Beyond Visible
Basics - Luminescence, Reflected Ultraviolet and Infrared Photography

Shane Elen

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

Reflected Ultraviolet Photography

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.

Schematic of reflected ultraviolet photography

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.

Reflected Infrared Photography

This technique selectively records the reflected IR light from a sample or scene.

Schematic of reflected infrared photography

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.

Luminescence Photography

Luminescence is considered a general term for the phenomenon of light emission that is not merely due to the temperature of a hot body (e.g. glowing hot iron). The type of excitation source used to stimulate emission identifies the various luminescence phenomena. Listed below are forms of luminescence that I have worked with:

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:

Phosphorescence - A Special Case
There is often confusion between the term fluorescence and phosphorescence that is not surprising as even the scientific community has varying definitions for phosphorescence. One definition is based on electron-spin state and others on a time factor. Unfortunately there is no consistent agreement on the time factor. The non-complicated approach, which is commonly used, is based on visual perception. This defines fluorescence as occurring during exposure to the excitation source, while phosphorescence occurs after the excitation has been removed. Although commonly associated with photoluminescence, phosphorescence describes all long-lived emission phenomena regardless of origin and is sometimes referred to as afterglow. However, afterglow (as is most visually observed phosphorescence) is more than likely a form of room-temperature thermoluminescence resulting from delayed emission due to shallow trapping of electrons or holes.

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).

This form of luminescence occurs as a result of excitation by light photons (ultraviolet and visible light) and is often referred to as fluorescence.

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.

Schematic of visible fluorescence photography excited by ultraviolet light

Visible fluorescence excited by ultraviolet light.

Schematic of infrared fluorescence photography excited by visible 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
This is probably the most common form of luminescence photography and typically requires an exciter and barrier filter. However the barrier filter must be carefully selected to provide a "window" which transmits all the fluorescence, while excluding all the excitation. This can often be difficult to accomplish and some compromise is often necessary. Although excitation can be LWUV or SWUV, LWUV excitation is easier to work with than SWUV as the barrier filters, glass or gelatin, tend to fluoresce significantly when exposed to SWUV.

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
The same problems exist here as in the previous section however it will now be necessary to ensure there is no infrared leakage from the UV source. This may require changing the excitation filter for an expensive one, or using a narrow band excitation source, also very expensive. In addition, the barrier filter should obviously transmit infrared and block UV and visible light.

Photoluminescence - SWUV excited LWUV fluorescence
Probably not a common form of fluorescence photography as it requires a very controlled SWUV excitation source, typically narrow band. A barrier filter that transmits LWUV and blocks SWUV is necessary for the lens. Unfortunately, most photographic lenses will fluoresce to some degree resulting in a foggy image. Lenses specially designed for reflected ultraviolet photography should fare much better (something I still need to try with the UV-Nikkor). It is not possible to use a readily available SWUV lamp in this application as they also emit some LWUV light.

Photoluminescence - Visible excited infrared fluorescence
Luminescence in the infrared can often occur in materials when they are exposed to blue or green light. This can occur in daylight but will go unnoticed due to its infrared nature. Blue and green lasers are often good excitation sources but their narrow band spectral quality may actually be a drawback if the object is not excited by the laser wavelength. Tungsten halogen lighting fitted with blue or green filters can also be used providing the filters do not leak infrared. Another older technique involved passing a strong light through copper sulphate solution. Now all that remains is to use an infrared transmission/visible blocking filter on the lens (sharp cut-off filters work best for this) and capture the image in the dark room to avoid stray infrared contamination from the environment, e.g. sunlight. However, it is important to know the cut-off point for the filter to avoid inadvertently excluding the infrared fluorescence.

Photoluminescence - Visible excited visible fluorescence
This is very similar to visible excited infrared, except of course the fluorescence occurs in the visible region of the spectrum. The excitation is typically blue or green light and the fluorescence red. However, fluorescence can be green, yellow or orange as exhibited by many "fluorescent" dyes and inks. Capturing visible fluorescence excited by visible light can be very challenging and often requires narrow band excitation sources and sharp cut-off filters. It is also important to exclude infrared from the excitation source or the environment.

This form of luminescence is the emission of light due to electron bombardment from a high energy electron beam. Many people have seen this in the form of cathode-ray oscilloscopes or old tube television sets. Two common types of instrumentation for observing cathodoluminescence in materials are, the electron microscope or the Luminoscope. The Luminoscope is ideal for 35mm photography as the sample chamber has a window through which to view or photograph the sample. There are a couple problems to overcome when photographing cathodoluminescence with this type of system. The observation window is a lead glass, to prevent the passage of any generated x-rays, and unfortunately tends to be a little yellow. The other problem is that stray light can occur when the energetic electron beam strikes air molecules in the sample chamber, which is under partial vacuum. This results in ionization of the air molecules producing a slight bluish-purple plasma discharge. Both these problems are only really noticeable for samples producing a weak emission and can be difficult to correct for.

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.

This is the emission of light due to the application of an electric field. Light emitting diodes (LED) provide an example of low-field electroluminescence (requiring only a few volts, direct current). Doped zinc sulphide demonstrates high-field electroluminescence and requires a strong electric field. The requirements for photography are the same as photographing phosphorescence, primarily a dark room.

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).

In progress

In progress

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Updated September 6, 2006