How to calculate illuminance from the output of a TCS34725 color sensor? I am using Arduino UNO with the sensor with the following programm.

#include &lt;Wire.h&gt;

// TCS34725 I2C address is 0x29(41)
#define Addr 0x29

void setup()
{
  // Initialise I2C communication as MASTER
  Wire.begin();
  // Initialise Serial Communication, set baud rate = 9600
  Serial.begin(9600);

  // Start I2C Transmission
  Wire.beginTransmission(Addr);
  // Select Wait Time register
  Wire.write(0x83);
  // Set wait time = 2.4 ms
  Wire.write(0xFF);
  // Stop I2C Transmission
  Wire.endTransmission();

  // Start I2C Transmission
  Wire.beginTransmission(Addr);
  // Select Atime register
  Wire.write(0x81);
  // Atime = 700 ms, max count = 65536
  Wire.write(0x00);
  // Stop I2C Transmission on the device
  Wire.endTransmission();

  // Start I2C Transmission
  Wire.beginTransmission(Addr);
  // Select control register
  Wire.write(0x8F);
  // AGAIN = 1x
  Wire.write(0x00);
  // Stop I2C Transmission
  Wire.endTransmission();

  // Start I2C Transmission
  Wire.beginTransmission(Addr);
  // Select enable register
  Wire.write(0x80);
  // Power ON, RGBC enable, wait time disable
  Wire.write(0x03);
  // Stop I2C Transmission
  Wire.endTransmission();
  delay(800);
}

void loop()
{
  unsigned int data[8];

  // Start I2C Transmission on the device
  Wire.beginTransmission(Addr);
  // Select data register
  Wire.write(0x94);
  // Stop I2C Transmission on the device
  Wire.endTransmission();

  // Request 8 byte of data from the device
  Wire.requestFrom(Addr, 8);

  // Read 8 bytes of data
  // cData lsb, cData msb, red lsb, red msb, green lsb, green msb, blue lsb, blue msb
  if (Wire.available() == 8)
  {
    data[0] = Wire.read();
    data[1] = Wire.read();
    data[2] = Wire.read();
    data[3] = Wire.read();
    data[4] = Wire.read();
    data[5] = Wire.read();
    data[6] = Wire.read();
    data[7] = Wire.read();
  }

  // Convert the data
  float cData = (data[1] * 256.0) + data[0];
  float red = (data[3] * 256.0) + data[2];
  float green = (data[5] * 256.0) + data[4];
  float blue = (data[7] * 256.0) + data[6];

  // Calculate the final lux
  float luminance = (-0.32466 * red) + (1.57837 * green) + (-0.73191 * blue);

  // Output data to serial monitor
  Serial.print("Red color Luminance   : ");
  Serial.print(red);
  Serial.println(" lux");
  Serial.print("Green color Luminance : ");
  Serial.print(green);
  Serial.println(" lux");
  Serial.print("Blue color Luminance  : ");
  Serial.print(blue);
  Serial.println(" lux");
  Serial.print("IR Luminance          : ");
  Serial.print(cData);
  Serial.println(" lux");
  Serial.print("Ambient Light Luminance : ");
  Serial.print(luminance);
  Serial.println(" lux");
  delay(500);
}


Is the output really in lux or is it counts? If it's counts, how can I calculate the illuminance in lux?
In the chip's datasheet on page 8 in the chapter "RGBC Operation" (emphasis by me):


  The RGBC engine contains RGBC gain control (AGAIN) and four
  integrating analog-to-digital converters (ADC)for the RGBC
  photodiodes. The RGBC integration time (ATIME) impacts both the
  resolution and the sensitivityof  the  RGBC  reading.  Integration  of
  all  four  channels  occurs  simultaneously  and  upon  completion  of
  theconversion cycle, the results are transferred to the color data
  registers. This data is also referred to as channelcount.


So, if you read the values from the data registers, you get simple count values, which were determined by the ADCs connected to the channels. To get a value in lux, you need to calculate that yourself.

Adafruit has a library for their breakout board with this chip. In their tutorial it seems, that the library outputs a value in lux. Maybe you can also use this library, or you can check, how they are converting the values from the sensor to lux values.