The 150w high pressure sodium (HPS) fixture illuminating the tree below keeps it healthy and growing. These HPS lights are used throughout the commercial horticulture industry because of their efficiency in regards to how many lumens are emitted per watt of input. This is explained in the diagram below which you may find on the back of a light bulb’s packaging that represent equivalent lumen output in terms of multiple of more familiar lighting sources.
Horticulture light applications would be trivial if lumen output is all there was to know when selecting lighting solutions. Lumens only give you a holistic perspective of the magnitude of light being emitted from a light source. Therefore, Lumens are only useful for comparing overall radiation performance of light sources and should not be used to communicate light intensity needed for optimum plant growth. Plants are only in need of light, or more specifically photons, with a wavelength between 400nm and 700nm. Photons with wavelength between these bounds are classified as Photo synthetically Active Radiation, or PAR.
High Pressure Sodium Spectral chart
The chart above shows relative intensities emitted by the HPS bulb that is being used on the tree above. The wavelengths are displayed on the horizontal axis. The vertical axis is normalized intensities to show relative magnitudes of intensity for different frequency photons that are emitted from these HPS bulbs. The vertical units are more clearly marked in the figure below. This spectral chart below is an illuminated Metal Halide (MH) bulb. When comparing these two spectral outputs, it is easy to observe a common understanding that HPS puts out more radiation observed as red and MH emits more radiation within the blue band.
Metal Halide SPectral Chart
To measure PAR we need to define a unit of measure to numerically quantify a measurement. To do this we must first define a surface with an area, A. Now we simply define how many photos, with select wavelengths between 400nm and 700nm would be striking this defined area. This can be measured on an instantaneous basic with a quantum meter that will typically read Photosynthetic Photon Flux Density (PPFD). Flux is an amount of particles permeating or penetrating a defined surface. In the figure above, the 2D surface area is defined as A, and the flux term in this figure is E, which signifies the one second long summation of penetrating photons.
Quantifying PAR over a plants lifecycle
Now that we have an instantaneous unit of measure, we can lend some understanding to how much relevant light a plant is receiving during the length of one day. The Daily Light Integral (DLI) is the amount of PAR received each day as a function of light intensity and duration. It is expressed as moles of light per square meter per day .The DLI concept is like that of a rain gauge. Just as a rain gauge collects the total rain in a particular location over a period of time, the DLI measures the total amount PAR ”captured” on a surface in one day. Once you have recorded the PPF from the sun striking the ground over the course of one day, you can chart it like below. The integral can easily be understood as simply summing this graphical area between the plotted PPF and the x-axis. DLI can be easily recorded with a quality quantum meter placed stationary of a period of time.