I am using the sketch as per your tutorial available hear http://learn.adafruit.com/tsl2561/
I am reading both luminosity and calculating the lux.
I note that these diodes were designed to measure ambient light - my project is using one of these sensors to measure the light from 3w led's but I also have the same problem in sunlight.
Unfortunately, both the IR and the Full Spectrum max out at around the 4935 figure, with the result that the visible light reduces to zero - as the lux is primarily dependant on the visible light it also reduces to zero.
I also found a reference to the same problem here http://itp.nyu.edu/physcomp/sensors/Rep ... uitTSL2561