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Outdoor LEDs also illuminate streets more efficiently than sodium not so much because of their superior lumens per watt but because they are highly directional, meaning that they focus Traffic Light mostly in one direction. Sodium lamps are gas-filled bulbs that emit in all directions. More than half of that light must be redirected downward by reflectors or lenses, reducing the lamps’ illumination efficiency.
A much trickier factor to quantify for street lighting is how the difference in color temperature between LEDs and high-pressure sodium affects the way we see at night. Our ability to see in a range of environments comes from two sets of sensors: a group of receptors known as cones that show us color in daylight, and night-time sensors called rods that are very sensitive to bluish Road Signs light but are less sensitive to red.
Our visual sensitivity shifts as the light grows dim because rods and cones respond most strongly to waves of different lengths. The collective response of cones makes the human eye most sensitive in the daytime to wavelengths of green-yellow light in the middle of the visible spectrum. Rods have a peak response to shorter blue-green wavelengths. Blue-sensitive cones, which are greatly outnumbered by other types of cones but are thought to play a role in sensing brightness at night, peak at wavelengths that produce indigo light.
The result is that at night the blue-rich light from an LED streetlamp looks brighter to the eye than the orangish Solar Panel System from a high-pressure sodium lamp—even if the two emit the same number of lumens, which are measured on a scale based on the eyes’ daytime response.
Given these facts, some experts touted bluer light for LEDs, noting that the relatively high color temperatures could enhance visibility at night. Some suggested that the use of bluish LEDs would let us see so much better at night that we could turn down the intensity of the lighting.
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