Can Lidar See Through Walls? What a Laser Scan Records

No. Lidar measures the distance to the first surface its light hits and returns from, and a wall is that surface. The question comes up because lidar has become a word for a lot of different instruments, from the sensor on a phone to the scanner on a survey tripod to the system under an aircraft, and because people have seen point clouds in which a building appears with its walls stripped away. This guide explains what a laser scanner actually measures, why that image is not a scan through walls, what can see into or behind a wall when a project needs it, and what a laser scan records of a building that no other survey does.

What lidar measures

A terrestrial laser scanner sends out a laser beam and reads the distance to the surface it hits, either by timing a pulse or by reading the phase of a modulated beam; a scanner of the Trimble X7 class times a pulse, by its datasheet. The scanner sweeps that beam over everything in its view and records a point for each return. A scanner of this class, by the same datasheet, uses a class 1 laser at 1550 nm, measures up to 500,000 points a second over a range of 0.6 to 80 m, and holds a 3D point accuracy of 2.4 mm at 10 m (one standard deviation). Every true point in the cloud sits on a surface the beam reached: a wall, a floor, a duct above an open ceiling, a tree, a person walking past. The beam does not continue past the wall, and the cloud contains nothing from the room behind it.

That is also why the point cloud of a building looks the way it does. The scanner is moved from room to room, and the stations are registered into one cloud, so the finished cloud holds every room’s surfaces together. Viewed from above with the ceiling points clipped away, the building appears as a plan with its walls standing and its rooms open, which is a view of many scans combined, not of one scan passing through anything. The thickness of a wall in that cloud is the distance between its two faces, each measured from its own side.

What the beam passes through, and what it does not

Clear glazing returns little of the beam, so the scanner often records what stands behind a window, a desk and a chair inside a storefront, while the pane itself is thin or missing in the cloud. A reflective or low-emissivity coating returns the pane instead and hides what is behind it. A mirror sends the beam back from whatever it reflects, so the reflected room appears as false points behind the mirror plane, and those are cleaned out of the cloud. Still water returns little. Dense vegetation hides what stands behind it, and a facade behind a tree is scanned from more than one side so the gaps in one view are filled from another. In each case the beam reflects from the first opaque surface or is lost.

Dust, smoke and rain scatter the beam and shorten its useful range, which is a reason to scan a demolition site after the dust has settled rather than during it.

What can see into a wall

When a project needs what is inside or behind a wall, there are instruments for it, and none of them is a laser scanner. Ground penetrating radar surveys read reinforcement, conduit and voids inside concrete and masonry from the surface, and locate utilities under a slab or a yard; they are a separate survey. Industrial computed tomography images the interior of an object with X-rays, as ASTM E1441 describes, and is used for parts rather than buildings; the guide to internal features covers it. A borescope through a small hole shows the cavity of a wall. And the oldest method, opening the wall, is still the usual one on a renovation, where the scan can be repeated once the finishes are off and the structure is in view.

The scan records the surfaces of the building to a stated accuracy; the radar survey, the borescope or the opening add what is behind them. The model built on the scan shows what the scanner saw, and where an element is drawn from an existing drawing because the scanner could not see it, the model marks it as unverified. The 3D laser scanning page describes what the scan delivers, and the MEP BIM page how the systems above a ceiling are captured when the tiles are lifted.

What a scan records that nothing else does

In a few minutes per station the scanner records every visible surface of a room, so a question that comes up months later, the height of a lintel or the offset of a column from the grid, is answered from the cloud without a second visit. A facade scanned from the street has every opening in place; a plenum scanned through lifted tiles has the ducts and pipes where they run; a building scanned mid-demolition keeps its structure on record before the new framing covers it again. Drawings and models built on that cloud show the building as it stands, to the accuracy the quote names, and say where the scan reached and where it did not.

Sources

• Trimble X7 3D laser scanning system datasheet: laser class and wavelength, range principle, range, scan speed, 3D point accuracy.
• ASTM E1441, Standard Guide for Computed Tomography (CT): what CT images and how.