setDirection( dir ) {

		// dir is assumed to be normalized

		if ( dir.y > 0.99999 ) {

			this.quaternion.set( 0, 0, 0, 1 );

		} else if ( dir.y < -0.99999 ) {

			this.quaternion.set( 1, 0, 0, 0 );

		} else {

			_axis.set( dir.z, 0, - dir.x ).normalize();

			const radians = Math.acos( dir.y );

			this.quaternion.setFromAxisAngle( _axis, radians );

		}

	}
Sets the direction of the helper.

@param {Vector3} dir - The normalized direction vector.