Yes, it is true that the wattage of your lighting fixture will determine many things about your growing area, from the dimensions to the stocking density and cooling requirements. However, as lighting technology becomes more powerful and efficient, some of the common rules of thumb are changing.
We have all heard the 4 by 4 ft. rule for a 1,000-W HID, but does this apply for 1,000 W of high-output T5 fluorescent tubes? This and other similar questions illustrate a need to understand the science behind the system—and this is where PAR and DLI light measurements come into play.
By utilizing a PAR light intensity meter, questions like the one above can be answered much more accurately, enabling more optimization in our growing environment.
In the United States, we typically hear units of light referred to in foot candles or lumens, but these units are not ideal when considering plant lighting needs. These units represent the amount of light perceived by the human eye, but plants only utilize a specific range of wavelength for photosynthesis. By measuring light in terms of the human eye we are not gaining a clear view of plant utilization.
Photosynthetically active radiation, or PAR, is light with a wavelength that measure between 400 and 700 nanometers, the spectrum that plants actually utilize to photosynthesize. Interestingly, this range of wavelengths is slightly smaller than that visible to the human eyes.
A PAR meter measures the instantaneous amount of photons (units of light) within the PAR range that are being delivered per square meter per second, this number is abbreviated to the unit "μmol•m-2•s-1." All other conditions being optimal, increasing the level of PAR can increase photosynthesis.
Now that you are convinced that PAR is the way to go, we have to do a little math in order to calculate the amount of light that our crop is getting over the course of the day because PAR is a measure of instantaneous light. This is where daily light integral, or DLI, comes in to our lighting calculations.
DLI is the measure of accumulated PAR over the time that our lights are on, and it is typically expressed in moles per day. The recommended DLI for fruiting crops like tomatoes or cucumbers is between 20 and 30 moles per day, whereas a crop like lettuce only needs around 17 moles per day.
We calculate DLI by multiplying our PAR reading by the total amount of seconds that our lights are on per day and then dividing that number by 1,000,000. So, we would multiply our PAR reading by .0864 if our lights were on 24 hours a day and we multiple that same PAR reading by .0432 if they are on 12 hours a day.
By looking at DLI across the growing area, we can determine the optimal light location vertically and horizontally. For example, when calculating the ideal growing area of a high-output T5 ballast compared to an HID, the optimal light footprint might be larger for the T5 than that of the HID, but the vertical penetration is lower.
This data will help influence the type of growing system you use or pruning that you do. Along the same line, DLI can be used to calculate the efficiency of your plant's canopy. If the top of your plant is receiving 25 to 30 moles of PAR per day, but the bottom is only receiving 10 to 15, you could consider thinning out your upper canopy to allow for better light penetration, or removing what will become unproductive flower buds from the bottom of the plant.
DLI can also be used to calculate the minimum amount of time of your lights are required to keep seedlings and clones productive while reducing electricity costs.
When integrated into a growing system, DLI measurement can even automatically determine when your plants need watering by measuring the accumulated light levels over a certain amount of time.
PAR meters come in a large range of prices and forms, from simple analog displays to digital meters than can integrate with your growing systems and automatically calculate DLI. From simple distance calculations to more advanced ones such as these, PAR measurements can go a long way to optimizing our indoor environment.