From American Cinematographer, Cinematography for Blade Runner by Herb A. Lightman and Richard Patterson (July 1982)
"One of the identifying characteristics of replicants is a strange glowing quality in their eyes. To achieve this effect, we'd use a two-way mirror-- 50 percent transmission, 50 percent reflection-- placed in front of the lens at a 45 degree angle. Then we'd project a light into the mirror so that it would be reflected into the eyes of the subject along the optical axis of the lens. We'd sometimes use very subtle gels to add color to the eyes. Often, we'd photograph a scene with and without this effect, so Ridley would have the option of when he'd use it."
- Jordan Cronenweth, ASC