Chapter 1: Highlight RTI

Highlight RTI (H-RTI) is the classical method of taking photos for RTI. The non-profit Cultural Heritage Imaging offers some pretty good manuals on how to capture photos for H-RTI. But I will offer my own guide here, as it is also specific to the tools we have available at the lab.

You might be asking why there are different methods at all. Well, before someone came up with the idea of a light dome, H-RTI was the way to go. Despite that, some objects are simply too big for or not reachable with a dome. H-RTI has its purpose and you should know how to do it, although doing RTI with a dome (D-RTI) is much easier.

In this chapter, we will have a look at the gear and how to set it up.

The gear

So for H-RTI, you need a lot of gear, most of which a usual archaeological department or project should already posess. You need more gear here in comparison to D-RTI, because it is not already built into a dome. The advantage however is transportability. The main gear consists of the camera, a tripod, light and some special spheres. There is also other equipment needed, but these are minor ones and will be explained below.

📸 Camera

So the better the camera the better the photos, right? Yes and this holds also true with RTI. The best choice would be a DSLR camera, where you can adjust the lens and aperture manually. Point-and-shoot cameras work as well, but you will achieve better results by using a better camera. Modern cameras have a large sensor and a good resolution and the higher the numbers, the better. Something that is often not said is, that you should opt for a mirror-less camera. Usually, DSLRs have a small mirror built in, that moves everytime you take a photo. That movement can create unwanted vibrations to which RTI is very sensitive to. DSLR in general have also another advantage. They save their data in RAW data files. This is very good, because with RAW data files, you have all the possibilities you need to post-process the images.

🔭 Tripod

The tripod is also important, because it holds your camera and it needs to hold it very steady. A heavy tripod is therefore better than a light one. There are also weights (sandbags or water canisters) that can make your tripod heavier, but a sturdy tripod to begin with is always the best option. Depending on your setup, you also might need a rod that can be attached to the tripod, so you can position your camera with a distance to the tripod (see image at the beginning of this chapter). This rod also needs to be very sturdy and it needs to be attached tightly to the tripod.

💡 Light

You also need some kind of light source. I personally use a detachable camera flashlight that I do not stick onto the camera, but that I wirelessly connect to the camera. It works really well and a good flashlight can be set to match the camera's settings (like ISO and F-Stop). You are still able to adjust the brightness and can adapt to many situations when recording photos for RTI. Sometimes you need a lot of light and sometimes you don't. In theory a normal flashlight (that you use at night) will also do, but you can't really adjust the light to different situations.

🪩 Spheres

Something very specific to RTI are spheres. You might have seen them in images that show RTIs, because sometimes you can't cut them out. These spheres are important to calculate the light direction in each photo, because they reflect the flashlight pretty well. These spheres therefore need to be very reflective and very round. This also means, you can't just simply take some plastic spheres you find at the hardware store. These spheres need to be as perfectly round as possible. Cultural Heritage Imaging are offering sets where you find very good black spheres in them. You could also search for spheres especially made for ball bearings, which usually are very good. These are normally metallic, which works, but spheres in a glossy black colour work much better.

➕ Additional equipment

In addition to the equipment above, you also need some smaller stuff. As I mentioned before, we do not want to move the camera when taking photos. This usually also means, that we can't simply touch the trigger button when we want to take a photo. We therefore need a remote control. These are usually very cheap, but specific to your camera. So search for something that can remotely trigger your camera.

I also mentioned that we move around with the flashlight in our hands. This flashlight needs to trigger automatically, when a photo is taken. This can be achieved with so-called radio remote triggers, that connect via Bluetooth. You put a sender on the camera and a receiver on the flashlight. As soon as you trigger the camera (even with a remote control) you also trigger the flashlight. Very convenient.

As usual with all photography, you also need a colour chart or grey card to adjust the colours in your photographs as well as a scale or a ruler to document measurements while taking photos. You will also need a 3D bubble level to position your camera perpendicular to the object.

The setup

So the setup for H-RTI is somewhat complicated, but if you have done it a couple of times, it gets easier. The first important thing is to visualise how and where to position the camera and if possible the object and the accompanying spheres. You then have to figure out how to move around the object and camera to hold the light from all directions. If you have thought this true, setting up should go fairly easy.

Positioning the object and spheres

So first, we should know if it is even possible to position the object. Sometimes you can't move the object or are not allowed to. In this case, everything else has to be positioned after the object. If you have to possibility to position the object, it is a good practice to lay it down on a table or the floor. If you have the possibility, it is also a good practice to lay the object onto something neutral, like black paper, so that in the end the photos look nice.

The spheres also need to be positioned. Depending on whether your object lies on the floor or is positioned somewhere else, you have to find ways to place the spheres correctly. In order to do so, you have to be clear what of the object you actually want to photograph. If you set up everything, your camera will take photos of the object. This doesn't have to be the whole object necessarily, but rather the area of interest. In any case, you have to imagine where your plane of focus lies. The spheres have to be positioned in a way, that their upper third is in that same plane of focus. It sounds complicated, but if you take a look at the screenshot, you probably understand what I mean.

It is also of advantage, if you position two spheres in the photo, if possible. Make sure both spheres are of the same size and do not sit next to each other. If the environment allows, it would also be good if you can at a later point cut the spheres from the photo, so don't position them too close to the object and imagine a crop frame around the object you are photographing. Also, think about what you will do in a moment. If you go around with the flashlight, everything will cast a shadow, especially the spheres. We naturally do not want the shadow of the spheres on our object, when we hold the light in a certain angle. So try to find a position that meets all the criteria above. It is sometimes difficult, but should be possible.

Lastly, you should also include a scale and colour chart to be able to post-process the photos and later on measure them. The colour-chart should definitely be cut away later, so don't put it too close. The scale might stay in the final RTI, that is up to you. Also, sometimes it makes sense to also include a small paper with the find-number, so you know which number each RTI has.

Positioning the camera and tripod

Ok, now that we have set up the object, spheres, scales, and colour chart, we can turn to the camera. First thing to know is, that the camera has to be positioned perpendicular to the object. If the object is on the floor, you need to position the camera facing down perfectly. For this, we can use a 3D bubble level. Before you do that however, make sure that you have connected all the equippent you need. For me that usually is the sender of the radio remote triggers as well as the receiver for the remote control. Make sure the battery is full and the SD card is empty, so you will not have to touch the setup once done.

You can either tighten the camera directly to the tripod or you use an extension rod, so that the camera can hang a bit on the side, which usually gives you more space to move around the object. In all cases, it is very important that the tripod and camera are fixed and secured. The smallest movement can ruin your dataset. With a weak setup, even walking by the tripod can do that, so make sure you put everything on solid ground, use a heavy tripod and screw and tighten all the connections in your setup. You can also use weights to weight your tripod down. Sometimes these come in the form of sand-bags, sometimes of water canisters. Everything is allowed, as long as you secure the stability.

Preparing the light

The light is usually easy to prepare. If you are new to RTI, you should definitely use a wooden rod and a string (I'll explain). If you are experienced, you can leave them aside. The important thing with the light is, that you have to position it for each photo and it should always point exactly to the object. So to take care of these two things, we can utilise some help.

In order to understand how you have to move around the object with the light, you can imagine a clock that shows 12 hours around the object. Imagine you take at least four images at each full hour. This would correspond to 48 images for a dataset and a 360° coverage. But at what angle do you take the photos at each hour? Well the lowest one should not be lower than 15° and the highest one not higher than 65°. So usually you start pretty low, do two intermediate angles and end high at 65°.

But how to point the light directly at the object? Well as written above, when you are experienced, it works quite well out of the hand. But if you are new, you should utilise some help. If you attach a wooden rod onto your flashlight, so that it points into the direction of the light, you can also attach a string (around 1 meter long) to that rod. Get a friend to help you. So while you are holding the light and pointing it to the object, your friend will hold the other end of the string (that is not attached to the flashlight) and hover it close over the object. The string helps you to see where to point the light. Also, the string should align with the wooden rod on your flashlight, so you make sure, the direction is absolutely correct. Before taking the photo though, your friend has to move away with the string, so that neither your friend nor the string cast a shadow on the photo. Have a look at the image at the very top, where one of my students is doing this by himself. Problem is: he hasn't got a third hand to finally push the trigger on the remote control.

The procedure

So how does taking the photos actually work. I already explained how to set up and what equipment you need. I also wrote, that you best imagine a clock around the object and you position yourself at the 12 o'clock position. Before starting taking the real photos however, we need to do some test shots. First thing is to make sure, that the camera is absolutely straight. You then take the object into focus. RTI images are made in Manual mode, meaning no Auto-Focus allowed. You can however use the Auto-Focus to to adjust and then switch to Manual mode afterwards. Whatever settings you take, you are not allowed to change them during a dataset. This is way we take some test photos. Hold the flashlight very low (around 15°) and take a photo. Hold the flashlight very high (around 65°) and take another one. Have a look at the pictures. The images should not be too bright, but also not too dark. Especially the low angle will turn out too dark and the high angle to bright. You have to adapt your camera settings (F-Stop, ISO) as well as your flashlight settings (Intensity) until you get two really good photos, where you have the object in focus, see the scale, colour chart, and most importantly the sphere(s). Both photos need to be neither too dark nor too bright. If you found your settings, you can start taking photos.

I usually start by taking one photo of my hand, so I always know where a dataset begins. You position yourself on the 12 o'clock position, start low and work your way up. Always make sure you point the flashlight directly to the center of the object and then hit the trigger button. Sometimes you need to take care, that no shadows fall onto the object (like from a tripod). When you have taken at least four photos, you continue to the next hour and restart. Don't worry about being too exact with your positions, it is more important to correctly point the light to the object and to keep always the same distance. When you have completed the whole 360°, your dataset is done. You can photograph your hand with thumbs up (or similar) to indicate the end of the dataset. You can now copy the photos on a computer and continue with post-processing the photos.