Quartz-Iodine   Lamps The quartz-iodine lamps (tungsten-halogen) are of the incandescent variety but bear little resemblance to conventional light bulbs. A quartz lamp is a short tube of quartz glass, housing a coiled filament that runs the length of the tube. In ordinary tungsten lamps, the tungsten evaporates from the filament and settles on the glass   and   gradually   darkens   the   bulb.   In   the quartz-iodine  lamp,  however,  iodine  vapor  combines chemically with the tungsten and causes it to redeposit on the filament. This prevents the tube from becoming tarnished  with  age.  The  intensity  and  color  temperature of the tube remain almost constant throughout its life. Although the quartz-iodine lamp is very small, it produces intense light that is particularly suited for copy work.  There  is  a  disadvantage-quartz-iodine  lamps generate extreme heat that could cause your original to curl. You should never touch a quartz-iodine lamp with your fingers. The oil from your hands can create a concentrated hot spot on the lamp, causing it to bubble and burn out. LAMP  REPLACEMENT.–As  lamps  get  older, their color characteristics and light intensity may change. Therefore, when one lamp in a set bums out, the new replacement lamp is usually brighter and has a different color temperature than the remaining lamps. You should replace all the lamps, not just one to avoid the  need  for  adjusting  the  new  lamp  to  get  even illumination. Replacement of all lamps in a set is particularly  important  when  you  are  copying  with  color film because the color temperature of the new lamps is higher than the old lamps. The variation in color temperature would be seen as an uneven color quality over  the  resulting  reproduction. VOLTAGE   VARIATIONS.–Fluctuations  in  the voltage or electric current affect the color temperature of copy lights. When the voltage to your copy lights varies, consult an electrician. The electrician can trace the source of fluctuation and recommend the best action to  overcome  the  problem. Parabolic   Reflectors An important element of the lighting equipment for copy work is the reflectors. Parabolic reflectors should cause the light to be evenly distributed over the surface area  of  the  original  and  not  cause  hot  spots.  Certain  types of lights, such as reflector photoflood lamps, have built-in reflectors. By use of the correct reflectors with artificial light sources, exposure times can be shortened. Daylight Daylight  can  be  another  excellent  source  of illumination for copying. When the sun is used, you should try to use the sun during the midday hours where a combination of daylight and skylight is present, because  of  the  shifting  of  color  temperature  throughout the day. The early morning and evening hours should be avoided when color film is used, because the lack of blue light present. Heavy overcast skys or copying in shadow produces  a  bluish  cast  and  should  be  avoided  or corrected  with  a  filter. Filters The use of filters was fully discussed in chapter 3. Both correction and contrast filters, as well as special filters, are used extensively in copy work FILMS FOR COPYING For copying, you can achieve the best results by selecting the correct film for the type of copy work to be done. Copy-type films are designed specifically to compensate for the compression that occurs in tone reproduction and it provides an improved highlight tonal separation.  Copy  films  are  available  only  in  70mm  and sheet film formats. Although 35mm film can provide acceptable results, you should use sheet film since it provides higher-quality enlargements and is easier to retouch. Film characteristics, such as color sensitivity and contrast, are important when you select the film to copy a specific type of original. Film characteristics were discussed in chapter 2. SELECTING  THE  PROPER  FILM The following factors should be considered when choosing  the  proper  film: The color of the original to be copied The contrast of the original The contrast of the film The type of product to be produced, that is, black-and-white   or   color   print,   duplicate negative,  color  or  black-and-white  transparency, and so on Color quality of the light source Types of film available 8-10