SpotLight in Three.js. Wrong lightened area and shadows

问题

I'm newbie at Three.js and OpenGL at all. So I can't understand lighting. I add SpotLight to my scene as follow:

var light = new THREE.SpotLight(0xffff00);
light.position.set( 1.5, 5.5, 0.9 );
light.shadowCameraVisible = true;
light.shadowDarkness = 1;
light.intensity = 2;
light.castShadow = true;
light.shadowCameraNear = 0.1;
light.shadowCameraFar = 50;
light.target = new THREE.Object3D();
light.target.position.set(5.5, 5.5, 0);
scene.add(light);

I turn on shadow map debug at WebGLRenderer and see following picture on my screen:

If I fly behind crates I see shadows only on 'red' zone of light source frustum. In 'yellow' zone there are no shadows and it looks very bad (look at next screenshot). '.castShadow' and '.receiveShadow' properties set to 'true'.

Help me understand what I'm doing wrong. Maybe I don't understand SpotLight source or shadow mechanism. I will be grateful for any advice. Thanks.

来源：https://stackoverflow.com/questions/22483713/spotlight-in-three-js-wrong-lightened-area-and-shadows