There are several ways of measuring the rate of photosynthesis in the lab. These include:
Several factors can affect the rate of photosynthesis:
Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide.
Increasing the light intensity increases the rate of photosynthesis, until some other factor - a limiting factor - becomes in short supply.
The rate of photosynthesis is directly proportional to the light intensity until another factor becomes limiting.
At very high light intensities, photosynthesis is slowed, but these light intensities do not occur in nature.