In an effort to determine what may be causing the unusual spikes in my Photometer results, I've run the Photometer almost evryday in a different orientation. Today, the Photometer was oriented with the DB9 connector pointing south.
This time the data shows two spikes, one before local noon and the other after local noon. But unlike prior data, the spikes are for different colors - blue in the morning and red in the afternoon.
Obviously, I have more tests to run. Perhaps I'll tilt the Photometer next.