Beam cutoff

Also called horizontal cutoff, cutoff line, sharp cutoff

A beam cutoff is a sharp upper edge to a light's beam, above which it throws almost nothing, and it is the optical feature that makes a wide beam usable when the air is full of dust, fog or snow.

What is a beam cutoff?

Point a lamp at a wall and most cheap off-road lights give you a bright middle that fades out gradually in every direction. A lamp with a cutoff gives you a hard horizontal line near the top of the pattern: bright below it, close to nothing above it. That line is engineered into the optics, and it is the difference between a lamp you can aim and one you can only point.

It matters because of where your eyes are. Light thrown above the lamp's own horizontal travels through the air at roughly your eye level, which is the worst possible place for it when that air has dust in it. A cutoff removes that light without narrowing the beam.

What the standard asks for

SAE J583, the American front fog lamp standard, writes the requirement in candela rather than in words. Set at its standard aim, a compliant lamp must produce at least 2,400 candela one and a half degrees below the horizontal, no more than 295 candela two degrees above it, and no more than 150 candela anywhere from ten to sixty degrees up. That is roughly an eight to one ratio across four degrees, written into a pass-or-fail test.

The standard also specifies the sharpness itself: a measured cutoff gradient of at least 0.08, and an inclination held within a fifth of a degree. A gradual fade fails, because you cannot aim to a line that is not there.

What a cutoff does not buy you

Permission to mount a wide beam high. Width and cutoff and height are three separate controls: width decides how much air the beam lights, cutoff decides whether that lit air sits above or below the lamp's horizontal, and mounting height decides where the lamp's horizontal falls relative to your eyes. A perfect cutoff on a lamp at eye level still puts a wall of lit dust across your sightline. It just puts a tidy edge on it.