Where Science Meets Cinematography: Shooting Infrared with the ARRI Alexa XT B+W
A few months ago, I partook in an exciting opportunity extended to me by the kind folks at ARRI Rental New Jersey: spending a day at their rental facility shooting with the ARRI Alexa XT B+W camera, ARRI’s monochromatic camera capable of shooting in infrared, which is available exclusively from ARRI Rental. Not satisfied with simply shooting some dry camera tests, I brought my friend Zach Terry on board as a director, and, along with a small crew, we proceeded to shoot two short videos with the goal of exercising the camera in real-world situations and putting its features to the test. In this article, I will explain a little bit more about the science behind the camera, break down some of our technical choices, and discuss some of the more startling results, which you can see in the videos below.
So How Does It Work?
The Alexa XT B+W camera is like any other camera in the Alexa line in terms of its design, ergonomics, and overall build, with a few minor modifications: the Bayer mask, optical low-pass filter (OLPF) and IR block filters have all been removed from the front of the sensor. By removing the Bayer mask, the camera does not see nor record any type of color information, instead creating a monochromatic grayscale image. As a result of removing the Bayer filter, each pixel on the sensor array records the entire spectrum of visible light (as opposed to just the green, blue or red portions of the spectrum), which significantly increases the resolution of the image, extends the dynamic range of the camera to over 15 stops, and increases the native sensitivity of the sensor to 2000 ISO.
The final piece of the puzzle is the IR block filter that is present in all digital cameras and is used to block out infrared light from hitting the sensor. In standard Monochrome mode on the Alexa XT B+W, this filter is still in place, limiting the camera’s sensitivity to the visible spectrum only and resulting in a “standard” (albeit much higher quality, sharper and more sensitive) black and white image -- the interior shots in the second video were filmed in this mode. However, what makes this camera truly unique is that this filter can be removed and replaced with a filter that blocks out all visible light, allowing only infrared light to penetrate through the filter and hit the sensor. This is the “Infrared” mode of the camera, which causes some of the more interesting visual anomalies that can be observed in the videos.
I should note that this camera does not see the entire infrared spectrum, but rather a very narrow portion of it that is just outside of the visible light spectrum referred to as “Near Infrared”. Specifically, the camera is sensitive to wavelengths that are between 800 nanometers and 1,100 nanometers. To boil it down to basics, in infrared mode the camera will only see objects that reflect or emit infrared light. And our first major demonstration of this principle came when we turned the camera on for the first time on one of the rental benches inside the ARRI Rental facility
At first, we were confused as to why we were not seeing anything on the camera -- the monitor looked completely dark. Squinting, we could barely make out the contours of some of the other rental benches and features of the room. We quickly realized that this was simply infrared mode in action. The industrial fluorescent lighting that ARRI uses to illuminate its rental floor emits a very limited spectrum of light between 550 and 650 nanometers -- and no infrared light whatsoever. And for the camera in infrared mode, no infrared light is essentially the same as shooting in the dark with no light at all, which is why we couldn’t see anything on the monitors even though the room appeared very brightly lit to the naked eye. After putting together our equipment package, we took our camera outside into the infrared-soaked sunlight to really start putting things to the test.
What You Don't See is What You Get
The most immediately noticeable characteristic of the infrared footage is that it tosses all preconceived notions of color and tonality out the window. Objects that reflect a lot of infrared light, plants and certain synthetic fabrics in particular, such as cotton, show up on camera as hot white. As you can see from the photos below, our lead actor from the first video was wearing a dark-colored jacket, which in the video shows up as white. Leaves, grass and the roses from the first video all appear as a very light shade of white as well. Interestingly enough, other materials seem to be significantly less infrared-reflective, such as the actor’s denim pants, which show up as black in the infrared footage as well as in the reference photo.
Another interesting observation is that pigments and coloration don’t show up in camera at all. The apparent tonality of an object depends entirely on how much infrared light it reflects: for example, the roses from the first video are of all different colors, but because they all reflect an equal amount of infrared light, the roses all appear to be the same shade of bright white in camera. The same goes for the dress worn by the actress: The flower patterns printed onto the fabric of her dress do not register on camera at all.
Eye pigmentation does not seem to register either. Though our actors in both videos had a variety of eye colors ranging from dark brown to light blue, in infrared their eyes all show up as the same uniform shade of deep grey, creating a very creepy “vampire eyes” effect. Hair undergoes a similar process, as demonstrated by the side-by-side comparison images of our 2nd AC, which you can see below. Human skin is also very infrared-reflective, and again, because discoloration and pigmentation don’t show up, various discolorations such as moles, freckles and more don’t show up in infrared at all, creating a sort of hyper-smooth, otherworldly texture to human skin.
Lighting and Exposure
For our exterior shots we predominantly relied on the natural sunlight, and utilized a single M18 HMI for a backlight or kicker. Though the HMI did not appear quite as bright in infrared as it looked to the naked eye, there was still enough infrared output in the HMI spectrum for it to register on camera. ND filters are incompatible with infrared filming, since the filters are designed to cut down visible light only, and do not affect infrared light at all -- as a result, the camera can see right through the ND. When held up in front of the camera, an ND filter appears to be just a clear pane of glass. The same effect can be observed with sunglasses, which in front of the infrared camera appear as empty frames. As a result, we had to rely mostly on ISO adjustment to control our exposure levels, and ended up shooting most of our exterior shots between ISO 160 and ISO 320. We also shot everything at 120fps, which provided us with a couple more stops with which we could get proper exposure without having to compromise depth of field.
Since newer cinema lenses are designed with digital cameras in mind and often include infrared-reducing coatings, we decided to rely on vintage glass for these two videos that we know had much fewer coatings and would allow the maximum amount of infrared light to enter the lens and hit the sensor. We ended up shooting with Zeiss Super Speeds on the first video and Super Baltars on the second.
One funny side effect of shooting infrared was also one that we did not anticipate at all: lenses are calibrated for the visible spectrum only and are not designed to account for infrared light, and the huge differences in the wavelength and nature of infrared light meant that the distance markings on the lenses were completely off while shooting in infrared mode. My 1st AC had to rely entirely on the monitor and pulled focus by eye, since the marks on the lenses were often off by a matter of a few feet, sometimes more. Additionally, we employed a full white Pro Mist filter in front of the lens for the flashback sequences in the first video, in order to provide an ethereal, otherworldly look to the footage.
Like all Alexa XT models, the camera shoots ArriRaw directly to on-board Codex XR mags, which we then offloaded through a Codex Vault system and transcoded using a custom Log-C gamma space into ProRes 4444 2.8K files. (Unlike other Alexa XT models, though, the Alexa XT B+W does not have internal ProRes or DNxHD recording options; it can only record to ArriRaw.) All post-production was done entirely in ProRes. Unfortunately we simply did not have the resources to work with ArriRaw on this project.
The footage was graded in DaVinci Resolve by our wonderful colorist Erik Choquette, who really only had to adjust contrast levels since the images were monochromatic. A significant amount of correction had to be done sometimes even within an individual shot, as we were shooting under spotty cloud coverage. Because of the sensitivity of the chip and the sheer volume of infrared light, the presence of direct sun versus cloud coverage resulted in pretty significant exposure shifts, far more extreme than if we were shooting normally.
All in all, it was a thrilling experience to put this incredible technology to the test, and every new visual curiosity sparked a little science lesson along the way as well -- trying to discern why the camera was reacting a certain way to some objects and another way to others, though there are still many textures and objects that we simply did not have the time to test on that day that I would love to shoot with this camera in the future. Overall, I think that infrared shooting provides a great opportunity to create some truly unique imagery, and while the technical and logistical limitations of the camera mean that you can’t shoot everything on it, I can certainly see it providing a very interesting and unique look to more stylized content such as commercials, music videos, or even a particularly pretentious dream sequence in a narrative film.
If you have any questions about the camera, the infrared technology, or anything that I neglected to cover, please don’t hesitate to chime in in the comments section or to reach out to me directly! My e-mail address can be found on my NFS profile page.