Variable light brightness can be used to blend items with their background and disguise the otherwise obvious silhouette.

This effect of counter-illumination was experimented with as early as 1916 and later during wartime in 1943 for military purposes and became known as Yehudi lights. These lights were mounted on aircraft and also projected onto ship hulls that automatically adjusted light brightness to match the background behind it.

It is interesting to note that this phenomenon also exists in nature in some squids, fish, and shrimp that have bio-illuminecent features, thus diminishing their silhouette.

This causes one to ponder what similar applications could be employed that might help to conceal private property or location to avoid attention or detection. Unlike the use of images or patterns, Yehudi lights would require power for illumination but might be more adaptive in their particular environment.

Daylight - When you are outdoors and need to know how much daylight you have left to make camp or to return to your car from a hike, stand facing the sun, extend your arm forward while holding your hand horizontally at a 90 degree angle facing you. Hold your hand so that the top of your thumb touches the bottom of the sun. The distance from the top to the bottom of your hand represents an hour. You can use your other hand below to measure another hour and so on. Once the sun sets, there is roughly a half hour of usable light remaining.

Distance - When you see lightning, thunder soon follows, because light travels faster than sound. By counting the seconds between the light and the sound, you can estimate the distance between you and the lightning. Sound travels roughly 1000 feet per second, so 5 seconds equal a mile. When the lightning storm approaches you, the delay grows shorter. When the lightning storm departs, the delay grows longer. Remember, if you can hear thunder, you are close enough to the storm to be struck by lightning, so seek shelter immediately.

Depth - Ever wonder how deep that pit or shaft is? Providing no one is below, you can drop a pebble and count the seconds before you hear it hit the bottom. The actual time is not linear as the object gains speed during it’s decent, however, the following linear time table is a rough guide to depth:

Elapsed Time / Velocity
1 second / -32 feet per second
2 seconds / -64 feet per second
3 seconds / -96 feet per second
4 seconds / -128 feet per second
5 seconds / -160 feet per second
6 seconds / -192 feet per second