FadeOut

using UnityEngine;
using System.Collections;

public class FadeOut : MonoBehaviour 
{
	public float fadeOutTime = 5f;

	private SpriteRenderer spriteRenderer { get { return renderer as SpriteRenderer; } }

	void Update ()
	{
		if (Input.GetKeyDown(KeyCode.Space))
			StartCoroutine(DoFadeOut());
	}

	IEnumerator DoFadeOut ()
	{
		float startTime = Time.time;
		while (true)
		{
			float fade = (Time.time - startTime) / fadeOutTime;
			var color = spriteRenderer.color;
			color.a = Mathf.Lerp (1f, 0f, fade);
			spriteRenderer.color = color;

			if (fade >= 1f)
				break;

			yield return 1;
		}
	}
}