Filming In Infrared and Thermal Spectrums
There can be some confusion over the differences between the different types of night vision: infrared and thermal imaging. This guide will explain the differences, advantages and uses of each.
The electromagnetic spectrum
First, take a look at this chart. It maps part of the electromagnetic spectrum, and should help visualise where the different spectrums are in relation to each other. Note how the units are in nanometres and micrometres (aka microns) as well as millimetres. This is to help illustrate how small the wavelengths are in a more common, comprehendible unit.
1 Micrometre = 0.001mm (one thousandth of a millimetre)
1 Nanometre = 0.001µm (one thousanth of a micrometre) or 0.000001mm (one millionth of a millimetre)
With the exception of visible light, all of the spectrums are coloured arbitrarily for illustration only – their true ‘appearance’ of course cannot be represented with what we know as colours. The spectrums with longer wavelengths than visible light have also been illustrated with their atmospheric transmission, meaning the gases in our atmosphere block them at different frequencies. You can see the distinct bands which can be conveniently grouped and labelled as different spectrums. Away from the visible spectrum, ultraviolet transmission rapidly drops off to zero and so hasn’t been illustrated in this way, but instead with a gradient to black.
To the left of ultraviolet are xrays and gamma rays. To the right of LWIR are microwaves and radiowaves. These of course we do not consider to be ‘light’, but are all made of photons and simply oscillate at a different frequencies (meaning the length of each wave varies). The shorter the wavelength, the higher the frequency, and the longer the wavelength, the lower the frequency.
What is "FULL SPECTRUM"?
Full spectrum, simply put, is the visible spectrum plus the ultraviolet and [near] infrared spectrums combined. This would therefore typically be from around 1000nm to below 300nm – as is illustrated on the expanded part of the chart above.
Cameras that are sensitive to the full spectrum can ‘see’ ultraviolet and infrared light in addition to visible light. This is all at once however, though filters can be used to limit what the camera sees to specific wavelengths. With the right filtration, a full spectrum camera can therefore selectively ‘see’ in ultraviolet or infrared, as well as retaining the ability to see in the visible spectrum too.
Starting with the shortest wavelengths: ULTRAVIOLET
Ultraviolet (or UV) is the spectrum of wavelengths shorter than those of visible light. They start at the end of the visible spectrum at around 400nm and extend down as far as 10nm. However, the atmosphere begins to block UV light from 315nm and is completely absorbed by 280nm.
While we humans can’t see in ultraviolet, there are some animals which can and so scientists are starting to discover the differing patterns which can be seen in UV light. For example, sunflowers look to us like they’re uniformly yellow, however when photographing in ultraviolet it’s clear to see that they actually have deep marks on their petals forming a ring around their stamens. Such markings are invisible to our eyes but scientists believe this is an extra way for the plant to attract pollinators, creating a bullseye effect to guide the insects into the flower.
UV photography can be difficult, as extra steps have to be taken to prevent all other forms of light from polluting the image. This also makes it extremely difficult to film in UV, as the short wavelength means longer exposure times are required than would typically be used when filming.
"Visible" light - how we see the world
Visible light is so named since it is the spectrum in which we can see the world. Most other animals see in this spectrum too, though there are some which can see in near-infrared or ultraviolet too.
We’re learning more and more that the world around us isn’t simply how we see it. We’ve had the technology for some time to produce images from gamma or x-rays and even radio waves, but until recently these tools were all pointed at space. As we look more at the life on our own planet, we’re discovering that our eyes are actually quite limited in the range of wavelengths that we can see and there’s literally a whole world out there which is entirely invisible to us.
NEAR INFRARED AND SWIR
Near infrared (NIR) is defined as wavelengths longer than those of visible light up to about 1000nm.
When the term infrared is used, it is usually referring to this spectrum. CCTV cameras, ‘night vision’ infrared cameras and IR lights such as those on the end of your TV remote are all between 700nm and 1000nm. IR lighting is commonly 850nm or 940nm. This means that the transmission peaks at these wavelengths; the actual output will be a little either side.
Think of near infrared in the context of ‘nearby’, as opposed to ‘nearly’; in other words, it is infrared that is adjacent to visible light – not light that is almost infrared. This is why you can often see a faint red glow from infrared lighting – this is where the limits of our eyesight and the emitted wavelengths overlap a little.
Short wave infrared (SWIR) is the spectrum from 3000nm (3µm) down to roughly 1000nm (1µm). Different definitions place the high frequency threshold for SWIR at 900nm, 1100nm or 1400nm.
It is similar to near infrared in many respects though is mainly used for scientific purposes and is not as prevalent in everyday technology as near infrared is.
When light becomes heat: thermal imaging
Thermal imaging is quite fascinating in that the camera is no longer recording light as we know it. Around 3µm (3000nm), the radiation emitted changes from light to heat, and so we must also alter how we attempt to record it.
‘Regular’ cameras can record in up to 12K (12,280 x 6,480) resolution by utilising small pixels. These cameras tend not to do so well in low light though, as smaller pixels mean that each individual pixel can gather less overall light than a sensor with larger pixels. This is why the Sony A7S and FX3 cameras are so good in low light while only having 12MP sensors – their pixels are the biggest size possible to gather the most light, while also producing a full frame 4K image.
For thermal imaging, the larger the pixel size (known as pixel pitch), the better. As primarily a military and scientific technology, bigger pixels mean that there’s a greater contrast between subjects which are warm versus those which are cool – plus subjects can be detected from further distances. This often means that thermal cameras have a much lower resolution than ‘regular’ cameras – the best the military currently has access to is just 720p!
Hire our industry leading thermal camera, the Pulsar Helion XP50, at the best price in the UK (from £360 per week).
MWIR AND LWIR
Medium wave and long wave infrared is what we know as thermal imaging.
NIR and SWIR ‘work’ in the same way that the visible spectrum does: objects reflect or absorb light. If there isn’t a source of light, then in these spectrums, nothing can be seen.
Medium and long wave infrared however, is not reflected light, but emitted thermal radiation. Everything with a temperature above absolute zero (0ºK/-273.15ºC/-459.67ºF) – in other words, everything in the universe – emits thermal radiation. This isn’t radiation in the nuclear or radioactive sense; it literally means to radiate heat. The higher the temperature, the more thermal radiation is emitted.
As MWIR and LWIR cameras are not dependent on any source of light, they can be used during both the day and night. The differences between the two are slight, with one or the other working marginally better in certain scenarios but are largely comparable and they do a similar job. MWIR is between 3-5µm and LWIR is 8-14µm.
How to film and photograph in these spectrums
Filming in thermal, near-infrared or ultraviolet generally requires specialist kit.
While all cameras inherently have the ability to ‘see’ ultraviolet and NIR, they require permanent modification in order for you to film and photograph in these spectrums. The reason for this is that camera manufacturers insert a special filter in front of the sensor which limits what the camera can record to just visible light. This filter is known as a ‘hot mirror’.
The hot mirror is there to prevent contamination from the infrared spectrum in particular; NIR is produced in abundant quantity from our sun and so without the hot mirror, all your photos would be tinted with varying shades of red. As removing the hot mirror is a permanent modification, most people (understandably) don’t wish to do this, as it invalidates manufacturer warranties. This is why full spectrum cameras are considered specialist kit.
However, to film or photograph in thermal spectrums, special sensors called ‘microbolometers’ are required, so you need to use a dedicated thermal imaging camera.
WHAT LENSES CAN BE USED?
Photography and cine lenses are of course designed for the visible spectrum. They’re made of glass and their coatings optimise transmission of visible light – sometimes they’re even designed to suppress infrared light.
That said, many regular lenses can be used for NIR, however some perform better than others – and not necessarily the ones you might expect. Some of the best lenses may not work very well in infrared, and vice versa. The most common problem is the introduction of hotspots – a bright area in the centre of the image. This will vary with the lens, zoom position or even focus distance. Generally they’re more evident at deeper (smaller) apertures and less apparent at wider stops. They’re also not necessarily consistent across the same set, series or era of lenses – so one good lens doesn’t mean all of its siblings will work well too. The best way to see what works is to test all of your lenses prior to a shoot and see what you deem as acceptable. That said, a number of lists already exist online that detail which lenses work better or worse, with this one from Kolari Vision being quite a good example.
Normal lenses could still be used for SWIR, however as this is further from the spectrum they were designed for, they tend to perform more poorly. Lenses designed for SWIR applications are generally used, though these are still made from glass.
For thermal imaging in MWIR and LWIR spectrums however, regular glass lenses will not work. Glass blocks the wavelengths of these spectrums, and so special lenses made of germanium must be used. These are expensive to manufacture and are one of the largest costs associated with thermal cameras. This means you can’t be seen through windows with a thermal camera – nor can you peek through any! Anyone wearing glasses will appear as if they’re wearing shades too.
IS INFRARED NIGHT VISION?
Assuming we’re only talking about the near-infrared and SWIR spectrums (and not MWIR or LWIR), then in a word, yes.
Infrared light is not visible to us or indeed most animals, so by using an infrared light and an infrared sensitive camera, then yes, we can use it to see in the dark.
Military night vision goggles, however, are not infrared. As an infrared light is necessary to illuminate the area, using goggles with infrared lights would be like using torches to like to anyone using infrared technology, defeating the purpose of concealment! Instead, military night vision uses light intensifier technology – which literally means it will brighten even the tiniest bit of light such as stars to visible levels. It therefore does require an existing source of light, no matter how small, but does not require active illumination.
IS THERMAL IMAGING THE SAME AS INFRARED?
Thermal imaging – in both the MWIR and LWIR spectrums – is indeed infrared, but not really within the common definition.
Infrared as we usually think of it, is black and white ‘night vision’. Both this ‘night vision’ and thermal imaging are infrared radiation – just that one forms as light and the other as heat.
So whilst it is thermal infrared, it is not night vision infrared.
HOW CAN I TRY INFRARED OR THERMAL IMAGING?
If you’d like to give infrared filming or photography a go – perhaps for night vision, or maybe just for some really cool, crazy colour effects, try our custom Full Spectrum Sony FX3.
If you want to find and film wildlife a little easier, (or maybe just want to see how hot you really are) we also have an LWIR thermal camera, the Pulsar Helion XP50.