What are Black Out Lights?
     The US Army devised a strategy to use the tail lights and marker lights on vehicles to not only evade detection from the enemy, but also to help with the problem of conducting vehicle movement at night under black out conditions.  These specially designed military lights are called Black Out Lights.  The original type used were called ‘Blue Louver” blackout lights.  They were sometimes also referred to as “Cat Eye / Cat’s Eye” Black Out Lamps, because the taillight had a black vertical stripe. These consisted of a glass lens at the front, followed by a black metal disc, and then a colored plastic inner lens. All the pieces were held together by a thick black rubber gasket mounted inside a pot metal bucket and metal bezel (cover).  The metal disc would be perforated with many parallel cuts.  The metal strips created by the slices were then bent forward to allow a tiny amount of light to pass through. (There were actually 2 metal discs, both with louvers, placed back-to-back and riveted or spot-welded together). The front marker lights had a blue gray colored lens, while the rear B/O <Black-out> Stop Light had 2 lenses, one of red, the other a blue gray.  Individual parts could be replaced. These lights were used on all types of vehicles including the prototype jeeps - The Ford GP, and the Bantam BRC-40.  The Blue Louver Lights did not see use on the standard production WWII Willys MB and Ford GPW jeeps.

The MB/GPW’s used the standardized B/O light of WW2.  These lights consisted of a bucket, and metal bezel that contained a sealed unit. The individual lenses, screen, colored lens and bulb were a sealed unit and had to be replaced as a unit if one part was damaged.  This made the unit much more water-resistant than the previous style.  The way the black out effect was achieved was also changed for these standard B/O lights. The thick rubber gasket, the metal louvers, and the black stripes were gone.  Instead the sealed unit contained a clear plastic lens on the outside. Then set back in from the front approx. ½ inch was a black plastic screen, which had upside down pyramids cut out. Following the black screen was a colored plastic lens in off-white, or red. Last, and set back into and soldered to the metal housing was the small incandescent light bulb of low output.

The key features were:
  1) The upside down triangles,
  2) The fact that the triangles were set back from the front face of the light's lens,
  3) The spacing of the triangles in relation to each other.
    The set back and the fact the triangle was upside down was important because it allowed any one at foot level the see the lights clearly. However, anyone flying over and trying to spot the vehicles or get their bearing from automobile lights on the road were unable to see them because the angle would not allow it. The higher the angle, the less of the triangle was visible because of the overhang. Low output bulbs limited the distance the lights could be seen. The next key was the spacing of the triangles.  The front marker light had 2 triangles. The rear stoplight had 2 pairs of triangles.
Configured like this:  Y Y       Y Y  
They were cut into the black plastic in pairs. The triangles of a pair would be separated by about 3/8”, and the pairs were separated from each other by about 1-½ inches.  This spacing was important.  They had done the geometry to determine the correct separation to make these triangles of light helpful when driving in a convoy at night.  As the driver of a vehicle located in the middle of the convoy driving at night under black out conditions, you had to rely on the B/O lights on the vehicle in front of you and behind you to maintain your speed and distance from the other vehicles in the convoy.  If you could see all 4 red triangles of the stoplight in front of you, you were following too closely. If all 4 red triangles merged into 1 red light, you were too far back. What you wanted to see was that each of the PAIRS of red lights merged.


From this (too close - under 60 feet): 
     Y Y      Y Y       Y Y      Y Y    8 points of light is too many
To this (correct distance - 60 - 180 ft): 
V    V V    V   4 points of light is correct
To this (too far back - over 180 feet): 
V V   2 points of light is too few
This means you would see 2 red lights per taillight. This allowed you enough stopping distance, and kept you from getting left behind as well.  As the driver, you were to also watch your rear view mirror and keep an eye on the guy behind you.  His front marker lights to be exact. Those 2 triangles should merge into 1 if he was following you at the correct distance. If you could make out individual triangles, then he was following too closely and you should tap your brake lights to get his attention. If the lights faded and couldn’t be seen, then you might be driving to fast, or there could be a problem that would require halting the column.
    Several months after standardizing the B/O lights, it was decided that a B/O Driving Light was needed.  This was a larger lamp assembly that was mounted to the left front fender of the jeep and other vehicles.  This light had a higher candlepower bulb inside. It was larger and used a lens with angles in it to direct the light in a horizontal pattern and minimize the light escaping vertically.  The lamp also had a metal hood similar to a baseball cap’s bill built into it to make it invisible from the sky.  The name B/O Driving Light confused many people.  The main purpose was not to help the driver see the terrain; rather it was for the people driving ahead of you. Your B/O Drive Light was right in line with the outside rear view mirror of the guy in front of you. As a driver, if you saw a rapidly approaching Black Out Drive Light in your rear view mirror, you could tap your brake lights as a warning or take evasive action as necessary. It would also allow pedestrians a better chance of seeing you coming and get out of your way. It’s value as a way of seeing where you were going is next to nil.  B/O Driving lights became standard issue starting in mid 1942.  Retrofit kits were issued to modify vehicles already in the field. See next question for photos.
1/4ton Bantam and Willys trailers (MBT / T-3) used the same black out lights.  However, switching between normal and blackout lights on the jeep pulling the trailer did not affect the status of the lights that were running on the trailer.  The trailer had its own light switch mounted to the front passenger side box frame. See photo right.  There was a small disc (door) that swiveled out of the way to reveal a small set screw that can be turned using the butt end of the Jeep H-700 Key to switch between running lights and B/O lites.

From: http://members.aol.com/brimiljeep/WebPages/JeepNamePage.html