Depending on a sample’s physical and chemical characteristics, light striking its surface may react a number of ways resulting in absorption, reflection, transmission, or emission (luminescence). Photography typically involves capturing reflected visible light, however ultraviolet and infrared, as well as luminescence, can also be captured with the appropriate equipment.Materials with differing physical and chemical characteristics can often exhibit similar reflectance in the visible region of the spectrum, but may show a distinct difference in the UV or IR region of the spectrum, or in their luminescence. Infrared, ultraviolet and luminescence photographic techniques record these differences and have significant application potential in materials characterization, failure analysis and forensic science. In the illustrations below the techniques are being applied to gemstones. More images and transmission spectra will be added as time permits.
Warning - Ultraviolet and Infrared light should be treated with caution as exposure can cause skin or eye damage
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 stray IR can contaminate the red channel 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.
This technique selectively records the reflected IR light from a sample or scene.
Most lenses will record reflected IR up to 1100nm and most digital SLR cameras perform well for recording IR images despite manufacturer's attempts to block IR.
Unless an IR modified camera is used, 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.
Most luminescence studies are concerned with the emission of visible light, however they can also include ultraviolet and infrared emission. Luminescence photography is an important materials characterization technique used in scientific and forensic applications. The contrast resulting from differing emission properties has two main applications:
Photographing phosphorescence is pretty straightforward requiring only complete darkness and long exposures, or multiple long exposures. The sample is exposed to the appropriate excitation source to "fill the traps", the source is then turned off and then the image is captured. No filters are required (except perhaps for correcting reciprocity failure if using film).PHOTOLUMINESCENCE
The term fluorescence is commonly associated with UV excited/stimulated visible emission (fluorescence) but it can also apply to emission occurring in the ultraviolet and infrared, as well as emission excited by visible light.
Visible fluorescence excited by ultraviolet light.
IR fluorescence excited by visible light.
Fluorescence photography is a little more complicated as it typically requires an excitation and a barrier filter. The excitation filter is placed over the light source to isolate the required excitation wavelength. This is often built into the light source by the manufacturer but may not necessarily have the desired, or expected, spectral transmission properties. The barrier filter is placed over the camera lens and has two functions, to reject the excitation wavelengths while transmitting any fluorescence from the sample. It is not necessary to use optical quality filters for the excitation filter but it is obviously a requirement for the barrier filter. While it is important to select excitation filters and barrier filters with the appropriate transmission characteristics, it is also necessary to ensure that they do not fluoresce themselves to incident or reflected excitation light. A barrier filter that fluoresces will produce a foggy image.
Equipment requirements vary depending on the wavelength range of the fluorescence and the required excitation. Most digital cameras and lenses can be used for the more common UV stimulated visible fluorescence, however UV and IR fluorescence requirements are more critical and are similar to those used in reflected UV and IR photography.Photoluminescence - UV excited visible fluorescence
The UV transmitting/visible blocking glass typically found on LWUV lamps, or the Nikon SW-5UV flash adapter, function as the exciter filter. The Kodak Wratten 2B or 2E can often be used as the barrier filter to exclude LWUV while transmitting visible fluorescence. The main problem is that most UV exciter filters also leak near infrared, and possibly a little red and blue light. Unless the barrier filter can effectively eliminate the IR leakage, which the Wratten 2B or 2E cannot, it will contaminate the image. Therefore sole use of the Wratten 2B or 2E is not recommended. A little blue or red contamination is an obvious problem when trying to capture visible fluorescence and in some cases can be easily eliminated if the sample does not exhibit fluorescence in these regions. However, the infrared may show up in any, or all, of the RGB channels to create additional problems. Changing the exciter filter for a better one, or combining two different types of barrier filter, or a combination of both of these solutions can significantly reduce these effects.
As LWUV lamps often use the cheaper (less spectral control) type of UV glass some gains can be made by replacing it with more expensive (tighter spectral control). The downside is that the filter can be difficult and expensive to replace, and it is likely to transmit less LWUV, so this is often not a viable option.
Using a combination of two barrier filters can significantly improve the situation but compromises might necessary especially for samples that fluoresce blue in the 400-420nm, or red in the 650-700nm region. Where blue fluorescence occurs in the 400-420nm region it will be necessary to select the appropriate Wratten filter that does not exclude any blue fluorescence. For red fluorescence in the 650-700nm region the situation is a little more difficult as this may overlap the red leakage from the excitation source. Here it is vital to know the spectral characteristics of the UV excitation source. Red and/or blue leakage can often be detected by placing a ball bearing in place of the sample. As the ball bearing does not fluoresce, any red or blue observed can be attributed to leakage from the excitation source. It may be possible to use a BG glass to prevent red leakage however this filter has a slightly unbalanced transmission in the visible region and due to a sloping cut-off towards the IR region it also cuts off some red. Better results can often be obtained using a sharp cut-off filter, however, the Tiffen Hot Mirror is not suitable for this as it transmits out to 800nm.Photoluminescence - UV excited infrared fluorescence
Natural and synthetic diamond crystals produce some very interesting cathodoluminescence, often with strong zoning representative of their growth planes. The growth planes are visible due to variation in concentration of impurities at the growth plane interfaces. It is the impurities which luminesce and the luminescence pattern is therefore representative of the growth planes. As the electron beam can only penetrate the top few microns of the sample, any luminescence zoning that appears is very well defined. Zoning observed in photoluminescence (fluorescence) caused by ultraviolet light is not so well defined, as the ultraviolet light can penetrate deeper into the sample, which results in the luminescence of multiple growth planes overlaying one another. The only exception to this is when the energy of the ultraviolet light is below the band gap energy for the material e.g. less than 225nm for diamond.ELECTROLUMINESCENCE
The most interesting sample I encountered was a synthetic diamond crystal that not only exhibited electroluminescence when touched with 50V DC probes, but also resulted in a strong long-lived phosphorescence. The electroluminescence and phosphorescence were localised to the area of the probe to such an extent, that it was possible to write my initial S on the crystal face (unfortunately no photos).THERMOLUMINESCENCE
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