diff --git a/libraries/ML8511/ML8511.cpp b/libraries/ML8511/ML8511.cpp
index d7f6a9c5..39151ccd 100644
--- a/libraries/ML8511/ML8511.cpp
+++ b/libraries/ML8511/ML8511.cpp
@@ -1,16 +1,19 @@
 //
 //    FILE: ML8511.cpp
 //  AUTHOR: Rob.Tillaart@gmail.com
-// VERSION: 0.1.5
+// VERSION: 0.1.6
 // PURPOSE: ML8511 - UV sensor - library for Arduino
 //
 //  HISTORY:
 //  0.1.0  2020-02-03  initial version
 //  0.1.1  2020-02-17  added _voltPerStep() to support more boards
 //  0.1.2  2020-06-21  refactor; add estimateDUVindex()
-//  0.1.3  2021-01-01  arduino-ci + unit test
+//  0.1.3  2021-01-01  Arduino-ci + unit test
 //  0.1.4  2021-04-23  fix for platformIO
-//  0.1.5  2021-05-27  fix arduino-lint
+//  0.1.5  2021-05-27  fix Arduino-lint
+//  0.1.6  2021-06-19  add get/setDUVfactor(), 
+//                     rewrite estimateDUVindex(),
+//                     add reset();
 
 
 #include "ML8511.h"
@@ -22,9 +25,8 @@
 //
 ML8511::ML8511(uint8_t analogPin, uint8_t enablePin)
 {
-  _analogPin = analogPin;
-  _voltsPerStep = 5.0/1023;
-  _enablePin = enablePin;
+  _analogPin    = analogPin;
+  _enablePin    = enablePin;
   if (enablePin != 0xFF)
   {
     pinMode(_enablePin, OUTPUT);
@@ -36,9 +38,17 @@ ML8511::ML8511(uint8_t analogPin, uint8_t enablePin)
   {
     _enabled = true;
   }
+  reset();
 };
 
 
+void ML8511::reset()
+{
+  _voltsPerStep = 5.0/1023;
+  _DUVfactor    = 1.61;      // https://github.com/RobTillaart/ML8511/issues/4
+}
+
+
 float ML8511::getUV(uint8_t energyMode)
 {
   if (!_enabled)
@@ -57,7 +67,7 @@ float ML8511::getUV(uint8_t energyMode)
 
   // see datasheet - page 4
   // mW/cm2 @ 365 nm
-  // @ 25 Celcius
+  // @ 25 Celsius
   // formula estimated on graph
   if (voltage <= 1.0) 
   {
@@ -69,18 +79,23 @@ float ML8511::getUV(uint8_t energyMode)
 }
 
 
-// experimental estimate DUV index  (not calibrated, USE WITH CARE !!)
+// experimental estimate DUV index ( ==> USE WITH CARE !!)
+// use setDUVfactor(float w) to calibrate
+//
 // input is power in mW per cm2
-// weight is pretty high 
 float ML8511::estimateDUVindex(float mWcm2)
 {
-  float weight = 1.0;   // this can be tuned to callibrate
+  // rewrite in 0.1.6
+  // https://github.com/RobTillaart/ML8511/issues/4
+  return mWcm2 * _DUVfactor;
+};
 
-  // convert to mW per m2
-  float mWm2 = mWcm2 * 10000;
-  // factor to normalize to an 0..10 scale see Wikipedia.
-  float factor = 0.04;  // 1.0/25;  
-  return mWm2 * weight * factor;
+
+bool ML8511::setDUVfactor(float f)
+{
+  if (f < 0.01) return false;  // enforce positive values 
+  _DUVfactor = f; 
+  return true;
 };
 
 
diff --git a/libraries/ML8511/ML8511.h b/libraries/ML8511/ML8511.h
index aee24e4c..c5b9a522 100644
--- a/libraries/ML8511/ML8511.h
+++ b/libraries/ML8511/ML8511.h
@@ -2,7 +2,7 @@
 //
 //    FILE: ML8511.h
 //  AUTHOR: Rob Tillaart
-// VERSION: 0.1.5
+// VERSION: 0.1.6
 // PURPOSE: ML8511 - UV sensor - library for Arduino
 //     URL: https://github.com/RobTillaart/ML8511
 //
@@ -18,7 +18,7 @@
 
 #include <Arduino.h>
 
-#define ML8511_LIB_VERSION (F("0.1.5"))
+#define ML8511_LIB_VERSION            (F("0.1.6"))
 
 
 class ML8511
@@ -27,14 +27,12 @@ public:
   // if enablePin is omitted, one must connect EN to 3V3.
   ML8511(uint8_t analogPin, uint8_t enablePin = 0xFF);
 
+  void reset();  // reset internal vars to initial value.
+
   // energyMode = HIGH or LOW;
   // returns mW per cm2
   float    getUV(uint8_t energyMode = HIGH);
 
-  // experimental estimate DUV index  (not calibrated, USE WITH CARE !!)
-  // input in mW per cm2
-  float    estimateDUVindex(float mWcm2);
-
   // voltage must be > 0 otherwise it is not set
   void     setVoltsPerStep(float voltage, uint32_t steps);
   float    getVoltsPerStep() { return _voltsPerStep; };
@@ -44,11 +42,32 @@ public:
   void     disable();
   bool     isEnabled() { return _enabled; };
 
+
+  // experimental estimate DUV index
+  // WARNING: USE WITH CARE
+  //
+  // input in mW per cm2    == typical the output of getUV()
+  float    estimateDUVindex(float mWcm2);
+  //
+  // https://github.com/RobTillaart/ML8511/issues/4
+  // discusses the calibration 
+  // see readme.md how to reverse engineer the factor for 
+  // the estimateDUVindex() conversion function.
+  // a value of 1.61 was found to be far more accurate
+  //
+  // returns false  if f < 0.01 (to force positive only)
+  bool     setDUVfactor(float f);
+  float    getDUVfactor()        { return _DUVfactor; };
+
+
 private:
   uint8_t  _analogPin;
   uint8_t  _enablePin;
   float    _voltsPerStep;
   bool     _enabled;
+
+
+  float    _DUVfactor;
 };
 
 
diff --git a/libraries/ML8511/examples/ML8511_DUV_index/ML8511_DUV_index.ino b/libraries/ML8511/examples/ML8511_DUV_index/ML8511_DUV_index.ino
index 9fd6370d..aabaf855 100644
--- a/libraries/ML8511/examples/ML8511_DUV_index/ML8511_DUV_index.ino
+++ b/libraries/ML8511/examples/ML8511_DUV_index/ML8511_DUV_index.ino
@@ -35,6 +35,9 @@ void setup()
 
   // manually enable / disable the sensor.
   light.enable();
+
+  light.setDUVfactor(1.80);    // calibrate your sensor
+
   Serial.print("\tmW cm^2");
   Serial.print("\tDUV index");
   Serial.println();
diff --git a/libraries/ML8511/examples/ML8511_determine_DUV_factor/ML8511_determine_DUV_factor.ino b/libraries/ML8511/examples/ML8511_determine_DUV_factor/ML8511_determine_DUV_factor.ino
new file mode 100644
index 00000000..eae9f133
--- /dev/null
+++ b/libraries/ML8511/examples/ML8511_determine_DUV_factor/ML8511_determine_DUV_factor.ino
@@ -0,0 +1,75 @@
+//
+//    FILE: ML8511_determine_DUV_factor.ino
+//  AUTHOR: Rob Tillaart
+// VERSION: 0.1.0
+// PURPOSE: demo ML8511 UV sensor - to determine DUV factor
+//    DATE: 2021-06-19
+//     URL: https://github.com/RobTillaart/ML8511
+
+//        BREAKOUT
+//      +-------+--+
+//  VIN |o      +-+| mounting hole
+//  3V3 |o      +-+|
+//  GND |o         |
+//  OUT |o         |
+//   EN |o       S |  Sensor
+//      +----------+
+//
+//   EN = ENABLE
+
+
+#include <Arduino.h>
+#include <ML8511.h>
+
+#define ANALOGPIN     A0
+#define ENABLEPIN     7
+
+ML8511 light(ANALOGPIN, ENABLEPIN);
+
+// for calculating the average
+float    sum   = 0;
+uint32_t count = 0;
+
+
+void setup()
+{
+  Serial.begin(115200);
+  Serial.println(__FILE__);
+  Serial.println("UV UltraViolet ML8511");
+
+  // manually enable / disable the sensor.
+  light.enable();
+
+  Serial.print("\tmW cm^2");
+  Serial.print("\tDUV index");
+  Serial.println();
+}
+
+
+void loop()
+{
+  Serial.print("enter reference DUV:\t");
+  // flush all
+  while (Serial.available() > 0) Serial.read();
+  while (Serial.available() == 0);
+  float DUV = Serial.parseFloat();
+  Serial.println(DUV);
+
+  float UV = light.getUV();
+  Serial.print("UV mW cm^2:\t\t");
+  Serial.println(UV, 4);
+
+  Serial.print("DUV factor:\t\t");
+  float factor = DUV / UV;
+  Serial.println(factor, 2);
+
+  // calculate the average
+  count++;
+  sum += factor;
+  Serial.print("DUV factor average:\t");
+  Serial.println(sum / count);
+  Serial.println();
+  Serial.println();
+}
+
+// -- END OF FILE --
diff --git a/libraries/ML8511/keywords.txt b/libraries/ML8511/keywords.txt
index 3ca229c5..907ed964 100644
--- a/libraries/ML8511/keywords.txt
+++ b/libraries/ML8511/keywords.txt
@@ -5,13 +5,18 @@ ML8511	KEYWORD1
 
 
 # Methods and Functions (KEYWORD2)
+reset	KEYWORD2
 getUV	KEYWORD2
+setVoltsPerStep	KEYWORD2
+getVoltsPerStep	KEYWORD2
+
 enable	KEYWORD2
 disable	KEYWORD2
-setVoltsPerStep	KEYWORD2
 isEnabled	KEYWORD2
-estimateDUVindex	KEYWORD2
 
+estimateDUVindex	KEYWORD2
+setDUVfactor	KEYWORD2
+getDUVfactor	KEYWORD2
 
 # Instances (KEYWORD2)
 
diff --git a/libraries/ML8511/library.json b/libraries/ML8511/library.json
index 6cc9a03c..1d45410b 100644
--- a/libraries/ML8511/library.json
+++ b/libraries/ML8511/library.json
@@ -13,9 +13,9 @@
   "repository":
   {
     "type": "git",
-    "url": "https://github.com/RobTillaart/ML8511"
+    "url": "https://github.com/RobTillaart/ML8511.git"
   },
-  "version": "0.1.5",
+  "version": "0.1.6",
   "license": "MIT",
   "frameworks": "arduino",
   "platforms": "*"
diff --git a/libraries/ML8511/library.properties b/libraries/ML8511/library.properties
index 40375709..72fceb4e 100644
--- a/libraries/ML8511/library.properties
+++ b/libraries/ML8511/library.properties
@@ -1,5 +1,5 @@
 name=ML8511
-version=0.1.5
+version=0.1.6
 author=Rob Tillaart <rob.tillaart@gmail.com>
 maintainer=Rob Tillaart <rob.tillaart@gmail.com>
 sentence=ML8511 - UV sensor - library for Arduino
diff --git a/libraries/ML8511/readme.md b/libraries/ML8511/readme.md
index bf9c30bd..76ce0b54 100644
--- a/libraries/ML8511/readme.md
+++ b/libraries/ML8511/readme.md
@@ -48,18 +48,40 @@ can set those with setVoltagePerStep()
 ```
 
 It is possible to always enable the sensor by connecting the EN to 3V3.
-The value of the enablePin in the constructor should then be ommitted 
+The value of the enablePin in the constructor should then be omitted 
 or set to a negative value;
 
 When connecting to an Arduino UNO one can use the 3V3 of the Arduino to power
-the sensor. However it is not possible to connect the enablepin directly to the 
+the sensor. However it is not possible to connect the enable pin directly to the 
 sensor. Use a voltage divider (10K + 20K) to convert the 5 Volts to ~3.3 Volts.
 
 
+## Interface
+
+- **ML8511(uint8_t analogPin, uint8_t enablePin = 0xFF)** Constructor, if enable is connected to 3V3 constantly one does not need to set the enablePin parameter.
+- **float getUV(uint8_t energyMode = HIGH)** returns mW per cm2, energyMode = HIGH or LOW
+- **void setVoltsPerStep(float voltage, uint32_t steps)** to calibrate the (Internal) ADC used. Voltage must be > 0 otherwise it is not set and the default of 5 volts 1023 steps is used.
+- **float getVoltsPerStep()** idem
+- **void enable()** manually enable
+- **void disable()** manually disable
+- **bool isEnabled()** get status.
+
+
+#### experimental
+
+WARNING: USE WITH CARE
+
+- **float estimateDUVindex(float mWcm2)** input in mW per cm2, returns a value between 0 and ~15(?)
+- **void setDUVfactor(float f)** set the conversion factor
+- **float getDUVfactor()** returns the set conversion factor (default 1.61)
+
+See below how to determine the DUV factor for your sensor.
+
+
 ## Sensor sensitivity
 
 Indoors there is very little UV light so use a known UV source like 
-a blacklight or go outside in the sun.
+a black-light or go outside in the sun.
 
 The formula to convert the ADC reading to mW cm^2 is based upon the graph 
 shown in the datasheet. As I have no reference source to calibrate the library
@@ -69,8 +91,26 @@ The sensor has its peak sensitivity ( >80% ) from λ = 300-380 nm
 with an absolute peak at λ = 365 nm.
 
 
-## Experimental
-(use at own risk)
+## Experimental DUVindex
+
+Note: this library is **NOT** calibrated so **USE AT OWN RISK**
+
+The DUV index can be used for warning for sunburn etc.
+
+#### DUV index table
+
+Based upon https://en.wikipedia.org/wiki/Ultraviolet_index,
+
+| DUV INDEX | Description |
+|:---------:|:------------|
+|   0 - 2   |  LOW        |
+|   3 - 5   |  MODERATE   |
+|   6 - 7   |  HIGH       |
+|   8 - 10  |  VERY HIGH  |
+|   11+     |  EXTREME    |
+
+
+#### 0.1.5 and before
 
 The formula for the experimental **estimateDUVindex(power)** is based on
 the following facts / assumptions:
@@ -82,19 +122,31 @@ This is the most lethal the sensor can sense > 80%.
 (Erythemal action spectrum) 
 As we cannot differentiate this is the safest choice.
 
-The DUV index can be used for warning for sunburn etc.
-Please note that this library is NOT calibrated so **USE AT OWN RISK**
+
+#### 0.1.6 
+
+The formula is simplified to a single factor that the user needs to determine.
+Below is described how to do the calibration. 
 
 
-Table based upon https://en.wikipedia.org/wiki/Ultraviolet_index,
+#### Calibrate estimateDUVindex()
 
-| DUV INDEX | Description |
-|:-----:|:----|
-| 0 - 2 | LOW |
-| 3 - 5 | MODERATE |
-| 6 - 7 | HIGH |
-| 8 - 10 | VERY HIGH |
-| 11+ | EXTREME |
+To calibrate the **estimateDUVindex()** function one needs to determine
+the DUVfactor. To do this you need an external reference e.g. a local or nearby
+weather station. You need to make multiple measurements during the
+(preferably unclouded) day and calculate the factor.
+
+```
+          DUV from weather station
+factor = --------------------------
+                  getUV();
+```
+
+you do this e.g. once per hour, so you get multiple values.
+You can then average them to have a single factor.
+
+Hardcode this found value in the library (in the constructor) or better
+use the **setDUVfactor(factor)** call in **setup()** to calibrate your sensor.
 
 
 ## More about UV
@@ -103,7 +155,8 @@ https://en.wikipedia.org/wiki/Ultraviolet_index
 
 
 ## Notes
+
 - 3V3 Sensor so do not connect to 5V directly.
 - do not forget to connect the EN to either an enablePIN or to 3V3 (constantly enabled).
-- library does not work with an external ADC 
+- library does not work with an external ADC. (todo?)
 
diff --git a/libraries/ML8511/test/unit_test_001.cpp b/libraries/ML8511/test/unit_test_001.cpp
index ee1d1417..9151894e 100644
--- a/libraries/ML8511/test/unit_test_001.cpp
+++ b/libraries/ML8511/test/unit_test_001.cpp
@@ -29,6 +29,7 @@
 // assertNAN(arg);                                 // isnan(a)
 // assertNotNAN(arg);                              // !isnan(a)
 
+
 #include <ArduinoUnitTests.h>
 
 
@@ -46,22 +47,10 @@ unittest_teardown()
 {
 }
 
-/*
-unittest(test_new_operator)
-{
-  assertEqualINF(exp(800));
-  assertEqualINF(0.0/0.0);
-  assertEqualINF(42);
-  
-  assertEqualNAN(INFINITY - INFINITY);
-  assertEqualNAN(0.0/0.0);
-  assertEqualNAN(42);
-}
-*/
-
 
 #define ANALOGPIN         0
 
+
 unittest(test_constructor)
 {
   fprintf(stderr, "VERSION: %s\n", ML8511_LIB_VERSION);
@@ -72,6 +61,9 @@ unittest(test_constructor)
   light.setVoltsPerStep(3.3, 4095);
   assertEqualFloat(3.3/4095, light.getVoltsPerStep(), 0.0001);
 
+  light.reset();
+  assertEqualFloat(5.0/1023, light.getVoltsPerStep(), 0.0001);
+
   assertTrue(light.isEnabled());
   light.disable();
   assertFalse(light.isEnabled());
@@ -79,22 +71,104 @@ unittest(test_constructor)
   assertTrue(light.isEnabled());
 }
 
-
+/*
 unittest(test_getUV)
 {
-  ML8511 light(ANALOGPIN);
+  // need god mode to fill the analogRead...
+  // does not work properly
+
+  GodmodeState* state = GODMODE();
+  state->reset();
+  int future[6] = {0, 0, 0, 400, 500, 600};
+  state->analogPin[0].fromArray(future, 6);
+
+  ML8511 light(ANALOGPIN);  // no/default enable pin
+
+  assertEqualFloat(0, light.getUV(), 0.0001);
+  assertTrue(light.isEnabled());
+  assertEqualFloat(0, light.getUV(LOW), 0.0001);
+  assertFalse(light.isEnabled());
+  assertEqualFloat(0, light.getUV(HIGH), 0.0001);
+  assertTrue(light.isEnabled());
 
   assertEqualFloat(0, light.getUV(), 0.0001);
   assertEqualFloat(0, light.getUV(LOW), 0.0001);
+  assertEqualFloat(0, light.getUV(HIGH), 0.0001);
+}
+*/
+
+
+/*
+unittest(test_getUV_2)
+{
+  // need god mode to fill the analogRead...
+  // does not work properly
+
+  GodmodeState* state = GODMODE();
+  state->reset();
+  int future[6] = {0, 0, 0, 0, 0, 0};
+  state->analogPin[0].fromArray(future, 6);
+
+  ML8511 light(ANALOGPIN, 4);  //  set enable pin
+
+  assertEqualFloat(0, light.getUV(), 0.0001);
+  assertTrue(light.isEnabled());
+  assertEqualFloat(0, light.getUV(LOW), 0.0001);
+  assertFalse(light.isEnabled());
+  assertEqualFloat(0, light.getUV(HIGH), 0.0001);
+  assertTrue(light.isEnabled());
+}
+*/
+
+
+unittest(test_setDUVfactor)
+{
+  ML8511 light(ANALOGPIN);
 
   light.enable();
-  assertEqualFloat(0, light.getUV(HIGH), 0.0001);
 
-  for (float uv = 0; uv < 1; uv += 0.1)
+  for (float factor = 0.10; factor < 2.01; factor += 0.1)
   {
-    fprintf(stderr, "%f\t", uv);
-    assertEqualFloat(0, light.estimateDUVindex(0), 0.0001);
+    light.setDUVfactor(factor);
+    assertEqualFloat(factor, light.getDUVfactor(), 0.0001);
   }
+
+  fprintf(stderr, "\nOUT OF RANGE\n");
+  assertTrue(light.setDUVfactor(0.577));
+  assertEqualFloat(0.577, light.getDUVfactor(), 0.0001);
+
+  assertFalse(light.setDUVfactor(0));
+  assertEqualFloat(0.577, light.getDUVfactor(), 0.0001);
+
+  assertFalse(light.setDUVfactor(-1.0));
+  assertEqualFloat(0.577, light.getDUVfactor(), 0.0001);
+  
+  light.reset();
+  assertEqualFloat(1.61, light.getDUVfactor(), 0.0001);
+}
+
+
+unittest(test_estimateDUVindex)
+{
+  ML8511 light(ANALOGPIN);
+
+  light.enable();
+
+  for (float mW = 0; mW < 10; mW += 0.1)
+  {
+    fprintf(stderr, "%f\t", mW);
+    fprintf(stderr, "%f\n", light.estimateDUVindex(mW));
+    assertEqualFloat(1.61 * mW, light.estimateDUVindex(mW), 0.0001);
+  }
+
+  light.setDUVfactor(1.0);
+  for (float mW = 0; mW < 10; mW += 0.1)
+  {
+    fprintf(stderr, "%f\t", mW);
+    fprintf(stderr, "%f\n", light.estimateDUVindex(mW));
+    assertEqualFloat(1.0 * mW, light.estimateDUVindex(mW), 0.0001);
+  }
+
 }