The Lux Meter is usually used to measure illumination. The illumination is how level of luminous flux is falling on a surface area. The luminous flux is visible component that is defined in radiant flux (light power) divided by relative sensitivity of human eyes over the visible spectrum. This means the Lux is well fit to light level from sense of human eyes.
However there is a difference between spectra response of ordinary silicon photo diode and human eyes, it cannot be used for the lux meter. Some photo diodes for illumination sensor have a color compensation filter on the window to correct its spectra response.
Photo diode outputs light current that is well proportional to input light power when it is used in short mode. In this lux meter, the output current is converted to voltage with an I-V converter, it is captured by a micrcontroller and displayed it in term of lux. The ppamp U1 in the Circuit Diagram acts as the I-V conversion circuit and its conversion rate becomes 50mV/µA according to a feedback register R5. The capacitor C5 is to gain correction, it cancels Ct of the photo diode (approx. 200 pF). In this case, larger capacitance may be good rather than 220 pF because high frequency signal is not needed.
The lux meter is powered by a 9V battery. To minimize battery space, it is made contact with two piece of springs made of nickel plated copper phosphorus alloy instead of a battery snap. However it is not anti-reverse insertion structure, any protection circuit is needed. Q1 is the reverse insertion protector which has not voltage loss like series diode. Q2 is a main swich constructs power hold circuit with Q3 and Q4.
U4 is a 7 segment LED display from HP. It is packed into a 14 pin DIP package. It is useful for handy equipments but it not suitable for battery operation because it consumes not small power.
Main function of the firmware is only captureing sensor output with A-D converter and display it to the LED display. The built-in A-D converter of AVR has 10 bit resolution but it is slightly insufficient for industrial measurement system. The A-D converter unit has a gain amplifier which can be inserted between multiplexer and A-D converter. This enables to improve resolution by changing the input gain automatically. To implement this funciton, capture input in gain of 20, if overflow occured, capture it again in gain of 1. As a result, can improve resolution at small input resion that lower relative resolution. Maximum light input is approx. 30000 lux due to A-D input raige of 0 to 2.56 V.
The result from A-D comverter is multiplied by a calibration value to LED display in term of lux. The LED will blink in low-battery condition.
The power switch is held by microcontroller to control power itself. This is useful to prevent to forget to turn power off. This lux meter turnes power off in 60 seconds.
Any reference light source will not be available in most case. We have to depend on only the data sheet to calibrate lux meter. The short current is Isc = 0.16µA/100lux, according to S1087 data sheet. It becomes 2µA at 1250 lux. When -100 mV is applied to test pin (TP1), it is equivalent to 1250 lux at photo diode, and I-V converter will output 100 mV.
To perform calibration, tie ISP pins via a several kΩ resister, turn on power, and will be calibrated to the reference input. Low range is calibrated when SCK-GND and -100 mV on TP1, High range is calibrated when MOSI-GND and -1 V on TP1. Ofcource photo diode must be masked while calibration, or the reslut will be invalid. When lux meter is housed into any clear case including light sensor, additional calibration is required to consider decrease due to case. When closed cover and lux decreased 10%, calibrate again with 10% decreased voltages.
If any reference lux meter is available, it is easy to calibrate, find places of 1250 and 12500 lux and calibrate at these places will do.