UV egg (surprising)

[Stefano]

Here Andrea suggested me to try a TriColour photograph of an egg, and here I am. I felt like it was better to create another topic, but I'm OK if this post is moved into the original topic.

 

Usual setup (full-spectrum Canon EOS M, SvBony 0.5x focal reducer lens), with the following filters:

 

Visible reference

Chinese BG39 (2 mm);

 

TriColour

Blue channel: double 310 nm Chinese bandpass filter + ZWB1 (2*2 mm);

Green channel: BrightLine 340/26 filter + ZWB1 (2*2 mm);

Red channel: BrightLine 387/11 filter + Chinese BG39 (2 mm);

 

Left: raw egg; right: boiled egg (both brown eggs);

 

Visible reference:

 

UV TriColour (there is some color fringing in the shadows, but the result is clear):

 

I would have never expected the blue color, it is very unusual for something to be "blue" in UV. I did a quick Google search and found this:

https://www.researchgate.net/figure/Average-spectral-reflectance-of-host-eggs-before-and-after-application-of-UV-block-a_fig2_281866377

https://www.researchgate.net/figure/Iridescence-of-eggshells-Specular-reflectance-of-a-T-major-eggshell-fragment-at_fig2_269398327

 

which may explain what I saw. It would be interesting if someone else tried the same experiment, preferably with a white reference (David did, but without the reference). Also, my paper tissue may not have a flat reflectance in UV, especially at 310 nm (if it darkens the egg appears brighter).

[Andy Perrin]

That lens seems sharp only in the middle. The results are certainly interesting. Why don't you use the usual PTFE for white balance? 

 

The Topaz plugin made the center a bit sharper:

 

[Stefano]

The lens has strong field curvature, I can actually make the edges sharper by focusing on them, making the center blurry. It is currently the only lens I have with good transmission at 310 nm.

 

If you haven't seen it, here there's a more detailed description: 

 

I am writing a lens simulation in MATLAB to design one day a better lens, but it will take time.

 

As for PTFE, I don't have it yet. It is one of the things I have never upgraded, I should search for some PTFE tape next time.

 

The Topaz plugin definitely made the image better.

[Andy Perrin]

Maybe you could focal stack with the SvBony?

[Stefano]

Yes, that would improve my images, although it would also take longer to take them. It might actually be a nice idea to try.

 

Spherical aberration in this lens is not too strong, the main aberration is field curvature (along with chromatic aberration).

[photoni]

A nice consideration. in some supermarkets it is fashionable to sell eggs of different colors (natural colors from hens of different breeds)

white, gray, light blue, green, dark brown, and the normal "brick" color

Another consideration are eggs with normal yolk, some very light (those raised in cages) and some of a very strong orange color because the hens are fed with beta-carotenes.

These yolks are usually used in pastry to give more color to creams and doughs ... in some cases the digital camera is unable to record the real color, in film there was no such problem.

[colinbm]

I like the colour of the UVA egg shell.

[Nate]

The egg yolk has certain appeal to it for sure. 

[Stefano]

It almost reminds me of century eggs.

[dabateman]

Stefano if you take my monochrome egg images and merge them does it look like that?

The monochrome ones at the beginning of the post shouldn't depend on white balance. 

[Stefano]

I will try that. The problem is that without a white reference if you exposed with the egg as your reference if the egg darkened in some bands it will look brighter than it should, because you would have compensated for that. But I still have to see how it looks.

[dabateman]

13 hours ago, Stefano said:

I will try that. The problem is that without a white reference if you exposed with the egg as your reference if the egg darkened in some bands it will look brighter than it should, because you would have compensated for that. But I still have to see how it looks.

Alternatively you could try a black reference.  The front half hard boiled egg is on a wood block painted black with Semple black 2.0.  It should be black in all wavelengths. 

This seems to be before I added a small PTFE cube in my images.

[Stefano]

From David's egg series:

Channels
Red: 390bp25;
Green: 335bp10;
Blue: 313bp25;

Images aligned and "black balanced".

 

They look similar, and the egg is a bit blue indeed.

[colinbm]

Do the eggs still taste OK like this ?
If you keep the black background a bit further back, it will be a darker black & not reflect the light so much.
 

[dabateman]

Wow thank you Stefano.  That unboiled egg on the far right looks like the best Easter egg dye job ever.

Glad they line up with yours doing the same method that you used. Two samples accross the globe matching is nice.

 

Actually Andy's image tells the story: 

The un boiled egg on far left reflects the deeper UV wavelengths and absorbed the near blue wavelengths.  So should reflect and look whatever color you assigned to deep UV.

[Alaun]

There seems to be one significant difference. While Stefano's egg shows the same blue hue all around in bright areas and in dark areas, David's eggs seem to to show a hue, which is changing with the amount of light?

[Stefano]

I think his light source was not still, it usually looks like that in this kind of images. @dabateman, were you holding your bulb in your hands?

[dabateman]

9 minutes ago, Stefano said:

I think his light source was not still, it usually looks like that in this kind of images. @dabateman, were you holding your bulb in your hands?

Not in my hands. I have one or two UVB bulbs in desk lamps over my subject. They are fixed and still, but the light doesn't uniformly illuminate my subject.  There might be a gradient from closest to further away from the bulbs.

[Alaun]

So one possible explanation would be: The reflected light has a different spectrum and in these areas the picture would need a different WB? Something like mixed lighting due to reflection.

[Stefano]

Strange, because an object should have a uniform hue with different intensities, not different hues, as the lighting should be the same regardless of wavelength (unless there are strong reflections from colored surfaces).

[Stefano]

2 minutes ago, Alaun said:

So one possible explanation would be: The reflected light has a different spectrum and in these areas the picture would need a different WB? Something like mixed lighting due to reflection.

Yes, that's what I thought.

[Stefano]

David, have you tried to photograph UVB using sunlight like I do?

[dabateman]

29 minutes ago, Stefano said:

David, have you tried to photograph UVB using sunlight like I do?

No, I always have photograph UVB using lights in the basement. 

I might try that though.