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Experiment with Photographing Metal Scratches and Embossed Letters


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#1 Andrea B.

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Posted 22 July 2014 - 15:45

In response to Luzer's request about whether UV would reveal some engravings on a metal vessel, I made a preliminary experiment in photographing metal scratches and embossed letters by choosing an old metal pie pan with the manufacter's name embossed on the bottom as my subject. There are also scratches on the metal and some patches of chemical deterioration (probably rust?).

Note that this pie pan was not shiny metal and so was easy to photograph without having to deal with reflections. Shiny metal makes for a lot of photographic complexity due to reflections and specular highlights. Be aware that the results here may not extend well to shiny metals without use of special gear.

We have noted that the shorter wavelengths of UV light are very good at bringing out surface details and so should be helpful to delineate scratches and embossings. That seemed so in my photographs, but not by much in the case of this particular metal pie pan. As you can see both the Visible and UV shots have almost the same amount of detail but there is slightly more detail in the bottoms of the embossed letters in the UV shots. The UV shot does show the chemical deterioration better than the Visible shot. Interestingly, I see that the longer IR wavelengths helped bring out some of scratches in a different way by penetrating underneath the patches of chemical deterioration. That is useful thing to learn.

Equipment [Nikon D600-broadband + Carl Zeiss 60mm f/4.0 UV-Planar]

All photographs were made in a room well-lit with sunlight. The subject was not in direct window light.

Visible Light [f/16 for 1.6" @ ISO 100 in ambient room light with Baader UVIR-Block Filter]
Attached Image: metalPiePanVis_ambientLight_072214swhME_23411pn.jpg

Ultraviolet Light [f/16 25" @ ISO 800 in ambient room light with Baader UV-Pass Filter]
Attached Image: metalPiePanUVBaad_ambientLight_072214swhME_23415pn.jpg

Ultraviolet Light [f/16 for 30" @ ISO 800 in ambient room light with UV-Precision UV-Pass Filter]
Attached Image: metalPiePanUVPrec_ambientLight_072214swhME_23428pn.jpg

Infrared Light [f/16 for 1/20" @ ISO 400 in ambient room light with B+W 092 IR-Pass Filter]
Attached Image: metalPiePan092IR_ambientLight_072214swhMpnE_23421pn.jpg

Edited by Andrea B., 23 July 2014 - 15:24.
corrected the length of the wavelengths

Andrea G. Blum
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#2 Alaun

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Posted 22 July 2014 - 17:02

Nice work done, Andrea, but you mixed up a little detail: UV: higher frequency => shorter wavelength :)

Werner
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#3 Andrea B.

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Posted 22 July 2014 - 20:55

I got reverse-itis there for a moment. Happened all my life. :)
I'll make the edit. And thanks for catching this Werner.
Andrea G. Blum
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#4 Shane

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Posted 23 July 2014 - 16:36

Quote

We have noted that the shorter wavelengths of UV light are very good at bringing out surface details and so should be helpful to delineate scratches and embossings.

This is true with respect to the penetration of light into a subject where IR may penetrate deeply resulting in poor surface detail and, UV penetrates minimally resulting in high surface detail. Since steel is a conductor, neither of these wavelengths will exhibit any significant penetration into the surface unless oxidized, the only benefit may be slightly higher resolution from the UV wavelength due to its short wavelength (which you appear to have noted).

The key to observing scratch type markings on metal is the angle of the lighting with respect to the camera. This is one of the more difficult "defects" to detect as it is often masked by other flaws. There are many choices of technique but are really variations of brightfield, darkfield and "oblique" (varying definition).

Angles are with respect to the Normal.
Brightfield mode involves observing the diffuse forward scattered light (not the specular reflection). Lighting ~20-30 degrees and Camera at minus ~20-30 degrees.

Darkfield mode involves observing the backscattered light (not the retro-reflection) and requires strong lighting. Lighting ~40 degrees and Camera at ~20 degrees.

"Oblique" mode can be a darkfield defect observed in a cross-web direction (perpendicular to the direction of light) resulting in an almost specular back reflection against a dark filed. Lighting variable position to produce darkfield with specular back reflection and Camera at 0 degrees.

I would try "oblique" mode. Put the subject on a turn-table (lazy-susan), in a dark room, and rotate it also varying lighting position until features of interest start to become visible by eye. A polarizer may also be useful.

Edited by Shane, 23 July 2014 - 16:37.


#5 OlDoinyo

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Posted 05 November 2014 - 01:16

Most metals have pretty uniform reflectance over a wide range of wavelengths; thus, there is little difference between UV and IR here. UV picks up the surface impurities a bit better. The magnification here is not sufficient for diffraction limitation to be an issue.