headlight, headlamp, 前照灯、前方を照射

Headlamp, Wikipedia

A headlamp is a lamp attached to the front of a vehicle to illuminate（＊照射）the road ahead. Headlamps are also often called headlights, but in the most precise usage, headlamp is the term for the device itself and headlight is the term for the beam of light produced and distributed by the device.

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The earliest headlamps, fuelled by combustible gas such as acetylene gas or oil, operated from the late 1880s. Acetylene gas lamps were popular in 1900s because the flame is resistant to wind and rain. Thick concave mirrors combined with magnifying lenses projected（＊投射）the acetylene flame light.[4] A number of car manufacturers offered Prest-O-Lite calcium carbide acetylene gas generator cylinder with gas feed pipes for lights as standard equipment for 1904 cars.

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The Guide Lamp Company introduced "dipping" (low-beam) headlamps in 1915, but the 1917 Cadillac system allowed the light to be dipped using a lever inside the car rather than requiring the driver to stop and get out. The 1924 Bilux bulb was the first modern unit, having the light for both low (dipped) and high (main) beams of a headlamp emitting from（＊から照射、発光）a single bulb. 

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Modern headlamps are electrically operated, positioned in pairs, one or two on each side of the front of a vehicle. A headlamp system is required to produce a low and a high beam, which may be produced by multiple pairs of single-beam lamps or by a pair of dual-beam lamps, or a mix of single-beam and dual-beam lamps. High beams cast（＊投射、照射）most of their light straight ahead, maximizing seeing distance but producing too much glare for safe use when other vehicles are present on the road. Because there is no special control of upward light, high beams also cause backdazzle from fog, rain and snow due to the retroreflection of the water droplets. Low beams have stricter control of upward light, and direct（＊向ける）most of their light downward and either rightward (in right-traffic countries) or leftward (in left-traffic countries), to provide forward visibility without excessive glare or backdazzle.

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Low beam (dipped beam, passing beam, meeting beam) headlamps provide a distribution of light designed to provide forward and lateral illumination（＊前方を照射）, with limits on light directed towards the eyes of other road users to control glare. This beam is intended for use whenever other vehicles are present ahead, whether oncoming（＊対向）or being overtaken（＊追い越し）.

The international ECE Regulations for filament headlamps[29] and for high-intensity discharge headlamps[30] specify a beam with a sharp, asymmetric cutoff preventing significant amounts of light from being cast into the eyes（＊目に照射）of drivers of preceding or oncoming cars. Control of glare（＊グレア、眩しさ）is less strict in the North American SAE beam standard contained in FMVSS / CMVSS 108.[31]

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High beam (main beam, driving beam, full beam) headlamps provide a bright, center-weighted distribution of light with no particular control of light directed towards other road users' eyes. As such, they are only suitable for use when alone on the road, as the glare they produce will dazzle（＊眩ます、眩しい）other drivers.

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Because wrong-side-of-road headlamps blind（＊眩ます、眩しい）oncoming drivers and do not adequately light the driver's way, and blackout strips and adhesive prismatic lenses reduce the safety performance of the headlamps, some countries require all vehicles registered or used on a permanent or semi-permanent basis within the country to be equipped with headlamps designed for the correct traffic-handedness.[34][35] North American vehicle owners sometimes privately import and install Japanese-market (JDM) headlamps on their car in the mistaken belief that the beam performance will be better, when in fact such misapplication is quite hazardous and illegal.[36][37]

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There are two different beam pattern and headlamp construction standards in use in the world: The ECE standard, which is allowed or required in virtually all industrialized countries except the United States, and the SAE standard that is mandatory only in the US. Japan formerly had bespoke lighting regulations similar to the US standards, but for the left side of the road. However, Japan now adheres to the ECE standard. The differences between the SAE and ECE headlamp standards are primarily in the amount of glare permitted toward other drivers on low beam (SAE permits much more glare), the minimum amount of light required to be thrown straight down the road（＊道路に沿って真っすぐ照射）(SAE requires more), and the specific locations within the beam at which minimum and maximum light levels are specified.