Removing the color cast in radioactive lenses – Results

I decided to place this article on its own, because I felt like the article comparing color casts was already rather long on its own.

As I have now updated in my article about radioactive lenses, the color cast is caused by so-called color centers which are in turn caused by the radioactivity itself.

Therefore, you do not need UV light to get rid of the color cast but a simple source of light will do just fine. While it usually takes up to a month or more time with the lens sitting outside in the sun, you can cure the color cast within a few days using the IKEA LED lamp Jansjö (IKEA part nr. 501.632.02).

24.09.2015 – Update:

Actually, this isn’t completely true as you can read in my next article. You will find out that the cure worked for the Takumar but not for it’s highly radioactive brethren.

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Why is the 2 W IKEA LED so much better than the sun?
A little thought experiment about solar energy:
In my area we get around 100 kWh/m² per month during summer. The rear lens element has a surface of maybe 4 cm², therefore it would be exposed to merely 0.04 kWh of solar energy per month.
The month has around 740 hours and that results in an average output of 0.055 W for the sun. (which is almost 0 at night and several times higher around noon, of course)

In comparison, my IKEA LED has 2W with a great efficiency ratio. It does, however, not emit any UV light (I tested that too!) and therefore might not be as well suited for the purpose at hand (to turn thorium carbonate back to thorium oxide).

Still, it seems that we found the reason for the much shorter cure duration required with the LED lamp (a few days) versus the sunlight (several weeks).

If you have a LED lamp that has more power than the Jansjö (or even a small UV-LED) it would be interesting to see whether the cure could happen faster.
I have a UV-LED flashlight that makes the UV-detector beads go crazy (even compared to the sun at noon!) but I have no continuous power source for it and the thing burns through AAA-batteries like I do with ice cream.

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Curing the yellow color cast on thoriated lenses

I have treated the S-M-C Takumar with the Jansjö lamp for 48 hours now and can report that it helped a lot!

I took the lens out from under the lamp, set my grey card and 5’500K lamp up again and used the Yongnuo to set the white balance.

Comparison before and after:

The second shot (right side) was taken at 1/25s and with the light source a little closer and less far off from the center, therefore the images on the right side are more evenly lit and a little darker.

Conclusion?

The light treatment got rid of most of the color cast, leaving the Takumar only 300K warmer than the Yongnuo (with only +6 on the green side).

Additionally, after correcting the white balance, there is no visible difference in brightness!

I now (July 22nd) have the Fujinon sitting under the Jansjö lamp and will hopefully cure that lens of its painful loss of 3/4 stops of light too. I expect a treatment of 3 days at least, before I test the Fuji and get the lamp to work on the Pancolar.

I will then let the lamp work on the Takumar a little longer to see if I can get rid of the green tint aswell.

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30.07.2015 – Update:

So, I had the Fujinon under the Jansjö lamp for a whopping 80 hours before I dared to measure it again.

Comparison before and after:

The “after” shots on the right side were captured at f/4 and iso 100 with 1/10s shutter speed. There still is some variance in brightness over the frame but you should be able to tell that the color cast is clearly reduced! Additionally, the images to the right are a little darker than the first ones I took, but that doesn’t change a thing.

Conclusion?

I reduced the color cast of the Fujinon from +1’350K to +650K but the tint was only reduced by 5 from +24 to +19 on the green side. My conclusion is that the Fujinon will spend some more time under the lamp.

What about the light loss?

Before, we had a light loss of 3/4 EV which basically turned this f/1.4 lens into an f/1.8 lens speed-wise.

After correcting the white balance the Fuji compares to the Yongnuo as follows:

You see that the difference is really marginal now. It measures in at a mere 0.2 EV and I’m quite happy with that result.

Right now I’ve got the Pancolar sitting under the Jansjö and will do a measurement on monday when I will also put the Fujinon back under the lamp.

For now, my goal is to get all three lenses on par with the Takumar.

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06.08.2015 – Update

I had the Pancolar under the light for 5 full days and am a little taken aback by the result. I’ve put the Fuji under the curing beam for one more day but there wasn’t much of a change.
I will repeat the procedure once I get back from the UK.

I only did a quick test, no tripod, iso 400 at f/2.8 and with daylight coming in through the window, it might not be very accurate.

Comparison before and after:

You can see that something changed in the Pancolar and that the Fuji now looks much more like “grey”.

In fact, at least the color Tint from the Pancolar was cut in half, whereas the warmth stayed.. also, the light loss was reduced to 0.4 EV (mind the slight vignetting of the Pancolar at f/2.8).

The Fujinon‘s color tint was again reduced by 5 down to +14, but the warmth didn’t change this time. The light loss still measures 0.2 EV but I’m quite happy with that.

I will let the lamp work on the Pancolar some more and do another set of precise tests after I get back. I’m confident that I will be able to cure the Pancolar somehow.

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Closing this article

Future updates to my curing efforts will be included in my next article, where I talk about giving up and will present my final measurements after a LONG time under the Jansjö lamp.