In Blade Runner, they show many time that Replicants eyes glows in unusual way:
Was it just for audience to indicate they are a Replicant or it's part of their cyborgish structure?
Because how can Rachael have glowing eyes and never realize herself being replicate before or why it can't be part of the test rather than the questionnaire?
Stealing @Möoz answer
It's a fourth-wall reference for the audience.
Given that we only ever see the 'glowing eyes' effect when the in-universe characters have not acknowledged it, it tells us that this must be a feature that is not apparent to them.
Additionally, according to Paul Sammon:
Ridley Scott maintains that this effect "was strictly a stylistic device, one more bit of detailing, if you like. If the replicant's eyes really did glow like that within the context of the story, then why would you need a VoightKampff machine to sniff them out?"
-Sammon, Paul M. (2000). "VIII: The Crew". Future Noir: THE MAKING OF Blade Runner.
And of the importance of eyes within the Blade Runner movie is emphasised by Mary Jenkins when she says:
[Ridley] Scott said of the replicants' sometimes glowing eyes: "that kickback you saw from the replicants' retinas was a bit of a design flaw. I was also trying to say that the eye is really the most important organ in the human body. It's like a two-way mirror; the eye doesn't only see a lot, the eye gives away a lot. A glowing human retina seemed one way of stating that".
-Jenkins, Mary. (1997) The Dystopian World of Blade Runner: An Ecofeminist Perspective, Cite note 6
