Measuring a pinhole with a Class 2 laser pointer

Purpose

To get one number — the diameter of a pinhole — out of the rings it throws on a wall, with the beam under control from the moment the pointer goes on to the moment it goes back in its case.

When to use it

At station 5 of making and measuring pinholes, for every plate entering the pinhole register; again whenever a plate is re-worked, and to inspect a suspect one, because the pattern shows a torn or elliptical hole before a loupe does.

It does not replace the scan or the calibrated eyepiece: the register wants three independent measurements, and this is the one that gets better as the hole gets smaller. The pointer is a measuring instrument here and nothing else — it does not leave the bench, and it is never switched on outdoors.

Before you start

You should already know where d = 2.44 λ L / D comes from and when it holds, from diffraction and the optimum pinhole, and how to state the uncertainty of a reading, from measurement and uncertainty. Neither is re-derived here.

The pointer. Use one marked Class 2 under BS EN 60825-1, with its wavelength printed on it; prefer red, 630–670 nm, to green. HSE place Class 1M, 2 and 2M lasers — naming low-power pointers as the example — among the sources that can harm only if used inappropriately, and begin their hazardous list at Class 3B. UKHSA publish the finding that governs everything below: pointers marked Class 2, or as under 1 mW, are commonly measured as Class 3B or Class 4, some carry invisible extra wavelengths, and no test a non-specialist can perform will tell you which sort you own. No eyewear is specified — for a Class 2 source the control is the beam path and the blink reflex, not a filter — so take off a watch and any reflective ring instead.

On the bench: the pointer and a block to clamp or tape it to; a clip that holds a plate upright; a tape reaching 3 m; a millimetre rule; a matt, light-coloured wall or matt card to stop the beam; the pinhole register.

Expected ring diameters for step 8:

Hole	D at 2.0 m	D at 3.0 m
0.20 mm	15.9 mm	23.8 mm
0.25 mm	12.7 mm	19.0 mm
0.30 mm	10.6 mm	15.9 mm
0.40 mm	7.9 mm	11.9 mm
0.50 mm	6.3 mm	9.5 mm

At 650 nm; a green 532 nm pointer gives rings 0.82 times these.

The room. Nobody in front of the pointer, no children or animals. Set the path so it cannot leave through a window: a beam reaching a road or a railway is somebody else’s dazzle, and, aimed at a vehicle, an offence under the Laser Misuse (Vehicles) Act 2018.

Level A on the classification rubric: nothing weighed, dissolved or heated, no waste, one energy source — a battery-powered visible pointer.

Procedure

⚠Before the pointer is switched on

Beam horizontal, below everybody’s eye level, terminated on matt card or a matt wall. Nothing reflective along it — glass, a phone screen, a steel rule, an unblackened brass plate. Never look into the beam, or back at the plate from the wall side.

1. Read the pointer’s own label and write down its class and wavelength. If it is not marked Class 1 or Class 2, or carries no wavelength, stop here.
2. Clear the beam path: reflective objects away, everyone briefed and behind the pointer, the window covered or the path turned from it.
3. Clamp or tape the pointer to the block so it cannot roll, aimed at the wall, below eye level.
4. With the pointer off, clip the plate upright about 100 mm in front of it, hole on the beam axis, blackened face towards the pointer.
5. Measure the throw L from plate to wall: 2 m for holes of 0.3 mm and under, 3 m for 0.4 and 0.5 mm plates, which keeps L far above the d²/λ limit the lesson sets (0.39 m for 0.5 mm at 650 nm).
6. Darken the room and give your eyes a minute. The outer rings are faint.
7. Switch on and look at the wall. If the pattern is not a clean set of concentric rings, switch off, square the plate to the beam and look again. Repeat until it is.
8. Measure D, the diameter of the first dark ring — the dark gap between the bright centre and the first faint ring — with the rule flat against the wall, read through the centre to the nearest millimetre. Take a second reading at right angles.
9. Switch the pointer off before you move the plate, the rule or yourself.
10. Compute d = 2.44 λ L / D in consistent units, from the mean of your two readings.
11. Write down the uncertainty you actually incurred: ±1 mm read on a 12.7 mm ring is ±8 per cent.
12. Repeat from step 4 for each remaining plate, pointer off between plates.
13. Finish: pointer off, battery out or the cap on, back in its case, out of a child’s reach.

How you know it worked

• The pattern was concentric rings, and the two readings of D agreed to the millimetre you could read.
• The computed d overlaps the scan and loupe intervals; five to ten per cent apart is normal.
• The register carries L, D and the wavelength, not only the answer.
• The beam never left the wall, and the pointer is off and cased before you leave the bench.

If a step fails

Step 1, the pointer carries no class or no wavelength. Do not use it. Measure by scan and eyepiece and record the diffraction cell as not measured: a seller’s listing is not a label, and the arithmetic is only as good as λ.

Step 5, the room will not give you 2 m. Use the longest throw it does, record it, and accept the larger uncertainty; below about a metre the scan is better.

Step 7, somebody is dazzled or reports an afterimage. Switch off; do not drive or use machinery until vision settles. UKHSA advise medical attention only if afterimages persist for hours or reading vision is disturbed.

Step 7, the pattern is a bright fan or a cross instead of rings. The hole is torn. Inspect it under the loupe; a torn plate is remade, not corrected. See burr smear for what it does to a negative.

Step 7, the rings are elliptical. Square the plate to the beam and look again. If the ellipse survives, so is the hole: its two diameters are two different numbers.

Step 7, the rings shift or clip as the plate moves. The beam is catching the carrier window. Re-centre the hole on the axis.

Step 8, you cannot find the first dark ring. A small pattern means a large hole: move the wall further away. A faint one means too much room light or too little dark adaptation.

Step 10, d disagrees with the other two methods by more than 20 per cent. Check the units of L, then that you measured the ring’s diameter and not its radius, then λ.

What to record

One line per plate in the pinhole register, kept with the plates: the pointer’s class and labelled wavelength; the throw L; both readings of D and their mean; the computed d and its uncertainty; a word for the pattern — concentric, elliptical, fan. Keep L and D raw: a register you cannot recompute is one you cannot correct.

Record the pointer once, as equipment: what it is marked, where it came from, the date. Replace it and the wavelength behind every earlier row changes with it.