The Doctor, Android Colonel Faraday and others in “The Android Invasion”

It's a robot detector. It lights up in the presence of androids.

...Thing must be faulty!

I don't think so. I see I was just too late.

[Suddenly entering] A pity you had to find out. We didn't want any shooting until our takeover was complete.

Hello, Doctor! We've been waiting for you.

Stand back, Doctor!

You know, the resemblance is astonishing! For a moment, I thought I was seeing double!