It controls the amount of light seen by the camera, So as not to flood out the image or cause it to be to dark.
some of them are sensitive to both visible light and infrared.
After the fuel ignition there would have been a very large spike in infrared energy that may have caused the iris to close down, darkening the image in visible light.
To see this effect, point your TV remote at a video camera and push a button while watching the image the camera sees, you will see the IR sender in the remote as a white light, But the camera doesn't know the difference.