# Arduino Light Intensity Meaurement

## Definition

The lux (symbol: lx) is the SI unit of luminance, measuring luminous flux per unit area. It is equal to one lumen per square meter. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface.

## Light Measurement

The easiest way to measure light with an Arduino is with an LDR. LDR’s (Light dependent resistors) have a low resistance in bright light and a high resistance in the darkness.

Doing that on an Arduino Analog port, would give a reading between 0 and 1024, which of course are really non-descriptive numbers.  What you would want is an output in Lux or Lumen. That is possible but mind you that LDR’s are not really accurate for precise readings. There is a somewhat rough formula that relates the resistance of an LDR to the light in Lux. That is:
Rldr=500/Lux, or
Lux=500/Rldr (in kOhm)

#### Component Used

1. Arduino Uno
2. Connecting Wires
3. LDR
4. 10K Ohm resistor

## Arduino Code for Light Intensity Measurement

```//Light Intencity Measurement
//www.circuits4you.com

{
//int lux=500/(10*((5-Vout)/Vout));//use this equation if the LDR is in the upper part of the divider
int lux=(2500/Vout-500)/10;
return lux;
}

void setup() {
Serial.begin(9600);
}

void loop() {
Serial.print("Light Intensity:");