If you are monitoring fluorescent lights, part of what you are seeing may be due to a fairly common attribute of fluorescent lights. When the filaments of a fluorescent lamp and the gas inside the lamp are not up to the normal running temperature (normally the first few minutes after startup), they are likely to flicker a bit.
Typically, the flicker during the first minute or two can be recorded by the data logger, due to the data logger's relatively fast response time. These data points are often unexpected and undesirable; unfortunately, there is no way to remove them from the data file, as they are actual events.
If your data shows more than just initial flicker, lower-level lighting conditions can exacerbate the effect of a fluorescent tube flickering on and off with each cycle of the 60 or 50 Hz line frequency. Electronic ballasts overcome this because they step up the frequency applied to the lamps to increase their efficiency and virtually eliminate lamp flicker.
The light sensor is directional and should face the light source. The light sensor is the clear LED on the side of the case; however, the sensor detects light directed at the front (label side) of the case. On the U9 there is a sensitivity control adjustment on the front of the logger that needs to be adjusted per application. When it is turned fully clockwise, the HOBO is at its most sensitive light level. Please read the section on setting the sensitivity in the User's Manual and be sure to run a test deployment prior to setting up your study.
When using the UX90, be sure to follow the calibration procedure in the manual: UX90-002 Manual
You may also need touse the option light pipe: Light Pipe Manual