Experiment: How much light penetrates a welding helmet?

Summary

Problem: How much light does a welding helmet block? They come in shades like '9' or '12' but the only documentation I can find defines what jobs to use what shade on. It never seems to say what the shades mean.

Conclusion: A basic 'shade 10' welding glass will block all but one part in 20,000 of low-frequency light.

A given shade number seems to allow 1/3 the light of one shade below it. So, for example, a shade 10 will allow 1/3 the light of a shade 9. Also, a shade 8 will allow 3x the light of a shade 9. A 'shade 1' appears to be equivalent to 'no shade'.

This appears to hold true only for light of wavelength green or longer. When viewing blue laser light we anecdotally saw the light appeared much dimmer than viewing green laser light of the same non-shaded intensity.

The experiment

Big thanks to Woody and Erin for helping design and do this experiment.

Original data:

Setup: We measured the shades of a Hobart welding helmet with a variable shade and a regular no-brand set-shade welding helmet. We used a LX1010B light meter inside the helmet, with cloth packed around it to shield light. Without any of the lights on, the sensor read '0' on it's most sensitive setting.

For each measurement, we took one of two flashlights and shone them through the helmet and centered on the light meter sensor. We also shone them directly on the sensor to get a no-shade measurement. For the variable-shade welding helmet we also needed to use an extra flashlight to activate the shade (normally it stays low-shade in order to allow the welder to see before they start welding and then changes it's shade when welding begins; it uses a light sensor on a different part of the face of the helmet to do this.)

We were only able to take a few measurements but they're a highly linear pattern, consistent from one light to the next, and the measurements from the variable-shade helmet closely matched the static-shade-of-10 helmet.

Results

The relationship between lux and shade looks very linear when the lux is plotted on the log scale. At a glance, it's about 1/3 the lux for every additional 1 of shade.

What's most interesting is if we assume this 1/3rd relationship going back down the shade levels, we predict that we'll see about the same as the 'unshaded' lux measurement when we reach shade level 1. This makes sense and reinforces that we've done the measurement at least approximately correctly since I can imagine an engineer/scientist choosing the shade level 0 or level 1 to be the unshaded level.