It fires light at a surface and times the return. That much is simple. The interesting part is what falls out of it, because the physics explains nearly every complaint owners have about these tools.
The tool emits a pulse of laser light, the pulse hits a surface, some of it scatters back, and a sensor next to the emitter catches it. Light travels at a known and very constant speed, so the time between sending and receiving gives you distance. Halve it, because the light made a round trip.
The problem is that light is fast. Over ten feet, the round trip takes about twenty billionths of a second. Timing that directly to the precision these tools claim would need electronics far more expensive than a sixty dollar handheld.
So consumer units mostly do something cleverer. Rather than timing a single pulse, they modulate the beam, varying its intensity in a wave, and compare the phase of the returning wave against the outgoing one. The shift between them resolves distance to a fraction of an inch without needing to time anything in billionths of a second. It is the same principle a tuning fork uses to reveal that two notes are slightly apart.
Both methods depend on catching enough returning light to read. Everything that goes wrong with these tools in practice traces back to that one requirement.
A matte white wall scatters light in all directions, so a useful fraction comes back toward the sensor. That is the ideal target, and it is also the surface manufacturers use when they measure the specifications on the box.
Change the surface and the returning signal collapses. Black asphalt absorbs most of what hits it. Polished tile, glass and still water behave like mirrors, bouncing the beam away at an angle rather than scattering it, so the sensor receives almost nothing. That is why measuring to a window usually returns either an error or the distance to whatever is behind the glass.
Angle does the same thing more subtly. Hit a wall square and the scatter comes straight back. Hit it at a steep angle and most of the returning light heads off sideways. This is why readings get unreliable in exactly the situations where you most want them, like measuring along a wall rather than across a room.
The sensor is trying to pick one specific returning signal out of everything else arriving at the same time. Indoors there is very little competition. Outdoors in direct sun, the surface you are aiming at is already flooded with light across the whole spectrum, including the wavelength the tool is looking for.
The returning pulse is still there. It is just no longer distinguishable from the background. This is why a unit rated to 165 feet indoors can fail at 60 feet on a bright driveway, and why owners of budget units describe the dot disappearing long before the rated range. Nothing has broken. The signal to noise ratio has simply collapsed.
Two things help. A green beam sits at a wavelength the human eye is roughly four times more sensitive to, which helps you place the dot even though it does little for the sensor. A target plate gives the beam a bright retroreflective surface to bounce off, which genuinely does raise the returning signal. On long range units like the Bosch GLM400CL the answer is different again: a camera and an on-screen crosshair, so you stop trying to see the dot at all.
The emitter and sensor sit somewhere inside the housing, not at its edge. So the raw measurement is from that internal component to the target, and the tool has to add or subtract a fixed offset to give you a distance from an edge you can actually place against a wall.
Manufacturers choose different edges for that offset, which is why the reference point setting exists and why getting it wrong shifts every reading by the length of the tool. It is not a quirk. It falls directly out of where the hardware physically sits.
Better units include an inclinometer, a small sensor that knows how far the tool is tilted from level. On its own that gives you a pitch readout. Combined with distance it gives you trigonometry.
If the tool knows it measured 30 feet at an angle of 20 degrees above level, it can calculate the horizontal distance and the vertical rise without you measuring either directly. That is how indirect and Pythagoras modes work, and it is why a unit with an angle sensor can tell you a ceiling height over a stairwell while a unit without one cannot.
Almost every laser measure claims plus or minus one sixteenth of an inch. That figure describes a reading taken indoors, square to a flat matte target, at a moderate distance, by equipment that is not moving.
Every departure from those conditions costs you something, and the physics above says exactly which departures cost the most. Less returning light means a weaker signal to resolve, which means less precision. That is why the same tool can be genuinely excellent across a living room and unreliable across a sunlit yard, without anything about it having changed. What that means for the numbers you actually get is worth reading next.
For the wider picture on choosing a laser measure, including what the specifications actually mean, start on our front page.