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Pentax Yellow Lenses
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PostPosted: Sat May 23, 2020 11:02 am    Post subject: Reply with quote
Teemō wrote:
Mike Deep wrote:
fiftyonepointsix wrote:
I cleared the Yellowing of my two 50/1.4 Pentax Super-Taks using a UV light (NOT a UV-C). I picked up a Hot-Glass Summicron that had been sun-cured, found the lubricants where baked in due to the heat. The UV light puts out much less heat. I set the lens and lamp in a box, let run for a few days. On the Pentax lenses: the rear module is easily removed from the lens, and is where the yellowing occurs. I unscrew them, then place in the box with the lamp.

They are fine lenses.


I guess it bears repeating any time this comes up: You do not need UV-anything to clear a yellowed lens. Any visible light will work.


You're correct and it can also be cured by annealing the glass again (not recommended lol), but not all wavelengths interact with glass in the same way nor do all wavelengths have the same energy. That optical glass is generally opaque to UV might have something to do with it, because that light is not being reflected or transmitted, it's being absorbed. UV is also a high-frequency wavelength, so it carries more energy and high-intensity UV lamps are readily available.
The radiation from the thorium adds electrons to the odd valence of the other glass atoms, which then absorbs light in the visible spectrum. So you could use visible light but would probably need a much greater wattage than you'd need with a UV-C lamp.

kymarto wrote:


It is absolutely not necessary to use thorium doped glass. It is also possible to use lenses with higher curvatures or other glasses with high indices of refraction, though they are usually more costly. Thorium was a way to make fast lenses with low CA more economically, as the crystalline structure of thorium oxide resembles that of fluorite, but is way less expensive and rare.


Indeed, there are many methods, each with their own problems. There is no magic bullet, unfortunately. More specifically, it's thorium dioxide.


Right you are, ThO2. Interestingly it is sometimes called thorium oxide, though clearly from the chemical formula it should be dioxide.


PostPosted: Sat May 23, 2020 11:51 am    Post subject: Reply with quote
What I wanted to avoid: Sunlight will heat up the lens, and causes it to get hot. This is bad for the lubricants. If you want to pull the optics out of the lens, and put them in sunlight- that would be better than letting metal get hot and bake it. I've taken apart Two Summicrons that were "Sun Bleached" by someone else. They were cheap- under $100. One required the front element be polished and recoated, which was done professionally

And I knew: Glass absorbs UV as it is travels through the optics. UV carries more energy. This makes it more efficient at bleaching the damage.

UV-C is not good as it gets absorbed too quickly as it travels through the glass, does not get past the surface. I selected a UV lamp used in a Lab for bleaching the optics. I remember reading someone using a UV-C lamp that did not work. I seem to recall that the one used was made to erase older memory devices. I have one of those lamps as well, used to erase the memory of older microcontrollers before reprogramming. Did not use it.

I looked it up, 253.7 nm is recommended to erase memory devices, UV-C. Memory devices use a Quartz window for this reason.


PostPosted: Sat May 23, 2020 4:02 pm    Post subject: Reply with quote
The important takeaway from not needing UV is practicality, I think: Flexible, low power, high output, white LED lamps are cheap and plentiful, and there are no special considerations for setting one up and leaving it for several days.

I certainly wish that was more widely distributed information when I was looking to clear the yellowing from a bunch of lenses.


PostPosted: Mon May 25, 2020 5:34 pm    Post subject: Reply with quote
http://www.photonstartechnology.com/learn/how_leds_produce_white_light especially those using phosphor...some uv get through? (Thanks to Sciolist for ask me that)


PostPosted: Tue May 26, 2020 4:35 pm    Post subject: Reply with quote
I used an LED flashlight on my first Takumar that was yellowed and it worked perfectly. Of course I did have to recharge it a few times but other than that, no issues. I have a second Takumar (different variation) that was a little more difficult to de-yellow so I looked online for the Ikea lamp everyone talks about and it's no longer being sold. I did find the one below from Amazon and works great. Luckily, the diameter of the lamp fits perfectly in the front of the lens. Not trying to shill for this particular lamp so I won't mention the name but seems to work well so far.


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