Seeing the Invisible: a European Researchers’ Night Lecture

European Researchers’ Night is probably the largest popular science event in Europe, which takes place every year on the last Friday in September. Presentations of scientific discoveries and attractions, exhibitions, public experiments and lectures are available to scientific enthusiasts throughout the day, especially in the evening and at night.

KAP Jasa – kite team Slovenia is a regular guest of the Researchers’ Night; twice we presented our kite-related discoveries in Banja Luka, and last year we gave a presentation on Assessing water pollution with a kite at the Šiška Library. And as we are an associate partner of the project, we participated in this unique scientific festival again on Friday, September 30, 2022.

We were invited to Sostro Primary School to show the pupils of the 8th and 9th grades how we can do real archaeological research with a kite, and this is what we told and shown them.

KAP Jasa in lecturer’s enthusiasm



The main theme of his year’s Researchers’ Night was the soil. Earth. For scientific enthusiasts, the soil is not really the most interesting and attractive topic – especially compared to atomic physics, space exploration and other glorious, “hard” sciences. But the earth also hides a lot of interesting things, and if we listen to it carefully, it can tell us of a lot of cool stories.

So this story is told by the soil, and we hear it through science: biology and biophysics, aerodynamics, infrared remote sensing, engineering… and archaeology.

Let’s start with aerodynamics – specifically, with kites. KAP Jasa is the oldest, the largest and, of course, the only society in Slovenia, that designs and makes kites, flies kites, does kite aerial photography and dabbles in kite science. Our kites operate on the same principle as the kites every kid has flown – the lifting body, the wind, the string – except that their forms are bolder, and they are much much larger. We fly these aerial sculptures with our kite friends on kite festivals all aroundthe world.

We entered the kite world because we wanted to take a more interesting, more ecological and, above all, cheaper way to photograph the world from above. Most aerial photographers do it with drones, but a kite has a lot of advantages over those droning nuisances. A good kite in good winds can easily lift a few kilograms of equipment; a kite flies as long the wind blows (for hours and hours); a kite flies quietly and gracefully – and above all, the regulations and restrictions for drones mostly don’t apply:you can fly with a kite practically anywhere.

To take aerial pictures from a kite, one needs a specially designed kite that has enough pull to lift the camera, and which is stable enough to keep the photos sharp. There are different types of kites for different winds and different photo targets – kites for weak winds and for stronger gusts, kites that fly far and those that fly high.

The camera cannot be attached directly to the kite, because the extra weight would threaten its stability and spoil its aerodynamic properties. So the kite is first released into the air about 30-50 meters above the turbulent layer of air near the ground, so it can to settle down and position itself in the wind – and then the camera is attached to the kite line by a special apparatus called a picavet.

Picavet – a pulley system that carries the camera

This system of pulleys ensures that the platform remains horizontal no matter how the wind changes and how the kite dances, so the camera remains focused on what we want to record. Then the camera is set up to take a photo every 20 to 30 seconds, and flown into the sky as high as the kite line allows.

Such kite aerial photos can be really beautiful, but beautiful pictures are just beautiful pictures, while we’ve always wanted to take interesting pictures. The view from above is perfect to discover something new, something surprising, something hidden.

The first such discovery of something unexpected happened during a kite aerial photography session above castle Kalc near Pivka, in the upper Pivka basin. The ruins are nothing special, the restored tower is nice – the surprise was hidden elsewhere.

These parallel grooves on the meadow – barely visible from the ground! – are the remains of an (early)medieval field division: we are looking at over five centuries old cultural landscape! The farmers cleared the karst meadow and set the stones into long grooves, thus creating very narrow and very long bands of arable land, one or two per family.

Such an ancient and so well preserved cultural landscape is a considerable rarity. The landscape changes fast with the development of farming techniques, but here we are fortunate that the natural and climatic conditions in the upper Pivka valley are not ideal: the fields have been abandoned and turned into pastures and meadows, and the boundaries between the plots remain visible to a kite with a camera.

And that’s how we got enchanted by history and archaeology … What other hidden wonders await us and our kites?

Aerial photography – including kite aerial photography – is one of the most powerful tools available to an archaeological amateur: a view from above can reveal what cannot be seen from the ground. And a flying camera is even more powerful if it doesn’t use just the visible light, but other parts of the spectrum, especially the infrared.

Researchers’ night is a scientific event, so we need to go deeper into science to explain why infrared light is so useful for archaeological research. And we’re going to start by asking why the plants are green.

This is basic knowledge: plants have chloroplasts in their cells, the chloroplasts contain chlorophyll, and chlorophyll is green. But this answer doesn’t tell us much (why is chlorophyll green then?), and we have to dig deeper. As a science lecture has to have at least one scientific graph in it, let’s take a look at this one:

We know that chlorophyll – both chlorophylls, a and b, to be precise – absorbs energy from sunlight so that the plant can make sugar out of air and water. This graph of the intensity of the absorption of light by the wavelength tells us that chlorophylls are quite picky and do not absorb just every photon that falls on the leaf of the plant. Chlorophylls like ultraviolet and purple light, and orange and red, while the light in between is reflected. And if we take purple, blue, orange and red out of the spectrum, the reflected light is … green.

For budding archaeologists, however, the interesting part of the graph is on the right, beyond the red colour. As we can see, chlorophylls don’t care about infrared light: they mostly reflect it completely. For a plant this is vital: if infrared light would be absorbed, the plant would overheat, consume even more water and die off faster in times of drought. But this fact is also important for archaeologists – especially since the ability of the plant to reflect infrared light depends on the plant’s health.

The healthier the plant, the better it reflects the infrared part of the spectrum. A withering plant reflects less, and a dying plant even less. This means that if we take a photo of a field or a meadow from the air with an infrared camera (and process the photo to highlight tiny differences in the plants’ infrared light reflectance), we actually create a map of the quality of vegetation in that field!

The quality of vegetation – plant health – depends mainly on the quality of the main character of this year’s Researchers’ Night: the soil. A map like this one above – a processed infrared photo – tells us where the soil is fertile and where it isn’t so much. A farmer can identify the less fertile parts of the field and fertilise only those parts instead of wasting the fertiliser on the whole field.

A map of soil quality is very very interesting for the archaeologist too. The quality of the soil depends on many factors, humans included. What’s more, some human actions that affected the soil centuries or even millennia ago can still be impacting the soil quality today – and, consequently, the health of the plants that grow on it.

Now imagine a stereotypical Medieval castle.

A castle with thick walls and tall towers, a defensive moat and a vicious knight living in the castle that torments and exploits “his” peasants. One day the peasants have enough, they invade the castle, they burn it down and destroy it completely, they bury the trench, they take the stones and the bricks away. The ruins of the castle are soon overgrown by grass and bushes, then covered by a layer of earth… and in a century or so there is no trace of the once mighty castle.

But only on the surface. Its remains are still there, hidden in the soil, and they still affect the plants that grow there.

Where there is a buried wall, the soil is shallow, the water drains faster and the plants there don’t really thrive there. Where there is a buried trench or a moat, the layer of soil is deeper, the water is retained longer and the plants grow better than their neighbours. And if we now photograph this area from the air with an infrared camera, we can make these subtle differences visible – and they show the floor plan of a castle that is long gone.

The photos below show the same lawn. The left one is taken in visible light, and there is no clue about what is hiding below. The photo on the right is in infrared and is processed to show tiny differences in plant health and soil quality – and the hidden foundations of a buried structure are spectacularly revealed.

So, in order to do some real aerial archaeological research, we need two things: an infrared camera and a way to lift it into the air to capture an area that is hiding something interesting. We have a way to lift – a kite – all we need is an IR camera.

A good infrared camera is actually quite affordable – every digital camera can be converted into a near-infrared one; all it takes is to remove a special IR block filter just in front of the sensor. With the blocking filter removed, and with a NIR 720 mm filter attached to the camera lens a cheap compact digital camera becomes an excellent infrared camera!

Pentax WG-10, converted into an infrared camera – a gift from dr. John Wells

For our first archaeological target we chose a field behind the church of St. Andrew in Kašelj. There are several reasons for archaeologists to suspect that there’s something old out there somewhere. Some time ago ago, a couple of Roman headstones and some spolia were found, which are now built into the exterior wall of the church. Random finds of shards of Roman pottery and roof tiles suggested that in Roman times a larger building, a farm, or even a small settlement must have stood here somewhere – along an important Roman road that led from Aemona to Zalog and down along the Sava river to Neviodunum and further east..

On one nice windy day we went there, flew the kite, put our modified Pentax on the kite line and lifted it some 150 meters into the air, and then Viktor walked up and down the the field so the camera would record the whole area.

In the end we had about two thousand photos like these two:

We had to stitch all the photos into a panorama of the whole area, and then we processed it to highlight the tiny differences in the intensity of the reflection of infrared light – which is due to the quality of vegetation – which is due to the quality of the soil and to things that might be hidden in it.

And look – this structure showed up:

Straight lines and right angles are scarcely a work of nature. If they show up like this, we can be pretty confident – this is something humans did. But what could this hidden structure (quite large, 30-40 meters long and over ten meters wide) be?

Turns out we most probably discovered the foundations of a Roman villa rustica, a large farm, a nearly 2.000-year-old homestead that stood here from the late 1st century AD to the late 4th, maybe early 5th century.

If we take a closer look at the foundations, we see the main building where they lived, cooked, ate and slept. The western (left) annexe was most likely a storage – for farming tools and produce.

What did the farmers grow here in the times of the Roman Empire? Pretty much the same stuff the grow there today: cabbage and beets, carrots and lettuce, wheat and oats. And they were selling their produce on the market in Aemona – just like the farmers of Kašelj sell their produce on the markets of Ljubljana almost two millennia later.

The rectangular thing on the southern side of the villa could be a stall, maybe a pigsty, but most probably a stable for horses. This villa rustica was close to a large cargo port on Sava river near Zalog. The barges were pulled upstream with horses – a Roman towpath carved into the rock can still be seen between Litija and Sevnica.

River Sava was an important strategic cargo route connecting the heart of the Empire with the eastern provinces and lands beyond. Barges were bringing all kinds of treasures of the east to the port at Zalog: gold and silver, Persian carpets, pearls of Sri Lanka, silk from China, exotic spices … As Ljubljanica wasn’t – and still isn’t – navigable between Zalog and Aemona / Ljubljana, the cargo was loaded on wagons and taken to Aemona, to Aquileia and on to Rome.

The Roman Empire crumbled and so did our villa rustica. Attila and the Huns destroyed Aemona in 452, the villa was abandoned, destroyed, and forgotten – and it would stay forgotten if its remains wouldn’t influence the quality of the soil, the health of the plants growing there, and their near-infrared reflectance.

Soil that seems so boring and dirty can tell us some really interesting stories – we just need to listen to it. It speaks to us via plants that reflect stuff hidden in it. If we take a kite and an infrared camera, we ca hear a story about Roman colones growing produce two millennia ago; a story about horses pulling barges and wagons full of treasures of the East; a story about life that thrived and disappeared – and returned again pretty much the same as it was back when the Roman Empire ruled these lands.

Soil is cool.

Big thanks to House of Experiments, dr. Miha Kos and Sabini Založnik Vidic, to Sostro primary school, Nini Zupanc, and the pupils of 8. in 9. class, to dr. John Wells for the infrared camera – and of course to Viktor who discovered the villa rustica in Kašelj with a kite!

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