Welcome to How’d They Do That? — a bi-monthly column that unpacks moments of movie magic and celebrates the technical wizards who pulled them off.

In an interview with Variety, director Ridley Scott addresses the old critique that his early films were too visual: “They said [my style] was too beautiful, too image-driven. And I thought, ‘What the fuck does that mean?’ Just because I could shoot better than most people, which is what made me such an employable commercial director, didn’t mean I wasn’t interested in story. I still feel that way. I’m not making a radio play, I’m making a movie.”

Blade Runner provides ample proof of Scott’s passion for visual storytelling. From the film’s neon-filtered debt to film noir to the Tyrell Corporation’s Myan Revival-inspired ziggurats, style and substance work together to tell a great story. And when we talk about Blade Runner’s visuals, we have to talk about eyes. More specifically, about the eerily luminous pupils of the film’s bioengineered androids, the replicants.

Unless you’re an expert (or have a Voight-Kampff test handy), figuring out if someone is or is not a replicant is no easy task. The “Nexus” series of replicants we see in Blade Runner are indistinguishable from regular human beings: they look like us, they act like us, they have our memories, and they have feelings. They also have reflective eyes: shining, golden pupils that glow when the light hits them just right.

Why do Deckard and other Blade Runners need a psychological test to bust replicants when the eyes give it away? Well, according to Paul M. Sammon, author of the Blade Runner making-of bible Future Noir, Scott maintained that the effect is non-diegetic; “one more bit of detailing, if you like,” for the narrative benefit of the audience.

This wrinkle reminded me instantly of John Carpenter’s The Thing, which features a similar, albeit reversed “shiny eye” trick to distinguish humans from non-humans. Like Blade Runner (which, incidentally came out the same year as The Thing), the way to tell if a character is a human or an alien copycat is by looking at their eyes. At least, while they still pass as a human, that is.

Here’s how the director of photography Dean Cundey put it in an interview with Blumhouse (via CinemaBlend), referring specifically to the blood-test scene: “We were looking for some kind of a subtle way to say which one of these [men] might be human. You’ll notice there’s always an eye light, we call it, a little gleam in the eye of the actor. It gives life.”

Human beings have a glint in their eye. The goopy, interdimensional body-snatcher does not. All to say: The Thing’s matte black pupils and the high beams of Blade Runner’s replicants are evocative examples of how powerful and communicative the human eye really is, and how this optical uncanny valley (of too much light, or too little) can be leveraged to bolster storytelling.

How’d they do that?

Long story short:

By reflecting light through a special mirror into the actors’ eyes.

Long story long:

As Ridley Scott puts it (as cited in “The Dystopian World of Blade Runner: An Ecofeminist Perspective”): the human eye is like “a two-way mirror; the eye doesn’t only see a lot, the eye gives away a lot.” Speaking of two-way mirrors, let’s dig into how Scott and his team pulled off this effect.

According to a wildly fascinating article in the July 1982 issue of American Cinematographer, the glowing quality of the replicants’ eyes was achieved live, on set, during principal photography, using a relatively simple technique. The filmmakers used a two-way mirror⁠ — 50 percent transmission, 50 percent reflection –and placed it in front of the camera’s lens at a 45-degree angle. This is better known as the ‘Schüfftan Process,’ a technique invented by Fritz Lang for Metropolis, which happens to be one of Blade Runner’s visual touchstones.

Next, the filmmakers shined a light into the mirror in the same path of the lens so that it would reflect into the eyes of the subject. The lighting device, also known as a pup, was very small and included a dimmer which could adjust the intensity of the light when necessary. The result being: light is reflected from the mirror into the subject’s eye while not being photographed directly in the camera.

The American Cinematographer article pulls from an interview with Blade Runner cinematographer Jordan Cronenweth, who describes how sometimes the crew would use very subtle gels to add color to the eyes. This coloring can be seen most clearly with the replicant owl in Tyrell’s office. “Often, we’d photograph a scene with and without this effect,” Cornenweth recalled. “So Ridley could have the option of when he’d use it.”

What’s the precedent?

While we’re talking about glowing robot eyes, I’d be remiss not to mention 1973’s Westworld. In the movie, the go-to trick for telling if someone is a robot is to look at their hands, which have imperfections that give them away. Yul Brynner’s murderous Gunslinger is a different story: his enhanced scanning implants give his eyes an unsettling silvery sheen. They also make him look like a creepy robot badass.

In an edited version of Michael Crichton’s “Shooting Westworld” essay. we learn that this trick was pulled off with mirrored contact lenses. The filmmaker explains: “Three problems were especially tricky. One was the robot eyes. I wanted eyes that looked only slightly unreal, not strikingly bizarre. After some experimentation, we settled on 80 percent reflectant mirrored contact lenses, which gave us flexibility to control the ‘kick’ by lighting. They also had the virtue of permitting the actors to see through them.”

Reportedly, one day a piece of wadding from a blank cartridge struck Brynner in the eye. This rendered him unable to wear his contacts, which 1) must have been very painful, and 2) delayed filming. Oh, and in case you’re wondering, the other two problems Crichton is referring to were the Gunslinger’s pixelated POV and the acid face-melt scene. More on those some other time, perhaps.

To wrap things up, let’s talk about 2001: A Space Odyssey. Our interests lie not with HAL 9000 (whose interface is, to be fair, just one big glowing robot eye), but with that leopard in the Dawn of Man scenes.

In this introductory sequence, we see a cunning leopard guarding a zebra carcass and terrorizing the vegetarian apes. In both of the scenes that the big cat appears in, its eyes have a fluorescent orange glow. This stunning effect was actually a happy accident of the front-projection system. Front-projection was being used for the desert backdrops because, among other things, rear-projection would have been too blurry for Super Panavision.

Put simply: front-projection combines foreground action with background footage through the use of carefully aligned projection (duh) and (you guessed it) an angled mirror. For 2001: A Space Odyssey, glass plates from on-location shots from Africa were made into 8 x 10-inch Ektachrome transparencies. Then, using a specially outfitted projector, these were projected from the front onto a highly reflective 40 x 90-foot screen. The ins and outs of 2001: A Space Odyssey’s front projection are fascinating, and perhaps we shall pour over them in more detail at a later date. But for now: what’s important is that light from the projector was being directed towards the subjects in the foreground.

This explains why the leopard’s eyes are glowing. The tissue in the eye of many vertebrates, but especially nocturnal animals like leopards, reflects visible light back through the retina. Differently put, Stanley Kubrick says it best: “I can only conjecture that the cat’s eyes must contain some substance having a reflectivity similar to that of the 3M material used on our screen, because the eyes picked up the front-projected light and reflected it almost as brightly as the screen itself.”

Let’s tie the room together, shall we? Blade Runner line producer Ivor Powell and special effects supervisor Douglas Trumbull were also crew members on 2001: A Space Odyssey. Regarding the eye effects, Powell has commented that the similarities are such that “it is feasible [he] suggested this idea to Ridley…unfortunately, [he] just can’t remember.” In a video for Wired, Scott confirms that Blade Runner’s eyes do indeed owe a debt to that 2001 leopard, and he credits Blade Runner’s variations on the technique to Trumbull.

So there you have it. The eyes: windows to the soul, but also biological lenses capable of reflecting light and outing you as a robot.