The most common problem is that the power supply driving the light source is noisy, causing a 60 or 120 Hz flicker of the light level from fluorescent, incandescent, or other lamps.
Your eye does not pick this up, but the sensor will. You may be thinking that you should not see this flicker, since you are sampling only slowly, but due to the way most of our interfaces collect data, the flicker may show up even at slow rates.
Even if you measured the supply voltage with a multimeter you might not see the noise; multimeters are designed to average the data and update their display only once a second or so. Sometimes the noise can be measured on the multimeter by using the AC mV setting, but a multimeter usually masks the effect by averaging several readings before displaying a value.
To see if this is the problem, try one of the following:
– Eliminate all artificial light sources, and try the experiment in daylight or with a battery powered flashlight. The reading should be stable.
– Perform a quick test with the light sensor hooked up and positioned as you plan to use it. Set the sampling to be 1000 points/second for 0.1 seconds. If the flicker is the problem, you will see a drastic variation in the light intensity with a period of 60 or 120 Hz (50 or 100 Hz outside North America).
– Sample at higher frequencies. When collecting data from a noisy signal like this, the noise typically does not look like a sine wave on the screen because LabPro’s sampling rate is set much lower than the frequency of the noise in the signal. However, faster sampling rates can alias the signal and produce a nice sinusoid at a much lower frequency than the sampling rate.
– If you cannot use a more stable power supply for your lamp, you will need to average the Light Sensor readings (using a calculated column) to get a quieter reading. The Light Sensor is simply reporting the actual variations in intensity.
– To minimize this problem in Logger Pro, turn up the Oversampling in dialog box where you control the data collection rate.
– In Logger Pro 2x, set the oversampling to as large a number as you can.
– In Logger Pro 3, just turn on Oversampling.
– You can also see improvement if you set the sample rate to a number that is not a factor of 60. (Using 30, 20, 15, 10, 5, 2 samples per second would be worse then using 17, 23, 27 samples per second.)