Darkness: how to capture a single crack where there is no light

A camera records collected light. In the dark it can only compensate by opening the shutter longer (blur), raising sensitivity (noise) or opening the aperture (shallower depth of field). For a moving drone, each of these can erase a hairline crack. Darkness also weakens camera-based positioning and obstacle sensing, which rely on seeing the surroundings. Darkness affects both image quality and flight safety.

There are two ways to deal with darkness: bring light with you, or use sensors that do not need it. Bringing light means onboard LED lighting so the camera can use short exposures. Light-independent options include thermal infrared cameras, which see temperature differences, and LiDAR, which emits its own laser. In practice the two are often combined.

Darkness is common underground — tunnels, sewers, utility tunnels, mine drifts — and in enclosed spaces such as storage tanks, boilers and furnaces, chimneys and ship tanks. Outdoors you meet it deep under bridge decks, in dam galleries, and beneath port piers where work is often possible only at night.

• Work back from required resolution: decide the crack width you must see, then set distance, shutter speed and lighting. If you know it will be dark, choose a lit aircraft.
• Prioritise shutter speed: slowing the shutter in low light causes blur. Add light, or stop and hover before each capture.
• Reshoot with a different light angle: imaging the same spot from different angles lets shadow direction separate cracks from stains.
• Check how the aircraft localises: camera-based position hold fails in the dark. Confirm the system can fly there — LiDAR SLAM, or vision combined with lighting. (Related: GNSS-denied)
• At night outdoors, watch the aircraft lights: orientation is hard to judge at night. Agree how the lights will be read and where observers stand.
• Keep extra battery margin: lights consume power. Plan shorter flights than the daytime equivalent.

In Japan, night flight is a “specified flight” under the Civil Aeronautics Act and generally requires permission, approval or another procedure (the treatment depends on aircraft certification and pilot licence). In the United States, night operations are allowed under Part 107 when conditions are met, such as anti-collision lighting visible for at least 3 statute miles and up-to-date pilot testing or training. Indoor spaces clearly separated from the outside, such as tunnel or tank interiors (even with openings), are outside the aviation flight rules in Japan — but dark underground spaces often overlap with oxygen deficiency & toxic gas and confined space hazards, so site-safety procedures still apply.

Related: dust & smoke, explosive atmospheres (lights and batteries can be ignition sources), world regulations.

• Lighting versus endurance: more light means less flight time. Fully lighting a large tunnel or void in one flight is still hard.
• Lighting in dust and fog: particles glow in the beam (backscatter) and reduce visibility. Offsetting lights helps but does not solve it.
• Limits of thermal imaging: without temperature differences, a thermal camera shows nothing. In thermally uniform underground spaces, cracks are nearly invisible.
• Consistent exposure over time: if lighting differs between visits, it is hard to tell whether colour changed or only the lighting did. Methods for standardising capture conditions remain site-specific.

Inspection aircraft routinely carry lighting that adjusts its angle to the shot and positioning that works in the dark. Because they capture from the same position under the same lighting as last time, overlaying images shows only what changed. Tunnel interiors and the undersides of piers at night yield records of daytime quality. “It was too dark to see” has disappeared from inspection reports. That is the editors’ outlook.