Blackout ( wartime ) A blackout during war , or in preparation for an expected war , is the practice of collectively minimizing outdoor light , including upwardly directed ( or reflected ) light . This was done in the 20th century to prevent crews of enemy aircraft from being able to navigate to their targets simply by sight , for example during the London Blitz of 1940 . In coastal regions , a " shore - side blackout " of city lights would also help protect ships from being seen in silhouette against the shore and attacked by enemy submarine s farther out at sea hidden from view . World War II . During World War II , the Air Ministry had forecast that Britain would suffer night air bombing attacks causing large numbers of civilian casualties and mass destruction . It was widely agreed that navigation and targeting would be more difficult if man - made lights on the ground could be extinguished . As early as July 1939 , Public Information Leaflet No 2 ( part of the Air Raid Precautions ( A . R . P . ) training literature ) warned of the need for popular discipline to ensure that the blackout regulations were fully enforced during the blackout periods . Blackout regulations were imposed on 1 September 1939 , before the declaration of war . These required that all windows and doors should be covered at night with suitable material such as heavy curtains , cardboard or paint , to prevent the escape of any glimmer of light that might aid enemy aircraft . The Government ensured that the necessary materials were available . External lights such as street lights were switched off , or dimmed and shielded to deflect light downward . Essential lights such as traffic lights and vehicle headlights were fitted with slotted covers to deflect their beams downwards to the ground . Shops and factories had particular problems . Factories with large areas of glass roofing found it impossible to install temporary blackout panels and permanent methods ( such as paint ) lost natural light during daylight . Shops had to install double " airlock " doors to avoid lights showing as customers arrived and departed . The blackout was enforced by civilian ARP wardens who would ensure that no buildings allowed the slightest chink or glow of light . Offenders were liable to stringent legal penalties . Blackout restrictions greatly increased the dangers of night driving and fatalities increased as a consequence . As a result , some aspects were relaxed and speed limits were lowered . The anticipated increase in crime rates did not occur . As German war - making capability declined , a " Dim - out " was introduced in September 1944 , which allowed lighting to the equivalent of moonlight . A full Blackout would be imposed if an alert was sounded . Full lighting of streets was allowed in April 1945 ; on 30 April , the day Hitler committed suicide , Big Ben was lit 5 years and 123 days after the Blackout was first imposed . The United States was not exposed to air attack , but along the Atlantic coast , the lack of a coastal blackout served to silhouette Allied shipping and thus expose them to German submarine attack . Coastal communities resisted the imposition of a blackout for amenity reasons , citing potential damage to tourism . The result was a disastrous loss of shipping , dubbed by German submariners as the " Second Happy Time " . Techniques . Lights can simply be turned off or light can sometimes be minimized by tar ring the windows of large public structures . In World War II , a dark blackout curtain was used to keep the light inside . Tarring the windows can mean a semi - permanent blackout status . Modern technology . The benefits of blackouts against air attack are now largely nullified in the face of a technologically sophisticated enemy . As early as World War II , aircraft were using radio - beam navigation ( see " Battle of the Beams " ) , and targets were detected by air - to - ground radar , ( e . g . H2X ) . Today , not only are night - vision goggles readily available to air crews , but sophisticated satellite - based and inertial navigation systems enable a static target to be found easily by aircraft and guided missile s .