如果加入到 scene 中，是能够显示出来的，但是加入到 camera 中 无法显示 背景还是一片漆黑
一下是代码片段：
//初始化相机

const aspect = window.innerHeight / window.innerWidth;
this.camera = new THREE.OrthographicCamera(
    -sceneConf.width,
    sceneConf.width,
    sceneConf.width * aspect,
    -sceneConf.width * aspect,
    -100, 100
);
this.camera.position.set(-10, 10, 10);
this.camera.lookAt(new THREE.Vector3(0, 0, 0));

//初始化 Scene

this.scene = new THREE.Scene();
let axesHelper = new THREE.AxesHelper(200);
this.scene.add(axesHelper);

this.scene.add(new THREE.AmbientLight(0xffffff, 0.8));//环境光

let shadowLight = new THREE.DirectionalLight(0xffffff, 0.3);//平行光
shadowLight.position.set(10, 30, 20);
let shadowTarget = new THREE.Mesh(new THREE.PlaneGeometry(0.1, 0.1),
    new THREE.MeshBasicMaterial({color: 0xF5F5F5}));//ShadowLight的照射目标
shadowTarget.visible = false;
shadowTarget.name = 'shadowTarget';
shadowLight.target = shadowTarget;

this.scene.add(shadowLight);
this.scene.add(shadowTarget);

//添加 background

const aspect = this.HEIGHT / this.WIDTH;
let geometry = new THREE.PlaneGeometry(
    sceneConf.width * 2,
    sceneConf.width * aspect * 2
);
let material = new THREE.MeshBasicMaterial({
    color: 0xd7dbe6,
    opacity: 1,
    transparent: true,
    side: THREE.DoubleSide
});
let background = new THREE.Mesh(geometry, material);
this.camera.add(background); //如果添加到 scene 上，是能够看到的
background.position.set(0, 0, -85);
this.background = background;