Being detected because of reflections has resulted in the loss of personnel, the failure of missions, and has even changed the course of battles.
The Japanese Army's second attack on Henderson Air Force base in WWII's Battle of Guadalcanal was turned because a U.S. Marine patrol saw reflections from a Japanese officer's binoculars on a hilltop to the south of the base, in what had been thought to be unoccupied, impenetrable jungle. Because of that sighting, the U.S. commander was able shift forces to his previously unprotected flank and defeat the enemy attack.
Today, no military force would think of going into the field without camouflaging their troops and equipment. Modern camouflage has made detection of equipment or the human form far more difficult. We have, however, become an optically-driven military.
Moshe Dayan, the Israeli general, got his famous eye patch when a sniper saw reflections of the sun from his binoculars.
Modern optical systems produce the highest quality images, give protection from an enemy's offensive lasers and allow troops to see into the thermal range. Unfortunately these same optics also have the ability to betray their own position by reflected light. Reflections from optical systems or vehicle lighting can compromise operational security, and OPSEC is about letting you - and not the enemy - determine the time and place of engagement.
In the Battle of Stalingrad, the top Soviet sniper, Vasili Zaitsev, won his famous three day duel with the top Nazi sniper, Major Koning, by looking for and targeting the reflection from the German's scope.
Anti Reflection Devices by Tenebraex Corporation provide a low-cost, retrofitable, fit-and-forget solution to glint and glare from optical surfaces, without affecting resolution. ARDs use a unique geometry of a high-tech honeycomb material with special anti-reflection coatings. Glint and glare are eliminated, and thus the operational security of a force maintained, without depending upon individual action.
Reflections from the Confederates' equipment alerted the Union's General Warren to their position below him on Little Round Top. The reinforcements he sent for were just able to turn the tide in that pivotal battle:
On July 2, 1863, the left end of the Union line was bounded by the hills Little and Big Round Top. The Confederates decided to seize those heights; from there, they would be able to attack the flanks and rear of the Union lines.
As the Confederates prepared to attack, General Gouverneur Warren arrived on the top of Little Round Top, which was occupied by a signal station. The signalmen reported seeing some movement in the trees below. General Warren sent a request for Captain Smith's 4th New York Battery to fire a shell into the area.
As Warren later testified before the Committee on the Conduct of the War:
He did so, and as the shot went whistling through the air the sound of it reached the enemy's troops and caused every one to look in the direction of it. This motion revealed to me the glistening of gun-barrels and bayonets of the enemy's line of battle, already formed and far outflanking the position of any of our troops; so that the line of his advance from his right to Little Round Top was unopposed. I have been particular in telling this, as the discovery was intensely thrilling to my feelings, and almost appalling. I immediately sent a hastily written dispatch to General Meade to send a division at least to me...
Battles and Leaders of the Civil War, Johnson and Buel editors, Thomas Yoseloff, Inc. 1956
Those reinforcements, led in part by Colonel Joshua Lawrence Chamberlain's 20th Maine, barely turned the Confederate attack and thus preserved the Union's possession of the hill. Chamberlain received the Congressional Medal of Honor for his actions.
Glint gave away the extent of the Confederates' line of battle. If it had not been seen, Union reinforcements would not have been sent; the Confederates would have taken the hill and turned the corner of the Federal's line. If that had happened, it can be argued that they would have rolled up the Union position on Cemetery Ridge and gone on to win the battle of Gettysburg.
For binoculars, rifle and other weapon sights in the visible and thermal ranges, laser-protective goggles, vision blocks, headlights and windshields.