RE: pasco light probe oscillations

["John Gastineau" <gastineau@mindspring.com>]

> explanation. With the polaroids at some arbitrary position, and everything
> left untouched, the reading from the probe oscillates with a
> period of about
> 15s and an amplitude about 5%. This goes on consistently for
> hours. I assume

This sure sounds like a data collection artifact.

What data collection rate are you using? With an incandescent light source
(and maybe a laser with a ripply power supply) you'll get a 120 Hz (100 Hz
in 50Hz AC power countries) flicker in light intensity. If your data
collection rate and the flicker rate are almost, but  not quite, the same,
you get beat frequency effects. In the US, a 100Hz data collection rate used
with a light sensor on a halogen light, AC powered, gives a nice 20Hz beat
wave. If the time base of the Pasco unit is just a bit different from line
frequency (bad luck), you might get a much longer beat period.

Of course, if you're using a battery for the 12V supply of the light, I'll
retract all this... Try a battery, and see if the effect is still there. Or
use a higher data collection rate.

The voltage reference in the particular interface you're using could also
have a time variation. I don't see a corresponding long-term variation with
my light probe used with a ULI.

JEG
 __________________________________

John E. Gastineau      gastineau@mindspring.com        	KC8IEW
900 B Ridgeway Ave.    http://gastineau.home.mindspring.com
Morgantown WV 26505    (304) 296-1966 voice    (304) 296-5035 fax
USA