Phalaenopsis, a member of the orchid family, stores CO2 at night (dark period) and consumes it during pho-tosynthesis the next day (light period). The critical moment, when nearly all stored CO2 has been consumed during daytime, can easily be detected with the sensor, as the photosynthesis-efficiency will decrease sharply.
These effects are illustrated In the following graphs. Graph 1 shows that extra light given around 12:00 has no effect on efficiency, because there is still enough CO2 available to be consumed. Graph 2 shows that extra light given at the end of the day has a strong negative effect on photosynthesis, because all CO2 has already been consumed. The extra light results in stress and internal damage, and even in lower photosynthesis the next day