In Blade Runner, the glowing effect in the pupils of replicants' eyes was a deliberate choice made by director Ridley Scott and cinematographer Jordan Cronenweth to create a sense of artificiality and to help the audience distinguish between humans and replicants.

To achieve this effect, Cronenweth would shine a light along the optical axis of the camera, creating a kickback in the replicants' retinas, which would then appear to glow on screen. According to Scott, this effect was a design flaw, but he embraced it as a stylistic device to emphasize the importance of the eye as an organ and its role in revealing a character's true nature.

The glowing eye effect was non-diegetic, meaning it was not visible to the characters in the film, but rather a tool for the audience to understand that they were looking at a replicant. This effect was almost always used after the characters had already assumed that the character in question was a replicant, reinforcing the idea that the glow was a visual cue for the audience.

Overall, the glowing eyes of replicants in Blade Runner served as a distinctive and memorable visual signature for the film, and helped to emphasize the themes of artificiality and humanity that run throughout the story.