Xamarin Android light sensor class (easy portable to Java)

LightMeter.cs

using System;
using Android.Graphics;
using Android.Hardware;
using Android.Media;
using Java.IO;
using YourNamespace.Utils;

namespace YourNamespace
{
	public class LightMeter: Java.Lang.Object, Android.Hardware.Camera.IPictureCallback, Android.Hardware.Camera.IPreviewCallback
	{
		/* 
		 * You can estimate the reflected light by taking a picture with your camera, and glancing at the exposure at given settings.
		 * If the exposure is decent, look at the meter settings: your used ISO, aperture, and shutter speed.
		 * Once you have a reading, you can do two conversions to reach the approximate LUX value.
		 * First, you need to convert to an EV (Exposure Value), then to LUX.
		 */

		int cameraId;
		Android.Hardware.Camera camera;
		string PICTURE_FILENAME = "";

		public void Start() {
			try{
				cameraId = findFrontFacingCamera();

				if (camera == null) {

					camera = Android.Hardware.Camera.Open(cameraId);

					Android.Hardware.Camera.Parameters p = camera.GetParameters();
					p.PictureFormat = ImageFormatType.Jpeg;
					p.Set("ISO", 100);
					camera.SetParameters(p);

					camera.Lock();
					SurfaceTexture surfaceTexture = new SurfaceTexture(10);
					camera.SetPreviewTexture(surfaceTexture);

					camera.StartPreview();

				}
			}
			catch(IOException e){
			}
		}

		public void Stop() {
			try {
				if (camera != null) {
				    camera.StopPreview ();
				    camera.SetPreviewDisplay (null);
				    camera.SetPreviewCallback (null);
				    camera.Unlock ();
				    camera.Release ();
				    camera = null;
				}
			} catch (IOException ex) {					
			}
		}

		int findFrontFacingCamera() {
			Android.Hardware.Camera.CameraInfo camInfo = new Android.Hardware.Camera.CameraInfo ();
			for (int i = 0; i < Android.Hardware.Camera.NumberOfCameras; i++) {
				Android.Hardware.Camera.GetCameraInfo (i, camInfo);
				if (camInfo.Facing == CameraFacing.Front){
					try {
						return i;
					} catch (Exception e) {
						// log or something
					}
				}
			}
			return -1;
		}

		public void TakePicture(){
			if (camera != null) {
			    camera.TakePicture (null, this, this);
			}
		}

		public double GetLux(){

			/* Illuminance 	      Surfaces illuminated by:
			 * 
			 * 0.0001 lux 	      Moonless, overcast night sky (starlight)[3]
			 * 0.002 lux 	      Moonless clear night sky with airglow[3]
			 * 0.27–1.0 lux 	  Full moon on a clear night[3][4]
			 * 3.4 lux 	          Dark limit of civil twilight under a clear sky[5]
			 * 50 lux 	          Family living room lights (Australia, 1998)[6]
			 * 80 lux 	          Office building hallway/toilet lighting[7][8]
			 * 100 lux 	          Very dark overcast day[3]
			 * 320–500 lux 	      Office lighting[9][10][11]
			 * 400 lux 	          Sunrise or sunset on a clear day.
			 * 1000 lux 	      Overcast day;[3] typical TV studio lighting
			 * 10000–25000 lux 	  Full daylight (not direct sun)[3]
			 * 32000–100000 lux   Direct sunlight
			 */

			ExifInterface exif = null;

			if (!PICTURE_FILENAME.Equals("")){
			    try {
				    File dataDir = Android.OS.Environment.ExternalStorageDirectory;
					exif = new ExifInterface(dataDir + "/" + PICTURE_FILENAME);
			    } catch (IOException ex) {

			    }
			}

			if (exif != null) {

				PICTURE_FILENAME = "";

				// (Exposure Value) EV = log2 N2 / t

				/* where:
				 * N is the relative aperture (f-number)
				 * t is the exposure time (“shutter speed”) in seconds
				 */

				double f_number = double.Parse(exif.GetAttribute (ExifInterface.TagAperture));
				double shutter_speed = double.Parse(exif.GetAttribute (ExifInterface.TagExposureTime));

				double EV = Math.Log ((f_number * f_number) / shutter_speed);

				// 0 EV = 2.5 lux

				// (Illuminance) E = 2.5 * 2^EV

				return 2.5 * Math.Pow(2,EV);
			}

			return -1;
		}

		void Android.Hardware.Camera.IPictureCallback.OnPictureTaken(byte[] data, Android.Hardware.Camera camera)
		{
			FileOutputStream outStream = null;
			File dataDir = Android.OS.Environment.ExternalStorageDirectory;
			if (data != null) {
				try{
					PICTURE_FILENAME = Utils.CurrentTimeMillis() + ".jpg";
					outStream = new FileOutputStream(dataDir + "/" + PICTURE_FILENAME);
					outStream.Write(data);
					outStream.Close();
				}catch(FileNotFoundException e){
					System.Console.Out.WriteLine (e.Message);
				}catch(IOException ie){
					System.Console.Out.WriteLine (ie.Message);
				}
			}
		}

		void Android.Hardware.Camera.IPreviewCallback.OnPreviewFrame(byte[] b, Android.Hardware.Camera c)
		{
		}
	}
}

// HOW TO USE

LightMeter cSensor;

protected override void OnCreate (Bundle savedInstanceState)
{
	base.OnCreate (savedInstanceState);

	cSensor = new LightMeter ();

}

protected override void OnResume ()
{
	base.OnResume ();
		
	cSensor.Start();
	
	// after start the sensor you have two methods:
	// cSensor.TakePicture() take a picture with the front camera to be used as source for ExifScan
	// cSensor.GetLux() get Lux (illuminance) value
				
}

protected override void OnPause ()
{
	base.OnPause ();

	cSensor.Stop();

}

// Utils class

using System;

namespace YourNamespace
{
	public class Utils
	{
		public static long CurrentTimeMillis(){
			return (long)(DateTime.Now - new DateTime (1970, 1, 1)).TotalMilliseconds;
		}
	}
}