This answer assumes that you have access to an optical calibration laboratory with Standards of Spectral Irradiance. You will also need to be familiar with the operation of the program Logger and the resulting tables that are produced.
The formula used by the Logger software is:
(raw data in volts – dark offset) / scale factor = calibrated data in µW/cm2.
The structure of the instrument's raw data makes the calibration a little more complicated than this formula suggests, because each optical channel has three feedback resistors that are automatically selected by the firmware, depending on the input signal level. These three different feedback resistors help give the instrument its dynamic range. Because there are three feedback resistors, there are three dark offsets, one for each resistor, and also three scale factors.
All of the information needed by the software is in the various tables that make up the Access (.mdb) file produced by the instrument. To illustrate the process, we have taken information from the data file 2010 08 08 000.mdb. Download this file "Deriving calibrated data from raw data.xls". This Excel file shows one particular 60 second line of data from the ACCESS data file. We chose this particular line because it shows different optical channels using each of the three resistors.
The first section is the raw data taken from the table for the 1 minute period at 10:50:01 on 8/8/2010.
The next section is from the Resistor Table and shows which resistor was being used by each optical channel for the same time period. PAR is using Resistor 1, which is the small resistor; 305 is using Resistor 3, which is the large resistor. All other channels are using Resistor 2, which is the medium resistor.
The third section is the channel table for the instrument and it contains the dark offsets (highlighted in yellow), and the scale factors (highlighted in orange) that correspond to the specific resistor (small, medium, or large) used by each channel during this 60 second period of data.
The final section is from the calibrated data table. If you click on the numbers in red, you will see the formula pointing to the individual raw data, offset, and scale factor used to derive the calibrated data.
If you want to collect data every second without averages, go to the Tools menu, select Options, then select the System Variables tab. You can adjust the data rate to 1 second and uncheck the box for Averaging Mode.