Fades light intensity Up/Down when you press the Up/Down arrow keys.

Fade.cs

// Fade.cs
// Fades light intensity Up/Down when you press the Up/Down arrow keys.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Fade : MonoBehaviour {

	// User Inputs
	public float fadeDuration = 1.0f;

	// Private Variables
	Light myLight;
	private float startIntensity;

	// Use this for initialization
	void Start () {
		myLight = GetComponent<Light> ();
		startIntensity = myLight.intensity;
	}

	void Update() {
		if (Input.GetKeyDown (KeyCode.DownArrow)) {
			StartCoroutine(FadeDown ());
		} else if (Input.GetKeyDown (KeyCode.UpArrow)) {
			StartCoroutine (FadeUp ());
		}
	}

	// Fades Light Intensity Down
	IEnumerator FadeDown(){
		float fadeStep = myLight.intensity / fadeDuration;

		while (myLight.intensity > 0.0) {
			myLight.intensity -= fadeStep * Time.deltaTime;
			yield return null;
		}
	}


	// Fades Light Intensity Up
	IEnumerator FadeUp(){
		float fadeStep = startIntensity / fadeDuration;

		while (myLight.intensity < startIntensity) {
			myLight.intensity += fadeStep * Time.deltaTime;
			yield return null;
		}
	}
}