text: Congo red was first synthesized in 1883 by Paul Böttiger, who had been employed at Friedrich Bayer Company in Elberfeld, Germany. He was looking for textile dyes that did not require a mordant step. The company which had a right of first refusal to his inventions was not interested in this bright red color, so he filed the patent under his own name and sold it to the AGFA company of Berlin. AGFA marketed the dye under the name "Congo red", a catchy name in Germany at the time of the 1884 Berlin West Africa Conference, an important event in the Colonisation of Africa. The dye was a major commercial success for AGFA. In the following years, for the same reason, other dyes were marketed using the "Congo" name: Congo rubine, Congo corinth, brilliant Congo, Congo orange, Congo brown, and Congo blue.  Once of economic significance, Congo red has fallen into disuse as have all benzidine-derived dyes, owing to their carcinogenic activity.  
  It is prepared by azo coupling of the bis(diazonium) derivative of benzidine with naphthionic acid.
  Congo blue, however, is in widespread international use, in gel sheet form, as a filter to place in front of theatrical, motion picture, television, church, and live event lighting instruments.  It is sold under the item name "181 Congo Blue" by Lee Filters.  It emits a deep rich saturated blue color with elements of red.  Depending upon the color temperature of the source lamp, the light from a lighting instrument with a Congo Blue filter reflected from a white surface can vary from very saturated blue to purple or violet.  The manufacturer reports that fluorescent light through a Congo Blue filter gives the appearance of black light.  Congo Blue filters are frequently used at live music concerts at an angle from behind musicians to cross back-light with a "warm" color gel like yellow, straw, gold, orange, or magenta, from an opposing angle, for a very dramatic effect.  Another use of Congo Blue filters by lighting technicians, is to cut a small strip from the gel sheet, which the technician looks through to make brightness adjustments to a video monitor displaying a standard color bar chart.  The Congo Blue filter effectively removes the color from chart and shows the separate bars only in terms of their differing incremental brightness levels.  This allows the technician to adjust the monitor to show a full and correct range of brightnesses.