switching  on  the  power  supply,  and  scanning  the forward area until the source of the sound was located or determined. Infrared  rays,  visible  only  to  the operator, were projected over the sighting area. The reflected rays, picked up by the electronic telescope, were  converted  to  a  visible  image  and  magnified according  to  the  power  of  the  telescope.  Sighted objects  appeared  in  varying  shades  of  green, regardless of their daylight color. Researchers chose the  color  green  because  green  light  stimulates  the cones without shutting down the rods. This enables the person using the night scope to look at a relatively bright picture without losing any night vision. Night  vision  capabilities  proved  to  be  particularly effective  in  combatting  enemy  infiltration  tactics conducted  during  periods  of  darkness.  However,  a major problem with the use of infrared instruments was that the user could be readily spotted by a foe who  also  was  using  infrared  instruments  or  detection equipment. To  combat  this  problem,  researchers developed passive devices, devices that only received light from a source and amplified that light to produce a picture on a screen.    In the following sections, we will discuss both active and passive devices that are in use  today. ELECTRO-OPTICAL  DEVICES All  electro-optical  devices  use  some  source   of power  to  produce  an  image  of  an  object  through illumination,   amplification,   or   thermal   imaging. These devices contain electronic components as well as electro-optical components such as light emitting diodes,  image  intensifier  tubes,  and  lasers.  Examples of  such  devices  are  night  vision  goggles  and  rifle sights,  laser  range  finders,  markers  and  designators, and  forward  looking  infrared  receivers. The night vision department of the Naval Surface Warfare  Center,  located  in  Crane,  Indiana,  is  the central   point   of   contact   for   all   electro-optical equipment used in the Navy. BATTERIES FOR NIGHT VISION EQUIPMENT Every   night   vision   device   uses   a   battery   to produce image intensification. The battery supplies voltage  that  accelerates  electrons  across  an  image intensifier tube. Various types of batteries are used in Navy and Marine Corps night vision equipment. Alkaline  batteries  are  inexpensive  and  readily available. However, their service life is the shortest of all  the  battery  types. Mercury batteries were used as power sources in earlier night vision equipment but, due to ecological problems, are now being removed from service. Nickel-cadmium batteries are used primarily for training  since  they  require  down  time  while recharging. Lithium batteries provide the longest service life at   temperatures   above   15   degrees   Fahrenheit. Because of high performance, this type of battery is used when possible. These batteries must be approved by  the  Navy  Lithium  Safety  Review  Board  before approval  for  service  use. Figure  1-1  shows  batteries  commonly  used  in night  vision  instruments. Beginning  at  the  upper  left  and  going  clockwise; 1. BA-1100/U Mercury Battery (6.5 volts) 2. TD-100A NiCd Battery Pack (14.0 volts) 3. BA-3058/U “AA” Alkaline Battery (1.5 volts) 4. BA-1567/U Mercury Battery (2.7 volts) 5. BA-5567/U Lithium Battery (3.0 volts) 6. BA-1312/U Mercury Battery ( 1.3 volts) HAND-HELD NIGHT VISION SIGHTS Hand-held sights are small, lightweight devices that are used for signaling or map reading. AN/PAS-6  Metascope The metascope, illustrated in figure 1-2, is a small hand-held  device  used  as  an  aid  in  detecting  infrared sources. The metascope has an attachable infrared light source (active) that can be used for signaling. With  the  light  source  attached,  the  metascope  can  be used for map reading. The metascope weighs 2.5 pound. It has a range of 1 mile for signaling purposes and 12 inches for map reading. Its receiver is powered by a 4.3 vdc battery and its light source is powered by two “AA” alkaline batteries.  The  field  of  view  is  26  degrees  and  the battery life is estimated at 40 hours at a temperature of 23 degrees Celsius. This unit is being replaced by the Nite-Eye   Pocketscope. 1-2