Lasers being used near glass panels, glass wall, glass balustrades or windows can often play tricks on the eye with multiple lines or bent lines because of the reflection and refraction of light. Refraction is caused as the light passes between two mediums of different density, in this case air and glass.
All glass reflects a certain amount of light as the light passes through it as a result of two rays of light; the incoming ray which hits the glass and an outgoing ray which is reflected as the light leaves the rear of the glass surface. As the light passes through the glass it straightens, then the outgoing reflects at a different level than the incoming light.
This can cause the laser to seem to project a bent, double or even multiple line.
How do I overcome glass refraction or reflection then?
When using a line laser, shift the unit to a new position where the area to be levelled is not between the glass and the unit. In most set-ups this is quite easy to achieve.
With a rotary laser, make sure the detector is in front of the glass and in a direct line to the unit.