PROJECTION   LAMPS.—Projection   lamps must  have  a  voltage  rating  equal  to  the  line voltage. The lamp is usually installed in a lamp house designed to accommodate either a 750- or 1,000-watt  projection  lamp.  Most  projectors  are designed to prevent the installation of the lamp in an incorrect way. They may have a small pin or  slotted  flanges  in  the  projection  lamp  base which  permits  the  insertion  of  the  lamp  in  only one  way. In many models a glass reflector is positioned precisely  behind  the  projection  lamp.  Since  an equal  amount  of  light  is  distributed  in  a  direc- tion  opposite  to  that  of  the  condenser  lens system, the reflector is used to return as much of this light as possible to the condenser lens. The Graflex 16 projector uses a type of lamp which   eliminates   the   need   for   reflectors.   A built-in   dichroic   reflector   reflects   visible   light forward  through  the  optical  system  but  allows the heat-producing infrared rays to pass through it to the rear of the projector. The lamp is a low power  consumption  21-1/2  volt,  250-watt  lamp; yet with the built-in dichroic reflector it is able to provide  screen  brightness  equivalent  to  a  high- voltage  1,200-watt  lamp.  This  projector  has  a lamp transformer built into allow matching with local  voltage  variation  for  optimum  lamp  per- formance  and  life.  So,  if  you  are  operating  the Graflex projector, the machine will not have the standard  projection  lamp  reflector  described  in the  preceding  paragraph. The  condensing  lens  assembly  contains  two lenses.  These  lenses  pick  up  the  light  from  the lamp  and  concentrate  it  into  an  intense  beam. This beam of light is directed through the aper- ture  across  which  the  film  travels.  As  the  light passes  through  the  film,  it  produces  an  image which  is  carried  along  the  beam  of  light  to  the projection  lens. The projection lens picks up and projects the image  brought  to  it  by  the  beam  of  light.  To meet   average   conditions,   most   projectors   are supplied  with  a  lens  that  has  a  2-inch  (50-mm) focal   length.   Other   focal   length   lenses   are available  for  projecting  various  sized  pictures and  for  projecting  at  various  projector-to-screen distances.  Some  projectors  are  also  equipped  to accept  a  field  flattener  for  maximum  corner-to- corner sharpness or a zoom lens with which you can enlarge the picture. Projector  lamps  are  blower-cooled.  In  most projectors  the  blower  fan  is  in  operation  at  all times   when   the   projector   is   turned   on,   even though  the  projection  lamp  is  not  lighted.  Cool- ing air is brought in and forced around the pro- jection  lamp  and  out  through  the  lamp  house top cover. The cooling air carries the heat away from  the  lamp.  Usually  the  arrangement  of  the lamp  and  motor  switches  is  such  that  it  is impossible for the lamp to light unless the cool- ing system is in operation. SOUND  OPTICAL  SYSTEM.—(See  figure 7-16.)  The  sound  track  on  the  film  must  pass around  the  sound  drum  (figure  7-1  5)  at  a  con- stant   rate   of   speed   if   good   quality   sound reproduction  is  to  be  obtained.  Usually  the sound  drum  shaft  is  stabilized  by  means  of  a flywheel to give it the constant speed necessary. The sprockets, immediately before and after the sound  drum,  control  the  path  of  travel  of  the film over the sound drum. As the film is driven through  the  projector,  the  sound  track  portion of the film passes between the sound optics car- tridge  and  the  photocell.  The  sound  optics  car- tridge  shapes  and  focuses  the  light  from  the exciter   lamp   into   a   rectangular   beam,   which passes  through  the  sound  track  into  the photocell.  Variations  in  the  sound  track  image density or area cause corresponding fluctuations in  the  amount  of  light  transmitted  to  the photocell and result in voltage changes which are applied  to  the  amplifier. The light for the sound track scanning beam originates  in  the  exciter  lamp.  It  is  beamed  by the  optical  parts  in  the  sound  optic  tube  which focuses the light in the shape of a narrow slit on the   sound   track   of   the   film.   After   passing through  the  sound  track,  the  light  impulses  are reflected  by  a  small  mirror  located  behind  the sound  drum  to  the  photoelectric  cell,  where they  are  converted  into  electrical  impulses.  The electrical impulses are then sent to the amplifier, where  they  are  amplified.  The  loudspeaker  con- verts the impulses into sound. Care and Maintenance Each  operator  of  a  16-mm  projector  should be   able   to   perform   simple   maintenance.   You should at least be able to clean the film path and 7-29