Depending on a sample’s physical and chemical characteristics, light striking its surface may undergo absorption, reflection, transmission, or excite fluorescence. Natural color gems and their treated color counterparts often exhibit a similar reflectance in the visible region of the spectrum, but may show a distinct difference in the UV or IR region. Reflected Ultraviolet and Infrared photographic techniques exploit these differences. Refining of these techniques may provide a quick method for identifying natural and treated color gemstones, either individually or in groups.
The observation of fluorescence color or zoning is often important in identifying natural and synthetic gemstones, as well as color treatments. Some fluorescence images may show subtle fluorescence zoning which can normally be difficult, if not impossible, to observe by eye.
This presentation demonstrates the possible application of these specialized digital photography techniques to identify natural and treated color pearls, however, they may be applicable to other gem materials.
Digital photography has provided an opportunity to access specialized areas of photography that were once difficult to work with. The techniques presented included reflected IR and UV, and fluorescence (emission) photography.
Success with these techniques requires specially selected cameras, lenses, filters and lighting that function in the wavelength regions required. Reference samples are also an absolute necessity.
The following equipment was used for this presentation: Nikon D70, Nikkor 105mm f/4 micro, Nikkor 55mm f/2.8 micro. El Nikkor 63mm f/3.5 and adapters, a variety of specialized filters, various light sources including LWUV lamps, a UV enhanced Nikon SB-14 flash unit*, and Adobe Photoshop CS software.
Images were captured in RAW file format and adjusted in the RAW converter with a custom white balance. Minor corrections for color, brightness, contrast, digital noise and sharpening were performed in Adobe Photoshop.
*Contact Author for more information.
This technique records the fluorescence from a sample. Although fluorescence is commonly associated with UV stimulated visible fluorescence, it can also apply to fluorescence occurring in the UV or IR.
|
Visible and IR fluorescence can also be induced by visible light. Equipment requirements vary depending on the wavelength range of the fluorescence and required excitation. Requirements are more critical for UV and IR emission and are similar to reflected UV and IR photography.
Most digital cameras and lenses can be used for the more common UV stimulated visible fluorescence, but a barrier filter is required on the lens to prevent UV light from the excitation source from entering into the camera. The examples presented were recorded with a Nikon D70 camera and excitation was produced by LWUV lamps.
 Fig.1a. |
 Fig.1b. |
Fig 1a: Three natural (N) and five treated (T) color yellow cultured pearls placed in the yellow-lipped P.maxima shell from which they are cultured.
Fig 1b: The fluorescence image of these yellow pearls shows the similarity of the fluorescence of the natural color pearls to that of the yellow nacre in the shell, whereas most of the treated examples are either too light in color, or tend to exhibit a slight orangey component.
 Fig.2a. |
 Fig.2b. |
Fig 2a: Three natural (N) and three bleached (T) white cultured pearls placed in the white-lipped P.maxima shell from which they are cultured.
Fig 2b: The fluorescence image of these white pearls distinctly identifies the three bleached pearls from the natural color. Note the similarity of the fluorescence of the natural color pearls to the shell nacre.
 Fig.3a. |
 Fig.3b. |
Fig 3a: Natural (N) and treated (T) color black cultured pearls in the P.margaritifera shell from which they are cultured.
Fig 3b: Fluorescence image of natural and treated color black cultured pearls. The treated pearls do not exhibit the reddish component of the shell nacre, however in this print it might be difficult to discern the difference between the shell nacre and the four treated pearls in the front row.
 Fig.4a. |
 Fig.4b. |
Fig 4a: Strand of treated color yellow South Sea cultured pearls from the P.maxima shell.
Fig 4b: Fluorescence image of the strand. When compared to the fluorescence of image of the natural color yellow pearls in the P.maxima shell, most of these appear color treated.
 Fig.5a. |
Fig 5a: In addition to fluorescence color, unusual fluorescence zoning indicates color treatment. The saturation and contrast has been enhanced in this image to accentuate the zoning.
 Fig.6a. |
 Fig.6b. |
Fig 6a: Treated color Tahitian black pearl in P.margaritifera shell from which these pearls are cultured.
Fig 6b: Fluorescence image of the same pearl in the P.margaritifera shell. This strong red fluorescence is not exhibited by natural color cultured pearls from this mollusk.
This technique selectively records the reflected UV from a sample. In addition to selecting the appropriate filters, it is important that the lens transmits UV within the range required and also that the camera CCD does not inherently block too much UV.
 |
For LWUV there are a few lens choices, but the El Nikkor 63mm f/3.5 is the most cost effective and also exhibits good LWUV performance. This specific version of the El Nikkor performs better than most of its counterparts. For SWUV there is little choice but a few very expensive obsolete production lenses or an expensive aftermarket lens. The Nikon D70 camera performs well for recording UV images.
A UV transmission/visible blocking filter is required on the lens to prevent all but the desired UV wavelengths reaching the camera. UV may be recorded in the blue, green or red channel on a digital camera. However the red channel can be contaminated by stray IR and many UV filters exhibit a small IR transmission window. Additional precautions are therefore necessary to prevent IR from reaching the camera CCD. The sun can be used as a weak UV source but a UV lamp or UV flash is preferred.
 |
Reflectance spectra of natural (right) and treated (left) color yellow South Sea cultured pearls from the P.maxima. The light purple bar indicates the transmission window of the UV filter used for the black and white reflected-UV image. The natural color shows a strong absorption in the ultra-violet at 365nm and therefore appears darker in the reflected-UV image. The authors believe these to be the first ever published reflected-UV photos of a pearl.
 Fig.7a. |
 Fig.7b. |
Fig 7a: Natural (upper) and treated (lower) color yellow South Sea cultured pearls.
Fig 7b: Reflected-UV image of natural and treated color yellow cultured pearls. The natural color (upper) appear darker due to their weak UV reflectance which is a result of UV light being absorbed by the pearls’ pigmentation.
This technique selectively records the reflected IR light from a sample.
 |
Most lenses will record reflected IR up to 1100nm and most digital SLR cameras perform well for recording IR images despite manufacturers attempts to block IR. A Nikon D70 camera was utilized for the example shown.
An IR transmission/visible blocking filter is required on the lens to prevent all but the desired IR wavelengths from reaching the camera. Tungsten, xenon, halogen or the sun can be used as an IR source.
Similar work was performed using color IR film on black cultured pearls in 1978 by Komatsu and Akamatsu , and on heat treated blue sapphires by Fjordgen in 1986.
 |
Reflectance spectra of natural and treated color black cultured pearls. The light red bar indicates the transmission window of the IR filter pack used for the black and white reflected-IR image.
 Fig.8a. |
 Fig.8b. |
Fig 8a: Natural (upper) and treated (lower) color black cultured pearls.
Fig 8b: Reflected-IR image of natural and treated color black cultured pearls. The natural color shows a strong absorption in the IR at 700nm and therefore appears darker in the reflected-IR image.