Light energy is relevant to other factors too.  The ET calculation ( evapotranspiration ) for irrigation scheduling uses solar radiation as a key variable.  Leaf wetness periods or high humidity, which affects disease pressure, can be mitigated with sunny days versus cloudy days. It is essential that growers understand this important variable in order to efficiently produce quality plants.

WHAT IS LIGHT?

Waves: Light is electromagnetic waves. The wavelength is measured in nanometers (abbreviated nm). Particles: Light is photons, which are a quantum, or individual unit. Since individual photons possess tiny amounts of energy, photons are measured in units of moles (abbreviated mol), which are each 6.02 x 1023 photons. Micromoles (abbreviatedare one-millionth of a mole

QUALITY

Photons have different amounts of energy, determined by their wavelengths. Light quality is the relative number of light particles at each wavelength. Light quality refers to the spectral distribution of light, or the relative number of photons of each portion of the light spectrum (visible and invisible) emitted from a light source.   

QUANTITY

Light meters measure light intensity – the instantaneous amount of light delivered to an area.  Units include foot candles and lux (for people), Watts/m2 (for solar radiation) and mole(for plants)

If photons were raindrops, light meters would show the intensity of a rainstorm. A five minute rainstorm may look impressive, but often provides less water than an all-day drizzle.  As cumulative rainfall is measured with a rain gauge, the cumulative quantity of light is measured using a light sensor with a data logger. 

Daily solar radiation is often measured in MJ/m2day.  The daily total of quantum light is called the Daily Light Integral, or DLI, and is measured in units of mol/m2day.  DLI quantifies the light available to plants to perform photosynthesis. On a sunny winter day in the middle latitudes, a plant receives about  9 moles/day. 

If it is cloudy, the DLI drops to  3 moles/day. In the summer, the DLI for a sunny day is about 26 moles/day and 12 moles/day for a cloudy day. Each type of plant has a different DLI range for optimal growth. DLI is directly correlated with plant quality, and a minimum amount of light is required for marketable plants. Measuring DLI over a growing season and comparing it to results can help a grower decide which varieties work for his or her location.    

In conclusion, the poly that covers your greenhouse is  fundamentally important to crop production. Plant growth and developments significantly influenced by both the quantity and the quality of light.