectral imaging, we are not just observing the environment — we are uncovering the detailed chemistry that drives it,” said David Blair, remote sensing general manager at Headwall Photonics.
“This level of insight is critical for tackling today’s environmental challenges with smarter, more sustainable solutions.”
Shining a light on water safety
Water contamination from industrial and agricultural runoff is a pressing concern; pollutants such as nitrites and nitrates
affect lakes, oceans, and drinking water.
For companies including Silanna and Marktech Optoelectronics, developers of rugged IR and/or deep-UVC LEDs for spectrometry systems, considerations regarding device portability and real-time results are paramount. Today’s water quality sensors and emission monitors use mercury vapor lamps, which have limitations. They require warm-up periods, cannot be pulsed, and produce high heat, leading to sensor lens fouling. They
also emit a broader spectrum of light,
including higher UV wavelengths that may cause damage to human end users. In contrast, newer 235- and 255-nm deep-UVC LEDs exhibit lower heat output and eliminate lens fouling.
“They do not contain toxic compounds, can be instantly turned on and off, and their pulsing capability further extends their long lifespan,” Campito said. “These advantages make deep-UVC LEDs and detectors a safe, efficient, and compact solution.”
Other optical approaches use colorimetry, fluorescence, scattering, and/or spectroscopic methodology to analyze the chemical components and particulates
in water samples. For example, a value of the turbidity (the amount of suspended particulate matter in a water sample) can be obtained by measuring the amount of scattering from an IR LED — typically 860-nm LEDs — combined with a silicon photodiode array. IR light minimizes color interference or absorption due to particles with different colors. In fluorescence water quality monitoring, meanwhile, fluorometers with an LED at a peak wavelength that is close to the excitation wavelength of the fluorophore is used. The photodiode detects the emission wavelength from the fluorophore. Optical filters can then be used on the LED and photodiodes so that only the fluorescence light emission is detected.
Ensuring seamless energy generation
Solar energy is one of the fastest-growing sources of renewable energy, owing to its cost-effectiveness and abundance. But it is inherently unpredictable due to fluctuations caused by weather conditions, time of day, and seasonal changes.
With the percentage of grid power from solar power plants increasing, and due to these variables, accurate solar forecasting is the subject of increased focus. Accurate forecasting enables grid operators to take necessary steps to balance supply and
demand as well as take measures to mitigate energy instability, outages, and overloads.
Recent advancements in fine sun sensors involve using quadrant photodiodes to precisely track the sun’s position. These sensors analyze sunlight intensity across four quadrants to determine the sun’s angle and enable solar panels to be optimally aligned. Fine sun sensors are adaptable both for terrestrial- and space-based systems, Campito said. Typically, silicon quadrant photodiodes are used for their sensitivity in the visible and NIR, while lithium-drifted silicon detectors and InGaAs photodiodes are preferred in high-precision or specialized applications such as aerospace or IR tracking. In the coming years, experts anticipate advancements in predictive technologies combined with AI analysis.
Optical sensing ubiquity
Photodiodes have steadily improved in size, speed, and sensitivity, driving
advancements across environmental
applications. Miniaturized photodiodes now power portable devices such as hand-held air quality monitors, while materials
such as SiC and InGaAs enhance responsivity for detecting faint signals in UV radiation monitoring. Faster response times enable real-time data processing, vital for dynamic scenarios such as tracking pollutant plumes.
There seems to be an ever-growing expectation that sensing solutions will continue to get better, faster, and cheaper. And so expectations keep rising.
For Saul Nuccitelli, director of the Texas Water Development Board’s Flood Science and Community Assistance Division, there is an increased expectation that lidar can better penetrate vegetation and other obstructions to provide a clearer picture of true bare-earth conditions. Today, SWIR LEDs coupled with InGaAs photodiode-based cameras image through clouds, smoke, dust, and certain plastics. “In flood damage assessment, optical sensing technologies have transformed how elevation data is collected,” Nuccitelli said.
Flood damage varies significantly with water depth. For instance, 6 in. of flooding may only require floor and drywall repairs. But 18 in. can damage electrical systems, drastically increasing cost. Traditional methods rely on estimating elevation based on building polygons or corner averages, which often lack precision.
Now, ground vehicles equipped with advanced optical sensors and AI capture images of individual homes, focusing on the bottom of front doors as a reliable marker for first-floor elevation. This innovative approach offers more precise data at a fraction of previous costs.
“As the cost to obtain this information drops and quality increases, this technology can be a game changer for developing more accurate flood damage estimates at a large scale,” Nuccitelli said.
To meet growing demand, integration
with AI is set to further boost data interpretation, as seen already in automated weather predictions. Even as this ascending technology blossoms, and as environmental challenges continue, LEDs and photodiodes will retain a growing role of their own in optical sensing for energy and climate solutions with energy-efficient designs aligning with ecological goals.
Photodiode Metrics: Performance and Characteristics
1. The size of the photodiode’s photosensitive area:
A larger active area captures more photons, which improves sensitivity, especially in low-light conditions.
2. Speed (response time/bandwidth) determines how
quickly the photodiode can respond to changes in light
intensity, usually measured in bandwidth or rise time.
3. Overall dimensions of the photodiode package:
Small dimensions simplify integration into dense electronic systems or optoelectronic chips.
4. Sensitivity: The ratio of electrical output (current or
voltage) to incident light power is usually measured in
amperes per watt. High sensitivity enables the detection of even weak light flux, which is vital for lidar scanning.
5. Noise characteristics: Intrinsic electronic noise, which limits the ability of the photodiode to detect weak signals, is
measured as the noise equivalent power or signal-to-noise ratio.
6. Wavelength range (spectral sensitivity) is usually
determined by the material from which the photodiode is made.
- Silicon photodiodes for the visible and NIR range
(250 to 1100 nm).
- Indium gallium arsenide (InGaAs) photodiodes for the NIR and SWIR (650 to 2600 nm).
- Indium arsenide antimonide, lead selenide (PbSe), and mercury cadmium telluride photodiodes for 2500-µm and longer wavelengths.