Photocells detect if light is needed. Photocells are light-sensitive sensors that respond to the amount of light detected. When the light is too low, such as at dusk or under heavy overcast skies, the sensor tells the computing unit within the streetlight to activate the flow of electricity. When the photocell detects too much light, the sensor will deactivate the streetlight (e.g., at dawn).
Electricity is sent through high-intensity discharge lamps. A high-intensity discharge lamp emits light by an arc of electricity created between two electrodes. The electrodes are in a transparent tube filled with gas and metal salts. The electrical arc generates heat, which works with the gas and metal to create light-emitting plasma.
Streetlights use bypass technology. Through use of either isolation transformers or film cutout technology, streetlights are able to pass the voltage through to other streetlights when they are burned out. Much like older Christmas tree lights, streetlights are connected in series design; the current to operate five streetlights on the same street flows from light 1 through 2, 3 and so on. In the past, when one light burned out, the streetlights after it would not power on since the current could not cross the dead bulb. Isolation transformers carry the current across regardless of a working or nonworking bulb. Film cutout technology uses a film of material that connects the contacts needed for current to flow. If a streetlight burns out, the current bypasses the burned-out circuit and runs along the film cutout.
Streetlights are carefully planned. Streetlight issues include light pollution of the night sky and interference with night vision of drivers. A sudden inability to perceive lighting and distance at night due to street lighting is because of the accommodation reflex of the human eye as cars move from a darkened area to an area illuminated by a streetlight. The pupils of the eye cannot adjust fast enough moving from dark to light and back again, causing issues. Light pollution is considered an environmental issue of street lighting. It includes excessive light bleeding onto private property, blinding glare and overillumination of areas and buildings. There is also the issue of unwanted voltage where a streetlight can project stray voltage spikes and injure those nearby. This is rare but possible, especially during electrical storms. To deal with the issues of night vision, light pollution and voltage accidents, only a certain number of street lights are installed within a given area, and they are designed for low-light exposure to drivers. Some lights are fitted with an alarm notifying nearby individuals of dangerous voltage issues until they are repaired.