diff --git a/wordclock/lightlevelmapping.m b/wordclock/lightlevelmapping.m
index 8174086..85282f4 100644
--- a/wordclock/lightlevelmapping.m
+++ b/wordclock/lightlevelmapping.m
@@ -1,4 +1,39 @@
 function lightlevelmapping()
+MINBRIGHTNESS = 1;
+MAXBRIGHTNESS = 10;
+
+LIGHTSENSOR_BOTTOM = 0;
+LIGHTSENSOR_TOP = 150;
+
+x = [LIGHTSENSOR_BOTTOM - 10:LIGHTSENSOR_TOP + 10];
+b = zeros(size(9));
+
+LIGHTSENSOR_ALPHA1 = 0.02;
+LIGHTSENSOR_ALPHA2 = 0.20;
+
+a = LIGHTSENSOR_ALPHA1 * LIGHTSENSOR_TOP / log(LIGHTSENSOR_ALPHA2 / ...
+	LIGHTSENSOR_ALPHA1);
+b = LIGHTSENSOR_ALPHA1 / a;
+c = MINBRIGHTNESS - a;
+
+y = a * exp(b*x) + c;
+y_ = round(y);
+
+[x; y_]
+
+figure
+plot(x, y, x, y_);
+A = axis;
+A(1) = LIGHTSENSOR_BOTTOM - 2;
+A(2) = LIGHTSENSOR_TOP - 2;
+A(3) = MINBRIGHTNESS - 2;
+A(4) = MAXBRIGHTNESS + 2;
+axis(A);
+grid on
+
+return
+
+function lightlevelmapping_old()
 MINBRIGHTNESS = 0;
 MAXBRIGHTNESS = 10;
 
diff --git a/wordclock/wordclock.ino b/wordclock/wordclock.ino
index 3f5b92a..d4a6ac7 100644
--- a/wordclock/wordclock.ino
+++ b/wordclock/wordclock.ino
@@ -75,7 +75,12 @@ void SWversion(void);
 #define LIGHTSENSOR_BOTTOM 0
 // top of light sensor (ambient light values at or higher than this level will
 // be mapped to MAXBRIGHTNESS)
-#define LIGHTSENSOR_TOP 900
+#define LIGHTSENSOR_TOP 150
+// slope of mapping function at bottom
+#define LIGHTSENSOR_ALPHA1 0.02
+// slope of mapping function at top
+#define LIGHTSENSOR_ALPHA2 0.20
+
 // weight for exponential decaying averaging (actual weigth is 2 ^ LIGHTSENSOR_WEIGHT)
 #define LIGHTSENSOR_WEIGHT 4
 // base of exponential mapping (must be > 1)
@@ -87,7 +92,7 @@ void SWversion(void);
 // start MAXBRIGHTNESS at DAYLIGHTHOUR (7 am)
 #define DAYLIGHTHOUR 7
 // NIGHT Brightness setting 0 = off, N_PWM_STEPS - 1 = full
-#define MINBRIGHTNESS 2
+#define MINBRIGHTNESS 1
 // start MINBRIGHTNESS at NIGHTLIGHTHOUR (7 pm)
 #define NIGHTLIGHTHOUR 19
 
@@ -180,6 +185,9 @@ void setup()
 {
 	int n;
 
+	// a, b, c and e are needed to compute lookup table for ambient light sensor
+	float a, b, c, e;
+
 	// initialise the hardware
 	// initialize the appropriate pins as outputs:
 	pinMode(LEDClockPin, OUTPUT);
@@ -245,6 +253,12 @@ void setup()
 	}
 
 	// compute ambient light level to brightness level mapping
+	a = LIGHTSENSOR_ALPHA1 * LIGHTSENSOR_TOP / log(LIGHTSENSOR_ALPHA2 /
+		LIGHTSENSOR_ALPHA1);
+	b = LIGHTSENSOR_ALPHA1 / a;
+	c = MINBRIGHTNESS - a;
+	e = 2.718281828459045297;
+
 	for (n = 0; n < sizeof(ambient_light_to_brightness); n++)
 	{
 		if (n <= LIGHTSENSOR_BOTTOM)
@@ -254,18 +268,15 @@ void setup()
 		else
 		{
 			ambient_light_to_brightness[n] =
-	round( pow((double) LIGHTSENSOR_BASE, (
-	(double) brightness_per_unit_light * ((double) n - (double)
-	LIGHTSENSOR_BOTTOM) / (double) (1 << LIGHTSENSOR_SCALE))) / pow((double)
-	LIGHTSENSOR_BASE, (double) (MAXBRIGHTNESS - (double) MINBRIGHTNESS)) *
-	((double) MAXBRIGHTNESS - (double) MINBRIGHTNESS) + (double) MINBRIGHTNESS );
-			/* Serial.print(n);
-			Serial.print(": ");
-			Serial.println(ambient_light_to_brightness[n], DEC); */
+				round(((float) a) * pow(e, ((float) b) *
+				((float) n)) + (float) c);
+			if (ambient_light_to_brightness[n] < MINBRIGHTNESS)
+				ambient_light_to_brightness[n] = MINBRIGHTNESS;
+			if (ambient_light_to_brightness[n] > MAXBRIGHTNESS)
+				ambient_light_to_brightness[n] = MAXBRIGHTNESS;
 		}
 	}
 
-
 	// determine whether we are running on old or new hardware
 	// old hardware tied the push buttons to ground using 4k7 resistors
 	// and relied on the buttons to pull them high