Dan is right, you need to look at the radout.sql in ResultsViewer or some other SQL viewer. But the illuminance data is also saved in a regular text format. Way down in your project's output directory is this directory:
[your_model]/run/1-Ruby-0/out/files/in/model/radiance/output/ts
In that directory you will find one directory for each space in your model, and in there is an ".ill" file corresponding to the name of your illuminance map for that space. In that ASCII text file is a header explaining the data format, the illuminance map's spatial definition/bounds, and the illuminance data itself, in CSV:
## OpenStudio Daylight Simulation Results file
## Header: xmin ymin z xmax ymin z xmax ymax z xspacing yspacing
## Data: month,day,time,directNormalIllumimance(external),diffuseHorizontalIlluminance(external),daylightSensorIlluminance,pointIlluminance [lux]
0.313154583129337 0.4019080153264 0.762 8.531689383129338 0.4019080153264 0.762 8.531689383129338 8.2589380153264 0.762 0.82185348 0.785703
1,1,01:00:00,0.0,0.0,0.000000000000000e+00,0.000000000000000e+00,0.000000000000000e+00,0.000000000000000e+00,0.000000000000000e+00,...
There is also a file called "merged_space.ill" in the /ts directory which is an ASCII text file where each row is a calculation point in your model, and each column is an hour in the 8,760 (annual hourly) dataset. Issues abound with this guy; first off, the buried location cited above. Next, the data is saved as ASCII. Last, the list of points is not linked with the points' spatial location. You will likely find the individual space illuminance map files more useful for plotting.