Vernier Tech Info Library TIL #1143

Question

The Light Sensor reading fluctuates, but the light appears steady

Answer

The problem is usually 60 or 120 Hz flicker of the light level from fluorescent 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.

To see if this is the problem try one of the following:
1. Eliminate all artificial light sources (except battery powered flashlights) and try the experiment.
2. Do the following 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).

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. On Logger Pro 3, just turn on Oversampling. You can also set the sample rate to a number not a factor of 60 to improve things. (Using 30, 20, 15, 10, 5, 2 samples per second would be worse then using 17, 23, 27 samples per second.)

Created by: dvernier on February 04 2003
Last updated by: jhopkins on April 28 2006