Normal welding lenses would be a good start. Test different ones for the correct exposure. My concern will be if the webcam can endure long exposure to the rays, even filtered.
I have already used the normal welding lenses. i have tried from DIN 4 to DIN 14. i haven't got satisfactory results.
And do you have any idea how long the webcam can be exposed to the rays?
The thing about welding is you really have to focus in on the weld to see the weld pool. I now have to use reading glasses! If you can't zoom in the webcam, magnifying welding lenses are available. Getting the webcam located and focused is the big thing, I'm sure. I don't know anything about webcams, just thinking about how strong and dangerous weld light is. Use cheap cams.
I would think a welding supply should be able to get the magnifying welding lenses. I understand your problem. To adjust your camera, substitute a quartz halogen lite bulb for the welding. When you can see the filament in your camera output, you should be properly focused. Then set up a fixture so the weld is at the same focal point. My eyes are sensitive to candles and other sources of infrared, but my vision loss is from age, I'm sure. I'm 62.
All I know is that once I saw an excellent tv show on welding.
The video clearly showed the weld pool, and even the subtle difference between the metal pool and the flux pool, which is a critical factor in learning to weld. This was stick welding, of course. It may have been the Discovery Channel, don't remember. Check camera shops for specialty lenses, you may be able to adapt one to your needs.