DRLs were first mandated in the Nordic countries, where ambient light levels in the winter are generally low even during the day. Sweden was the first country to require widespread DRLs in 1977. At the time, the function was known as "varselljus" ("perception light" or "notice light"). The initial regulations in these countries favored devices incorporating 21 W signal bulbs identical to those used in brake lamps and turn signals, producing yellow or white light of approximately 400 to 600 cd on axis, mounted at the outer left and right edges of the front of the vehicle. Finland adopted a daytime-light requirement in 1972 on rural roads in wintertime, and in 1982 on rural roads in summertime and 1997 on all roads all year long; Norway in 1986, Iceland in 1988, and Denmark in 1990. To increase manufacturer flexibility in complying with the requirement for DRLs, the daytime illumination of low beam headlights was added as an optional implementation. Given the headlamp specifications in use in those countries, such an implementation would produce approximately 450 cd axially.