A computer based light intensity logger is made up of the following components

• Sensor - a photodiode (BPW34)

• Data acquisition module

• Computer with programming software (LabVIEW)

The light sensor used is the BPW34. This is a high speed and high sensitive silicon PIN photodiode in a miniature flat plastic package. A photodiode is designed to be responsive to optical input. Due to its waterclear epoxy the device is sensitive to visible and infrared radiation. The large active area combined with a flat case gives a high sensitivity at a wide viewing angle.

Photodiodes can be used in either zero bias or reverse bias. Diodes have extremely high resistance when reverse biased. This resistance is reduced when light of an appropriate frequency shines on the junction. Hence, a reverse biased diode can be used as a light detector by monitoring the current running through it. Coupled to a 10Kohm resistor, and given the specification of the BPW34 a simple relationship between lux (light intensity) and voltage is given by

lux = 1333 * V_o

With the Data Acquisition Module (DAQ module), a regular computer can now be used to measure this voltage. The DAQ module contains an 24-bit ADC (analog to digital converter). During analog to digital conversion, a digital value can correspond to a range of analog values. Any analog signal within the zone of one least significant bit (LSB) will have the same digital value. This error is known as quantization error. The relationship between this error and the bit resolution is given by

error = 1 / 2^n where n is the resolution in bits of the ADC

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