To extend shaderMaterial with three.js based lights, passing the light as uniform to the shader.

extendShaderMaterial.js

// To extend shaderMaterial with three.js based lights, passing 
let light = new THREE.DirectionalLight(0xff0000);
  light.position.set(0, 0, 1);
  scene.add(light);

  uniforms = THREE.UniformsUtils.merge( [
		THREE.UniformsLib[ "lights" ],
    {
      u_time: {
        type: "f",
        value: 1.0
      },
      u_resolution: {
        type: "v2",
        value: new THREE.Vector2()
      },
      u_mouse: {
        type: "v2",
        value: new THREE.Vector2()
      },
      u_fragMouse: {
        type: "v2",
        value: new THREE.Vector2()
      },
      u_texture: {
        value: new THREE.TextureLoader().load("ambience.png") // new THREE.VideoTexture( video )
      },
      u_scale: {
        type: "v2",
        value: new THREE.Vector2()
      },
      u_zpos: {
        type: "f",
        value: 0.0
      }
    }
  ]);

/*

In vertex shader, add this -
  
  #if NUM_DIR_LIGHTS > 0
      struct DirectionalLight {
          vec3 direction;
          vec3 color;
          int shadow;
          float shadowBias;
          float shadowRadius;
          vec2 shadowMapSize;
       };
       uniform DirectionalLight directionalLights[ NUM_DIR_LIGHTS ];
  #endif
  
  use this to access light as vec3 -
  
  vec3 color = directionalLights[0].color;
  
*/