Light is arguably the most important environmental parameter for plant growth and thus, aquaponics. This project creates an inexpensive lux meter using a TSL2561 ambient light sensor. Fun fact, the TSL2561 was actually designed for use in cell phones for features like auto-brightness and turning the screen off when you are talking on the phone. So what is a lux? One lux is equivalent to one lumen (luminous flux unit) per square meter - the total "amount" of visible light emitted by a source. In other words, lux is a measure of how brilliant something appears to the human eye (the human eye's sensitivity to the light source).
The project here does not create a true lux meter, rather, the manufacturer of the sensor has compared the sensor output against professional lux meters to come up with equations which approximate lux (and to a good degree). These equations are written into the Arduino library used to interface with the TSL2561 and thus the output will be in lux. Therefore, I will refer to the TSL2561 breakout board as a "lux sensor".