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What do mirrors and windows do to a LiDAR scan?

Mirrors send the scan into a room that does not exist, and clear glass disappears entirely. Here is why it happens, how to spot it in the drawing, and the four fixes that solve it.

Why mirrors are the worst case for a scan

LiDAR measures distance by sending out a short pulse of light and timing how long it takes to come back. That relies on the surface scattering the light diffusely back towards the phone — the way a painted wall, a rug or a wooden panel does.

A mirror does not do that. A mirror bounces the light onward in a new direction, back into the room, where it hits something else and only then finds its way back. The phone has no way of knowing the light took a detour. It simply calculates: a long travel time means a long distance. So it places the surface far behind the mirror — exactly where you yourself see your reflection standing.

In the drawing, this shows up as:

• A bathroom with a mirror above the sink that has grown a small niche or a hole in the wall behind the mirror.
• A wardrobe with mirrored doors that has become twice as deep — the whole room is reflected out on the other side of the wall.
• An entrance hall with a tall mirror, where the wall is gone and a strange empty room sticks out into the neighbour's flat.

The extra room is not noise or a random glitch. It is a systematic, predictable error: the phantom room is always exactly as deep as the real room in front of the mirror.

Why windows disappear

Clear glass is almost invisible to LiDAR. A large share of the light pulse passes straight through the pane and hits whatever is outside — the hedge, the neighbour's gable, a car. If something usable comes back from there, the phone measures the distance to that. If nothing usable comes back at all, the wall simply gets a hole in it.

There are two typical outcomes of a large glazed wall in a home scan:

• The wall gets a hole where the patio door sits. The drawing looks as if a wall is missing.
• The room sticks out into the garden. The scan measured the hedge 4 metres away and took it for a surface inside the room.

What saves the window wall is the frame. The frame is wood or plastic, it is matte, and it sits in the plane of the wall itself. Scan the window wall at an angle from the side rather than straight on, and the phone picks up plenty of frame and only a little glass — and the wall lands where it should.

In the evening, the window is a mirror too

This one catches many people out: scan in the evening, when it is dark outside and lit inside, and the pane behaves like a mirror seen from inside. You get both errors at once, on the same surface — some light passes through, and some is bounced back as a reflection of the living room.

A living room scanned on a winter evening at 7pm can therefore end up reflected out into the garden, while the same room scanned at 11am simply gets a hole in the window wall. If you need to scan a home with large glazed areas, do it in daylight — or draw the curtains.

The four fixes that solve it

1. Cover the mirror

The simplest and best fix: hang a towel over the bathroom mirror, and close the sliding doors if the wardrobe has mirrors on the inside. A towel is a matte surface in exactly the plane where the mirror sits — so the scan measures the wall where the wall actually is.

If the mirror cannot be covered (a fixed mirrored wall), avoid pointing the phone straight into it. Scan the room by walking along the mirrored wall with the camera angled away from it.

2. Draw the curtains across large panes

A curtain or blind that is drawn turns the glazed wall into a matte surface. The measurement comes out essentially right — the curtain typically hangs a couple of centimetres in front of the glass, and that is an error you know about and can subtract, unlike a hole in the wall.

3. Scan glass and mirrored walls at an angle

Never point the phone straight into a pane or a mirror. Walk past at an angle instead, and the LiDAR light hits the surface obliquely, picking up frame, sash, handle and window reveal along the way — the matte surfaces that carry the real measurement.

4. Check with a tape measure — once per room

A room with a mirror or a large glazed wall is the one room in the home you should verify. Measure one wall from wall to wall with a tape measure and compare it with the scan.

Example: the bathroom is 195 cm wide by tape measure. The scan says 251 cm. The 56 cm difference is not measurement uncertainty — it is the mirror over the sink adding a niche's worth of depth. An error like that is obvious the moment you have one real measurement to hold it against.

Other glossy surfaces that do the same thing

Mirrors are the worst case, but they are not alone. Anything glossy enough to show a reflection disturbs a LiDAR scan to a smaller degree:

• Freshly varnished or wet floors — the floor level becomes uncertain, and so does the room's ceiling height.
• Glossy floor and wall tiles in bathrooms and utility rooms.
• Stainless steel — an extractor hood, a fridge front, a cooker.
• Glass tabletops and glass cabinets.
• High-gloss kitchen doors.
• Large flat screens in standby — a black, glossy surface that is both mirror and light-absorbing at once.

None of these ruins a scan on its own. But if the whole bathroom is glossy tile plus a mirror, that is the hardest room in the home, and that is where you need the tape measure.

How to spot the error in the drawing afterwards

You do not have to guess whether a mirror tricked the scan. Look for these three signs in the floor plan:

1. A room that sticks out into a neighbour or into open air, with nothing actually there in reality.
2. A room that is suspiciously deep compared with how it feels to stand in it — the mirror error typically doubles the depth.
3. A wall with a hole in it where you know there is a window or a glass door.

All three are errors you can spot on the drawing without measuring anything. If you see them, the room in question should be scanned again with the mirror covered — not fixed up by hand.

What HouseSense does — and does not do

HouseSense builds the floor plan on the geometry the phone actually measured, and states measurements as internal measurements from wall surface to wall surface. That means a mirror phantom room shows up in the drawing instead of being quietly smoothed away — and that is deliberate. An app that silently smoothed out suspicious rooms would also smooth out real niches and bay windows, and then you could no longer tell an error apart from a detail.

The principle in HouseSense is that the drawing may be dressed up, the numbers may not. So the best defence against mirrors and glass is still the three minutes it takes to hang up a towel and draw a curtain before you start. And if the home has a mirrored wall that cannot be covered, you can draw that room on the app's graph paper instead and type in your own tape-measure figures — there are no mirrors there.