Blue light, subtle and dramatic, surrounds us, its special properties serving many purposes. When it comes to light perception, glare and brightness are both functions of wavelength; the short wavelength of blue light appears relatively bright to human eyes, making this among the most energy-efficient colors of light to produce. The bright bluish light emitted by high-intensity discharge headlamps thus increases visibility while using less energy than halogen headlamps, but that brightness also can heighten glare for oncoming drivers, particularly elderly drivers, who may already have trouble seeing at night. Now-ubiquitous compact fluorescent lamps (CFLs) similarly produce more light with less energy compared with incandescent lamps, and the bluer the CFL (“daylight” bulbs have the bluest color balance), the more energy efficient. More dramatic blue light is found in dental offices, where blue curing lights are used to harden amalgam material (orange goggles and filters provide eye protection against the intense light). The specific wavelength and intensity of the curing light stimulates a photoinitiator in the amalgam to decompose and initiate polymerization of the compound. But don’t think blue light is all work and no play—some, like that cast by the sea of Christmas lights at Tokyo Midtown, serves little purpose other than sheer enjoyment.