How to choose a line laser: accuracy, range, planes

What does “3 mm at 10 m” accuracy mean?

It is the largest stated departure between the projected line and true horizontal, measured ten metres from the instrument. The error grows with distance: an instrument stated at 3 mm at 10 m drifts in theory by 1.2 mm at four metres, and by 6 mm at twenty metres.

For laying tiles, lining up furniture or putting up stud walls in a home, that order of magnitude is more than enough. It is over long distances, outdoors or in a workshop, that the departure starts to count.

What range do you actually need?

There are two ranges, and they have nothing to do with each other. Range by eye depends on the ambient light: it collapses the moment you step outside. Range with a detector is electronic, and so indifferent to light — and it is the one that counts outdoors.

On our 16-line laser level, the manufacturer states 20 m by eye and up to 50 m with the detector, in pulse mode. That is the usual gap: the detector multiplies the usable range by two and a half.

Three planes or four: what does it change?

A plane is a complete circle of light over 360°. Three planes give one horizontal and two crossed verticals: enough to work a floor, a wall and a corner. The fourth plane adds a second horizontal, useful for setting out a ceiling without moving the instrument.

The number of planes changes nothing about accuracy. It changes the number of trips back and forth: on repeated setting out — plasterboard track, tiling over a large area — that is time gained, not correctness gained.

Green laser or red?

The human eye is markedly more sensitive to green than to red at equal brightness. At equal power, a green line therefore reads far more easily. In exchange, a green diode draws more and costs more. The subject deserves its own guide: green laser or red laser.

What else to look at before buying?

Four points, in the order they count on site: the self-levelling range in degrees, the ingress rating — IP54 is the useful minimum —, the tilt mode for setting out a deliberate fall, and the mount: 1/4" or 5/8" thread, magnetic bracket, fine-adjust plate.

• The self-levelling range, in degrees: the largest tilt the pendulum takes up on its own. Beyond it, the instrument must warn you, by flashing for instance.
• The IP rating: IP54 means protected against dust and against splashing water. That is the minimum for site use.
• Tilt mode, for setting out a deliberate slope by switching off automatic levelling.
• The mounting: 1/4" or 5/8" thread, magnetic mount, fine-adjustment plate. Look at the accessories before you choose.

How do you check your own instrument's accuracy?

Set the instrument about five metres from a wall and mark the horizontal line. Turn it through 180° on the spot, without moving it, and mark again. Half the gap between the two marks is the real error at that distance. If it exceeds the stated value, the instrument needs adjusting.

See the green-beam laser levels