If you’re reading this, it means that I have given up on the curing of my radioactive lenses.
For now, that is.
You may want to read about the color cast found in most radioactive lenses in this article here first. I wrote about radioactivity in general and radioactivity found in lenses for photography in particular in this article here. The measurements for my radioactive lenses can be found here.
When I bought my radioactive lenses I was well aware of the fact that they would also bring along a yellow tint in the glass. I was, however, also aware of this tint being removable by exposure to sunlight.
After reading up on the topic, I learned that the yellow color is caused by so-called color centers in the crystal structure of thorium dioxide crystals, which were in turn caused by radioactive particles. These color centers swallow light and thus the glass doesn’t appear clear anymore (Read about this in my article about curing the glass).
I learned upon further research that it wasn’t necessarily sunlight that cured the color cast but simply exposure to light itself. Therefore I followed the example of a savvy guy from Florida and bought a LED lamp from IKEA.
While the lamp worked great on my Super-Multi-Coated Takumar 50mm f/1.4, removing most of the yellow color within only 48 hours, it didn’t have the desired effect on the Fujinon or the Pancolar.
My final measurements
Let’s remind ourselves of what the situation looked like at the beginning:
Fujinon: +1’350 K, Tint +24 green, 0,75 EV darker
Pancolar: +1’300 K, Tint +47 green, 0,66 EV darker
In the end, the Fujinon has spent slightly over 200 hours under the Jansjö lamp and the Pancolar was exposed to Thors lightning for almost 400 hours (yeah, I pay the energy bills myself, though that results in less than 1kW and therefore less than 20 cents on the bill).
Let’s look at the current state of these two lenses
You can instantly tell that I ended up with a rather greenish tint instead of a warm yellow one. To be completely honest, I kinda adored the golden color cast of the Fujinon in the beginning, but I simply couldn’t accept it swallowing 3/4 stops of light at the same time.
The absolute values for this last measurement:
Fujinon: +300 K, Tint +19 green, 0.2 EV darker
Pancolar: +650 K, Tint +30 green, 0,2 EV darker
As you can see, I got rid of most of the light loss (both lenses are now only 0,2 stops slower than the Yongnuo.) and I could reduce the temperature offset of the Fujinon to a mere 300K, which is comparable to the Helios.
However, the color tint, the shift towards green, seems to be untouchable. I already had the Fujinon down to +19 after the first 3 days under the lamp (when the temperature was cut in half only) and back then I also already got the light loss down to 0,2 EV. Therefore, further curing under the lamp simply resulted in the temperature getting closer to neutral.
With the Pancolar, the situation is even more dire. I could reduce the temperature shift by half but the color tint remains extremely high, resulting in pictures looking green instead of “warm and cozy” as they did before.
I will do another measurement using actual sunlight as a light source, because my 5’500 K lamp was measured as 4’950 K yesterday (before it usually measured 5450 or 5500). It may be because of the reflective surface on my lamp head (being not pure white but silverish, cold) but I want to make sure.
Maybe this will show different values for the tin, but I wouldn’t get my hopes up.
Though, all hope is not lost yet, for I have adjusted the white balance and added a comparison of the three lenses below:
Test conducted in daylight (overcast sky)
Well, I just did another test to confirm my previous measurements. First of, it was not conclusive, not at all. While the brightness changed a lot inbetween shots (taking the pictures took a mere 10 minutes but the overcast sky seems to be somewhat unstable I figure) I could adjust for that afterwards. All pictures have the same brightness (aligned on the histogram so that the rightmost part touches the center line) and the white balance was adjusted to match the one measured by the Yongnuo.
I will just show you the measurements then:
You can see that the Color-Ultron isn’t a perfect match anymore. It has the same temperature but it ended up being slightly more greenish (+4). Though, this could also be due to a change in the clouds during the ten minutes that were inbetween these two lenses.
What struck me most though, was the fact that the Fujinon and the Pancolar seem to have switched places. The Pancolar here was only 400K warmer and +15 on the green side, whereas the Fujinon ended up being a whopping 900K warmer than the Yongnuo and also added +26 on the green side. Basically, we’re back to the warm golden/bronze shimmer..
The Takumar is currently spending some time under the Jangsjö lamp, because it clocked in at 450K warmer (more than the Pancolar, mind you) and with +7 on the green tint.
I had the Helios under the lamp for 24 hours, just to see if something would change. Unfortunately, it didn’t. It’s still 250K warmer and has +9 on the green side.
I can conclude from this series of measurements, that they aren’t all that conclusive. Obviously, the temperature of the light source itself plays a major role for these measurements and using the sun seems to be a bad idea because its temperature changes a lot during the days, therefore rendering the measurements incompareable between each other.
My 5’500K lamp still seems to be the best choice, though I have to figure out what the reflective hood does to it, because the last measurements were down to 4’950 K..
One thing I can take out of these two measurements would be, that the colour tint changes with the colour of the light, therefore “results may vary” especially when you’re not able to adjust the white balance.
What are my alternatives now?
I could try treatment using heat, but I don’t have an eccsicator at home and I absolutely don’t trust the temperature indicator on my oven. I would hate to mess up the lubricants inside the lenses.
I could also look for a strong UV lamp and try this approach, but the cost involved seems somewhat unreasonable.
I have now given up trying to cure the Pancolar and the Fujinon from their green tints until I’ve found the reason for it being so much more stubborn than the Takumar.
As you might figure, I’m open for suggestions.