by Tessa de Alarcon

Since we have posted often about visible induced infrared luminescence (VIL) and the equipment we use at the Penn Museum, we on occasion get emails from other conservators and museum professionals asking about what equipment to buy and the costs associated with this photographic technique. This technique is often used for imaging Egyptian blue, but it can also be used for Han blue and Han purple. Much of the same equipment is also used for infrared reflectance.

Making specific equipment recommendations though are tough because there are a lot of options and a lot depends on your budget. Basically though, what you need are the right lights, a camera (and lens), and a long pass filter for the camera to capture in the infrared. I thought I’d do some testing to show how some these different elements impact the results in the hopes that it might help others figure out budgets, and to show that it’s also possible to build this equipment up piece meal, starting with equipment you might already have or is very low cost. Some elements though are pricey and things can add up.

All of the VIL images in this blog post also have a Spectralon standard in them (99% infrared reflectance standard). This is not low cost, and not required for the technique. It is often required for publication, and is useful for trouble shooting or developing new techniques. I’m mentioning this because it is useful to evaluate the data presented here, but it isn’t something that is strictly required. We did not have one for a long time, but were able to do this type of imaging. We waited as it was a big investment for us (approx. $500). These standards also range in price depending on the size and calibration.

An infrared (IR) filter is a requirement for this technique. I only tested one IR filter (though we have two). They range in cost depending on where you get it and the quality of the filer and the size of the filter. Ours is a B+W and is an 830nm IR long pass filter and is 62mm ($130). There are cheaper ones available as well as more expensive ones too. We got ours to fit our macro lens, and have adaptor rings (all our rings are generic brand low cost ones that were each under $10) to fit it onto other lenses that we have.

The right lights are a critical factor for this technique, but not necessarily one that has be high cost. You need a bright light with no infrared radiation, red lights are commonly used and LED bulbs are preferred as they produce no infrared radiation. I tested out three different lights for visible induced infrared luminesce. These include at the high end of the budget a Mega 64 Profile Plus RGB + UV Par light at maximum intensity with the red LEDs on only. This is a roughly $200 light. The other lights I tested are both low cost options. I tested one FEIT electric one red LED light bulb I put in one of our extra reflectors and photo light stands. I bought this bulb for $6 from my local hardware store, and a two daylight LED bulbs (UL certified) that I got from our facilities department to replace burnt out bulbs on our copy stand. I used both bulbs in the regular copy stand set up for imaging. I don’t know what the bulbs I got from facilities cost, but there are no brand name stamps on them so I’m guessing they weren’t expensive, that UL logo on it just means that it is a UL certified bulb.

I tried both our full spectrum modified camera, and our regular digital SLR (unmodified). Neither of these is an inexpensive camera, but should show generally the difference between using what ever digital camera you already have, and getting a similar cost one as a modified camera. Neither is new, we use our cameras until they can’t be repaired. The unmodified camera is a Nikon DSLR D5100 and the modified camera is a Nikon DSLR D5200. We bought our modified camera new, and sent it to Life Pixel to remove the internal filter, but they now sell used cameras that they will modify for you (there are options). Ours was modified to be a full spectrum camera. At the time of writing this post, the most economical option I saw on life pixel for a used camera with this modification package cost a total of $449. So even used not cheap. But lets get to the testing and start looking at results.

Lets get to the results from the modified camera. I have a visible reference image in the set, and the VIL images using each of the different lights. All the lights worked, but the brighter MEGA par 64 gave the best results, especially at exciting traces of Egyptian blue. Though the daylight no-name brand LED bulbs were not bad. These have a range of camera settings, and the benefit of the modified camera is that I could see the results in live view, focus the image and adjust the settings with minimal bracketing. The spectralon should not be visible, and if it is usually means that the image capture settings are not quiet right.

Next up, the unmodified camera. The images are arranged in order of the lights used the same way as for the modified camera for easy comparison. So I did get results with all of the lights. The down side is that the live view shows nothing so the focus can’t be corrected. These images are all slightly out of focus because the focus in IR is different from the focus when capturing in the visible range and I focused the image before putting on the IR filter. All of them had to be taken at the longest possible exposure of 30 seconds. This made data collection easy since I had no choice in settings. And for the low costs bulbs it looked like they were just black with no data until I got them in adobe camera raw and converted them to grey scale by adjusting the saturation to -100. Then I could see something, but I did also adjust the exposure for the images you see here. You can see the spectralon and the background in all of the images and interpretation might be harder with these than the images taken with the modified camera. I will say I think this could be used for detecting Egyptian blue, but it’s important to note that the unmodified camera only got the thick areas of Egyptian blue, and didn’t have the sensitivity to pick up the traces visible in the images taken by the modified camera.

For fun, I also tried putting the IR filter over my cell phone and using the MEGA par lights took a photo. This is just to show that even a small sensor like what is in a cell phone camera can work. It is out of focus though and like with the unmodified Nikon DSLR you aren’t getting traces of Egyptian blue. But it did show something, and I could see the results in the live view. This is also an avenue that I know others are working on: producing low cost modified cell phone cameras with built in filter wheels. Sean Billups has presented at AIC on this topic.

To wrap things up, I think it is possible to build this equipment up overtime. You can start with the camera you already have for documentation, and then get better lights and a camera as you can afford them. The IR filter can also be used for IR reflectance, this is also possible with any digital camera with a long exposure and using any light that produces infrared radiation. There is much less difference in data quality between a modified full spectrum camera and an unmodified camera for this method, though again there is no live view and sharp focus is hard to do with an unmodified camera. We use incandescent photo floods (real bright and toasty) but any light that gets warm probably produces infrared radiation and could be used. Daylight for example works real well too (and is free).