Yes, interesting & worthy of note. Since bright daylight didn't work, but the light from the LED torch did, it makes me wonder if the effectiveness of the method is related to the spectrum and / or the polarisation of the light that reaches the item under inspection. If so, then that suggests that other light sources could also be useful for this task.
I did try this when I learned of it a a year or so ago. I tried several different LED torches in a dark room on several old valves, but unfortunately, it didn't work for me. I guess there has to be some residual markings to illuminate even though the may not ordinarily be visible, and it may well depend on the method of marking. In my case they were all valves with B9 bases, rather than pre-war valves such as the HL2 in the youtube video, but nothing ventured - nothing gained! The results in the video are certainly impressive.
Anyone tried it with success?
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
This is being discussed over at UKVRRR, and the question of polarised light was suggested, but LEDs don't emit polarised light. UV was my favourite, but a member there is fairly convinced it's not that.
However, if the numbers were etched, that might be clue. The very intense light from an LED torch, light which has a peculiarly spectrum as well, could easily show up if there are minute physical disturbances to the surface of the glass...
