First let's explain the different elements of an auto darkening welding lens and then we can look at how they combine and work together to give you auto-darkening technology.
1) The UV/IR filter (The purple section)
The UV/IR interference filter removes high levels of UV/IR radiation whether the Auto Darkening Lens (ADL) is activated or not. The UV/IR filter consists of a thin glass substrate with a number of metallic layers. The metallic layers are made up of 5 layers of silver and 6 layers of aluminium oxide. These metallic layers help to reflect and absorb 99.99% of the IR radiation within the shade range of the ADL and not only protect the eye, but also protect the liquid crystal panels (described below) from heat damage too. For UV protection, the metallic layers work in conjunction with the polarising filters (explained below) in the ADL to absorb 99.9997% of UV radiation (UVA, B & C) within the shade range of the ADL. The thickness of these 11 metallic layers is only 0.7 microns! Glass is also a natural UVB filter (UVB causes sunburn)… this is why it is unlikely that you will get sunburnt through a closed window. The layers of glass used in the ADL filter absorb quite a lot of the harmful longer wavelength UVB radiation… However, the more penetrating UVA radiation can get through glass and is less easy to detect… that is why the polarising filters and UV/IR filter are important. The combination of these metallic layers give the lens its reflective metallic purple colour and give the light passing through a green tint as it absorbs light at the blue end of the spectrum.
2) Polarisation Filters
Used in conjunction, polarises darken the visible light. You'll notice that the first two polarises to the right are arranged in the same orientation while the polariser closest to the UV/IR filter is at a 90 degree angle compared to the other two polarises. When looking through two polarises when arranged at 90 degree angles they are at their darkest. Conversely when looking through polarises arranged in the same orientation they will only darken the light slightly. As you twist the polarises either way, they will darken or get lighter dependent on which way you are moving them. This will be explained in more detail later - just as long as the orientations effect on the light makes sense...
3) LC Cell
Liquid Crystal Cells (LCC) have the ability to turn the light. When liquid crystals are lying flat they twist the light by 90 degrees. However, when these LCC's are stimulated by electricity you can manipulate how far they bend the light - even to the point where they don't refract the light at all.
Putting it all Together
You will notice that the 2 Polarising filters nearest the eye (furthest to the right in the diagram above) are arranged in the same orientation… when switched off, the liquid crystals in between the polarising filters twist the polarised light by 90 degrees. This means that when the ADL is switched off, the lens will be dark (shade 5-6). This is a safety feature, in the event of ADL failure the eye will be protected from very bright light.
When the ADL is switched on, the LC panel between the first two polarises un-twists the polarised light waves and the lens shade drops down to shade 3 before the arc is struck (so if this doesn’t happen the welder is alerted to a problem and shouldn’t continue welding).