Thunderbird 9 with signal mirror
Maj. (Dr.) Chris Scheibler, Thunderbird 9, holds the signal mirror he used to mark show center for Thunderbird 1 during the USAF Thunderbirds' aerial performance at the "Thunder over the Empire" airshow at March Air Reserve Base in Riverside County, CA on April 17, 2016.
TB1 can't see the mirror itself, of course - it is 2"x3" - smaller than a business card. What TB1 sees is the concentrated ray of sunlight reflected by the mirror - 4,000,000+ candlepower of it - more than 20 times brighter than an air traffic control signal light gun.
Between airshow performances, Thunderbird 9 is the Thunderbirds' flight surgeon, responsible for monitoring the Thunderbird flight and support teams, and evaluating and briefing guest passengers. During the Thunderbirds' aerial performance, Thunderbird 9 is responsible for signaling Thunderbird 1 with the signal mirror to mark the "show center" point for the aerial maneuvers.
After the Thunderbirds took off, Thunderbird 9 walked out onto the runway, and stayed there for 30 minutes with his radio headset and signal mirror, signaling on almost every pass. Other photos in this album show him signaling to Thunderbird 1 with the signal mirror.
This signal mirror is a USAF issue 2"x3" glass MIL-M-18371E Mark 3 Type 1 emergency signaling mirror with a retroreflective mesh aimer. The MIL-M-18371E is standard USAF pilot survival gear, also used for ground-to-air communication. On a clear day, with the sun overhead, this mirror can reflect a 4,000,000+ candlepower beam to any point in the sky, a beam that is naked-eye visible at 20+ miles - see the video here: www.youtube.com/watch?v=sxOvip1gtB4
The beam is so bright because the power is concentrated in a very narrow beam - about 1/2 degree wide - the angular diameter of the sun. A retroreflective aimer such as this vastly improves the probability of hitting a distant target with the very narrow sunbeam reflected by the mirror. Since the sunbeam is only 1/2 degree wide, the difference between a dead center hit and a clean miss is only 1/4 degree of error, or 1.3 ft at 100 yards. The Mark 3 was first issued in 1949, and is issued to the current day. We teach Boy Scouts how to make similar retroreflective aimer signal mirrors for the "Operation On-Target" peak-peak signal mirror event.
When the user looks through the retroreflective mesh aimer, they see a virtual image of the sun produced by the retroreflective glass beads on the mesh, in the direction of the reflected column of light from the mirror. The virtual image of the sun appears as an extremely bright, round fuzzy dot, or "fireball" about 1.5 degrees in diameter, visible only through the aimer. To use it, Thunderbird 9 holds the mirror in one hand, and reflects the light on their other hand. Keeping the reflected light on their other hand, he looks through the aimer at the light on their other hand. In the center of that light, he sees a shadow cast by the aimer, and the "fireball" in the heart of that shadow. He then removes his other hand, and tilts the mirror until the very center of the dot crosses the target. These instructions are on the back of the mirror (bottom right), and I've posted an illustrated set of these instructions here
The MIL-M-18371E is currently made by S.I. Howard Glass, and is available to civilians through distributors. Similar commercial glass signal mirrors are made by Coghlans , and (formerly) by Vector 1 and Rescue Reflectors. Beward of imitations - glass signal mirrors with nonfunctional, non-retroreflective aiming grids that don't produce the targeting "fireball", described here.
I found an even nicer public domain photo of the back of Thunderbird 9's mirror posted by the Thunderbirds, and processed it to make the text on the mirror clearer (top right). I found a pristine one in my collection from the same purchase contract: DLA 400-81-C-0743 to Revere Glass Company (bottom right), to provide a much clearer view of the operating instructions and the high-angularity retroreflective beads on the aimer mesh.
I'm releasing the photo to the public domain under the CC0 copyright. If you use it, you may want to start with the full resolution photo, which is here.