Highlight-Based RTI Equipment

The equipment required for highlight-based Reflectance Transformation Imaging (RTI) is essentially the same for all captures; a light source which can be moved (or a dome of multiple light sources) and a fixed camera perpendicular to the object being imaged. However the actual objects being studied do inform the exact choice of equipment. My current work is on Ancient Egyptian Predynastic palettes, as my next research project will be too, which typically are around 20-40cm (even the so called Narmer palette is only 64cm in its longest dimension). If you’re working with larger objects then some of my problem solving wouldn’t fit your needs.    

Figure 1 – H-RTI flashgun setup

Typical highlight-based RTI (H-RTI) setups use a string tied to the flash to ensure a constant distance between the head of the flash and the surface of the object. Rather than put string onto the flash which needs measuring for each set up (and can untie itself during capture and need more measuring), I’ve mounted a 1.50m (60 feetsies for my American colleagues) fabric tape measure with a plastic tip. It’s mounted to the flash on the underside by Velcro to a Honl SpeedStrap, but you could just use any Velcro mounting onto the flashgun. Whilst string gives you more versatility and distance, for the smaller objects I am currently studying this solution gives me a few less steps in setup – which an RTIsta will tell you takes much longer than the capture itself and any streamlining is welcomed. I found two minor issues with the tape measure set up when I first used it. Firstly the tape measure needs to be set back slightly from the front of the flash, or it interferes with the light spread, and secondly the black plastic tip is extremely hard to see in low light environments – thankfully something easily remedied with a piece of white plastic tube glued onto the tip.  I had toyed with the idea of having two laser dots to line up to get the correct distance, somewhat like a modern Dambusters, or even using an ultrasonic rangefinder but decided that simple was better. Although if/when I work on larger objects and need to be further away I might revisit the ultrasonic rangefinder idea instead of using metres of string.   My lighting setup is using an Olympus FL-35 flash and Phottix Atlas triggers, for no reason other than I already owned them. Any flash which allows the setting of light intensity setting and remote trigger, or continuous lighting, set up will work.  

Figure 2 – Engraved Thoth hieroglyph

The grip is a motorbike grip over a solid aluminium round bar, which was drilled and tapped to accept a ¼ inch (20 TPI) male to ¼ inch (20 TPI) male connector that then screws into the trigger unit. I don’t have access to a lathe, or it would have been nice to have turned the end of the bar down and threaded it to screw into the trigger (similar to the handle supplied in the CHI starter kit). The aluminium bar was chemically blackened and I’ve hand engraved a Thoth hieroglyph into the end of the bar (Figure 2).    

Figure 3 – Quick release camera mounting bracket, on adjustable copy stand column

My museum based research is on smaller objects and often in location where it’s not possible to set up a large stable tripod, and so a copy stand is much more suitable as it has a significantly smaller footprint. Traditional copy stands usually have their own baseboard and potentially their own legs, making them less ideal for transport to different research locations, and they also tend to be rather expensive. To mitigate both issues I’ve built myself a mobile copy stand which has a desk clamp rather than the traditional style stand (Figure 5).   This stand is built from a Dusrt M670 darkroom enlarger column and base with a compatible geared winder, mounted to a computer monitor stand desk clamp. The horizontal camera arm (Figure 3) was created with a 150mm long mild steel tube with a 25mm internal diameter, with an M6 wingnut style hand knob bolt to secure it to the winder and with a solid mild steel plug interference fitted into the end (with a 4mm grubscrew to ensure zero possible movement) which was drilled and tapped to ¼ inch UNC (20 TPI) and bolted to a Manfrotto quick release mount. Using the ¼ inch UNC (20 TPI) thread makes the unit compatible with standard photography equipment, such as tripod heads or camera brackets.  

Figure 4 – Desk clamp

The column was mounted to a desk clamp (Figure 4) using a 5mm thick mild steel interface bracket, which both clamp and column base were bolted to. This then allows the whole unit to be camped onto a desk, with the camera height being easily adjusted.

Figure 5 – Copystand with camera mounting bracket