three.js/src/extras/helpers/DirectionalLightHelper.js

/**
 * @author alteredq / http://alteredqualia.com/
 * @author mrdoob / http://mrdoob.com/
 * @author WestLangley / http://github.com/WestLangley
 */

THREE.DirectionalLightHelper = function ( light, size ) {

	THREE.Object3D.call( this );

	this.light = light;
	this.light.updateMatrixWorld();

	this.matrixWorld = light.matrixWorld;
	this.matrixAutoUpdate = false;

	size = size || 1;
	var geometry = new THREE.PlaneGeometry( size, size );
	var material = new THREE.MeshBasicMaterial( { wireframe: true, fog: false } );
	material.color.copy( this.light.color ).multiplyScalar( this.light.intensity );

	this.lightPlane = new THREE.Mesh( geometry, material );
	this.add( this.lightPlane );

	geometry = new THREE.Geometry();
	geometry.vertices.push( new THREE.Vector3() );
	geometry.vertices.push( new THREE.Vector3() );

	material = new THREE.LineBasicMaterial( { fog: false } );
	material.color.copy( this.light.color ).multiplyScalar( this.light.intensity );

	this.targetLine = new THREE.Line( geometry, material );
	this.add( this.targetLine );

	this.update();

};

THREE.DirectionalLightHelper.prototype = Object.create( THREE.Object3D.prototype );

THREE.DirectionalLightHelper.prototype.dispose = function () {
	
	this.lightPlane.geometry.dispose();
	this.lightPlane.material.dispose();
	this.targetLine.geometry.dispose();
	this.targetLine.material.dispose();
};

THREE.DirectionalLightHelper.prototype.update = function () {

	var v1 = new THREE.Vector3();
	var v2 = new THREE.Vector3();
	var v3 = new THREE.Vector3();

	return function () {

		v1.getPositionFromMatrix( this.light.matrixWorld );
		v2.getPositionFromMatrix( this.light.target.matrixWorld );
		v3.subVectors( v2, v1 );

		this.lightPlane.lookAt( v3 );
		this.lightPlane.material.color.copy( this.light.color ).multiplyScalar( this.light.intensity );

		this.targetLine.geometry.vertices[ 1 ].copy( v3 );
		this.targetLine.geometry.verticesNeedUpdate = true;
		this.targetLine.material.color.copy( this.lightPlane.material.color );

	}

}();