DIY Laser levelling using webcam and laser level

[AndyUK]

How satisfying to read a 21st century physicist suggesting the use of a technology which may have been used by the Egyptians when building the pyramids!

Haha. When it works it works... :) I once had a trainee who looked very thoughtful when we showed them this method, and after about thirty seconds, came up with the question: "But how do we know the water is level?"

Thanks Andy. Yes i know the laser light will travel straight, what I meant was the straightness of the line that is generated by the cylindrical lens that is presumably a function of how accurately machined the lens is? Is testing this what you meant by twist in the horizontal projection? How do you measure this at work, do you use a water level?

Yes, this is what I mean by checking the horizontal by moving left and right. I was covering all bases with the later comment about light travelling in straight lines - you never know who you're talking to... ;)

Measuring absolute level is rarely done at work, because its more important the lasers are planar with one another than they're absolutely level as such, but this is how we do it when needed. The beauty of course is that the accuracy increases with distance, so its best used over distances over a couple of metres.

When we need absolute level in a slightly different situation, we use a large tank of water with a travelling probe. The probe has two wires, one longer than the other, that dip into the water and they measure the resistance between them. The probe moves from air down into water; when the second shorter wire touches the water's surface, the resistance decreases substantially. This is performed at three corners of the tank, from which the angle between the probe's movement mechanism and the water's surface in two directions can be ascertained.

Before starting this laser project I did something similar with the image sensor focused on a clear tube that was part of a water level, although I could easily repeatably locate the bottom of the meniscus to around 0.01mm resolution I had a problem getting much measurement accuracy due to the meniscus 'sticking' to the glass tube due to capillary action so gave up.I only used a short u-bend setup, maybe this was the problem and having much more volume of water in the system would overcome this problem.

Yep - hence my comment about the 'depth' of the meniscus being the error in the measurement. Try to use reasonably large diameter pipe, and a touch of washing up liquid in the water will reduce the surface tension.