A vision unit which can be easily assembled into the opening in a door such as a fire door which is constructed so that it can withstand high temperatures without disintegration. The vision unit is readily assembled into an opening in the door by one person working alone. The vision unit is composed basically of two frame members, two lower support elements and a biasing element which extends between the wall surface at the top of the opening and one of the support arms of the frame. The support arms include sharp edge portions for contacting a transparent panel member. Securing means such as threaded nuts and bolts or screws pass through the door opening and fasten the frame members to the door as well as to the panel member.