NIR: Dark Water - Dark Skin? (... and a mysterious artefact)

[Kai]

 

On the NIST website you can find a contribution to the reflection spectrum of skin:
https://www.nist.gov/programs-projects/reflectance-measurements-human-skin

 

The illustration reminds in part of the absorption curve of water that Andy (Perrin) showed in 2019:
https://www.ultravioletphotography.com/content/index.php?/topic/3505-dark-water-and-the-nir-absorption-peak/

 

To see whether skin actually appears darker in longer-wave NIR, I made the following experiment:
 

I took three photos showing my hand on PTFE.

Illumination with a 150W tungsten red light lamp.

Canon EOS 6D-FS, EF 1.8 50mm STM
As filter I used a ZWB2 (720-820 nm), an IR 950 (890-1040 nm on the sensor) and an IR 950 combined with a UV-IR cut filter (app. 1120-1180 nm on the sensor).
The image in the longest wave area is very foggy. I only adjusted the brightness of the RAW images on the Teflon. No further image processing.

 

Observation
At 950 nm, there is practically no darkening of the skin compared to the range around 770 nm.

Around 1160 nm the skin actually appears a little darker. 

The piece of black carpeting in the background on the right-hand side appears lighter with increasing wavelength. 

 

 

Complement
With a glass of water, the effect is much stronger - as others showed here a few years ago.
Even with me (sunlight, same equipment):

The "long wave pictures" show an artifact ("butterfly"), which is also created without a lens!

 

[Andy Perrin]

The effect is much less subtle in SWIR. I'm normally a pale-skinned brown-and-gray-haired person, but at 1550nm, it's all very different:

 

 

[Bill De Jager]

Nice self-portrait!

[Kai]

Interesting!
Which camera? Which lens?

[lukaszgryglicki]

Looks like > 1.5 um

 

[Andy Perrin]

TriWave camera with germanium sensor, and as I said, 1550nm. You can’t photograph 1550nm with a normal converted camera. Silicon cuts off at 1100-1200nm or so. 

Kai, it was a few years ago but I think it was the ThorLabs 25mm lens (on a half inch sensor, so there is a large crop factor).

 

[GaryR]

Interesting topic; Striking photo Andy!

[Cadmium]

Andy, That sunburn is really going to hurt in the morning! 

[Andy Perrin]

I knew it would be bad when my eyes turned black Cadmium!

[Fandyus]

On 11/5/2021 at 2:11 AM, Andy Perrin said:

The effect is much less subtle in SWIR. I'm normally a pale-skinned brown-and-gray-haired person, but at 1550nm, it's all very different:

 

 

That looks actually horrifying..

[Cadmium]

Yes, I wonder if they have ever used such in any sci-fi or horror films?

[Andy Perrin]

I doubt it, just because of the low resolutions currently available. Even the expensive cameras are in the 1.5 megapixel range for the most part. But if you just want dark skin, dark eyes, light hair, you can do it cheaply with CGI and makeup.

[Andrea B.]

I couldn't figure out in which photo was the "butterfly"??

 

And wanted to say that this was a very cool experiment. Thanks for posting it.

[Kai]

4 hours ago, Andrea B. said:

I couldn't figure out in which photo was the "butterfly"??

 

And wanted to say that this was a very cool experiment. Thanks for posting it.

All the extreme IR recordings on the edge of the sensitivity of the silicon (> 1100nm) show this lighter "double shadow" in the form of two surfaces reminiscent of butterfly wings.
The pattern can be found on three cameras, also with a filter and pin hole (i.e. without any lens).
Seems to be a structure on the sensor that only appears well beyond 1000 nm.
The pattern can be seen in all pictures, especially clearly with a homogeneous background: 
 

[dabateman]

Isn't that the eye of Sauron? 

[Kai]

10 hours ago, dabateman said:

Isn't that the eye of Sauron? 

I had already suspected deeper connections ...;) 

[OlDoinyo]

In the US, there is an urban legend about certain sinister "black-eyed children" who suddenly show up at people's doors. Who would have thought that it might have originated with IR photography!