Controlling light pollution
In an era of growing environmental awareness and urban innovation, cities across Europe are investing in smarter infrastructures and smarter data to support them. In France, this shift has been reinforced by national legislation like the “Decree of 27 December 2018 on the prevention, reduction and limitation of light pollution”, one of the world’s most ambitious policies to minimise light pollution. The decree limits outdoor lighting hours, restricts upward-directed light emissions, and strictly regulates color temperature. These measures are intended to reduce energy consumption and protect nocturnal ecosystems.
Flying into the night
Among the companies at the forefront of this transformation is APEI – now part of the Hexagon group – a French airborne company with over four decades of experience and a long-standing partnership with Leica Geosystems. In recent years, APEI has been called on for increasingly specialised and technically demanding missions, including nighttime flights to monitor light efficiency.
Nighttime orthophotos offer a unique window into how cities consume energy after dark. They reveal more than just where the lights are—they show how lighting is distributed, where it’s too intense, where it’s uneven, and where it’s spilling into places it shouldn’t, like parks or natural areas.
For city planners and environmental agencies, this data is essential. It helps determine wasted energy, highlight areas where lighting can be improved or reduced, and measure the broader impact of light pollution on people and wildlife. Insights from these flights also guide smarter upgrades, such as switching to LED technology or adjusting lighting schedules to reduce environmental impact.
Ortophotos captured at night provide valuable insight into urban energy consumption and the broader effects of light pollution on the environment.
The hidden challenges of night surveys
Operating at night presents unique operational and technical challenges. “Cities want images before and after the public lights are turned on or off,” – said Bruno Callabat, CEO of APEI – “Which means we have to do at least two flights in the same night.” This doubles the workload and increases complexity, not only in timing but also in ensuring consistency across data sets. Strict aviation regulations, like maintaining a high side lap, contribute to adding constraints.
In addition to flight requirements, three technical challenges must be overcome for night surveys to get sharp images. The first major challenge is long exposure time: with limited light at night, cameras need more time to capture images. This leads directly to the second issue: motion blur. Even the slightest movement during a long exposure can blur the image, especially because survey aircraft can fly at speeds over one hundred meters per second. The third challenge is stability. Reduced visibility and atmospheric turbulence at night make it hard to keep the aircraft steady. Together, these factors can significantly impact the sharpness and accuracy of nighttime airborne imagery.
Airborne technology that makes the difference
To meet the challenges of night flights, APEI relies on the Leica DMC-4 airborne imaging sensor, equipped with Leica MFC150 technology and mounted on the Leica PAV200 gyro-stabilised mount. This combination provides the precision and adaptability required for night missions.
Leica DMC-4 airborne sensor provides over 31,500 pixels across swath, delivering superior image fidelity in all lighting conditions
“The exceptional quality and versatility of the Leica DMC-4, fitted with MFC150 technology, made it the perfect sensor for night survey,” said Quentin Imperiale, Task Specialist at APEI.
A key factor in collecting accurate data at night is the Forward-Motion-Compensation (FMC). The FMC technology, which is common to all Leica Geosystems’ airborne systems equipped with MFC cameras, physically shifts the sensor in perfect sync with the aircraft’s forward movement. This ensures the sensor is aligned over the same spot on the ground during the entire exposure, significantly reducing or eliminating the blur caused by the aircraft movement.
“The MFC150 camera, with Forward-Motion-Compensation, actively counteracts any aircraft movement along the flight line” – explains Quentin – “This advanced technology basically freezes the scene during exposure and gets rid of what could be over 25 pixels of blur.”
The image on the left, captured without FMC on, is less sharp than the image on the right, taken with FMC.
This is especially critical during long exposures, when even small shifts can ruin an image. However, FMC alone isn’t enough; Leica PAV200 gyro-stabilised mount compensates for any motion on a roll, yaw, and drift, ensuring a steady flight and the most accurate data collection.
Together, these technologies enable APEI to deliver sharp, consistent, and reliable nighttime imagery, even under the most difficult conditions.
Supporting sustainable cities
While the technical feat is impressive, the ultimate goal of these missions is impact. Urban lighting accounts for a significant portion of municipal energy use, and cities are under pressure to cut waste and improve sustainability.
“Monitoring of light pollution and energy consumption has become essential for creating sustainable and efficient urban landscapes” – says Callabat –“Our night surveys provide a unique source of data that supports change toward energy efficiency and greater sustainability.”
It’s a mission APEI takes seriously. “We are proud to play a part in improving our cities for future generations,” he adds.
Capturing precise urban imagery by night, APEI exemplifies how innovation, expertise, and purpose can come together to shape a better world.
Orthophoto captured in Moulins, France, with Leica DMC-4
Through their continued partnership with Leica Geosystems, they are helping cities move from reactive to proactive, building data-driven strategies for smarter, greener cities.
France's fight against light pollution
Data captured by APEI at 10:00 PM (left) and 1:00 AM (right) in Lyon, France. By comparing these orthophotos with the public lighting map provided by Lyon’s municipality, APEI was able to classify the light intensity of both public and private buildings.
According to the OPECST (Parliamentary Office for Scientific and Technological Assessment), artificial light now affects 85% of metropolitan France, and the brightness of the night sky has doubled over the past decade. Since 1990, the number of public lighting points has increased from 7.2 million to 11 million—a 53% rise. This contributes significantly to energy waste, greenhouse gas emissions, and the disruption of biodiversity and human circadian rhythms. Public lighting accounts for up to 37% of a city’s electricity bill, and light pollution is widely recognised as a growing threat to both environmental sustainability and public health.
The number of public lighting points has increased from 7.2 million to
million
Turning insight into action through airborne data
Airborne data doesn’t just highlight where issues exist—it gives cities the tools to take action. By delivering detailed insights into how and where artificial light is used, images captured at night support the practical implementation of environmental regulations. This includes spotting non-compliant lighting systems, tracking progress on emissions reduction, and evaluating the impact of measures like dimming schedules or switching to warmer, low-impact LEDs. Aligned with national recommendations to move from blanket illumination to context-sensitive, needs-based lighting, airborne imagery plays a vital role in designing smarter urban lighting strategies that balance safety, energy efficiency, and environmental protection.
What is forward motion compensation?
Forward Motion Compensation (FMC) is a technology exclusive to all Leica Geosystems’ airborne systems equipped with an MFC camera module. It is designed to reduce image blur caused by the aircraft’s forward motion during data capture. When the shutter is open, the aircraft continues to move, which can result in blurred images, particularly during longer exposures. FMC addresses this by physically shifting the camera sensor during exposure in the opposite direction of the aircraft’s movement. The speed of the sensor shift matches the aircraft’s speed, keeping the ground scene aligned with the sensor throughout the capture. This compensation is essential for achieving consistently sharp images, ensuring high image quality across all lighting conditions and flight speeds.
