Related Clues:

• Hole in the head?
• Vent, e.g.
• ---
• Camera shutter
• Open space
• Optical opening
• Light setting
• Light limiter
• For CIE, I look for an opening
• A bodily opening.
• Opening, hole
• A bodily opening
• Opening, mostly small
• An opening that admits light
• An opening, mouth of a cavity