Identify best light level of healthy plants Photosynthesis & Plant Growth depend on the sun or specially designed lighting systems. But of that light reaching the plant, only specific types of energy (in the lighting spectrum) drive the photosynthesis process. Since light is the most important single factor affecting your plants' life cycles, quality light meters are an essential tool in determining accurately if your crop is receiving the maximum light needed for healthy growth. A light meter allows commercial growers an accurate means of adapting to their plants' needs as the light conditions change during a full growing season.