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If you could measure the quality as well as the quantity of natural light entering your greenhouse or growing dome, you could supplement with the kind and amount of artificial light needed for optimal plant growth. The colors preferred by plants for rooting, stemming, branching, leafing, flowering and seeding vary just as they do for the temperature, atmospheric pressure, gas envelope, hormonal balance, and nutrient complement.
While there are many moderately priced systems for measuring the amount of light, measuring spectral quality has long been elusive for hobbyists and start-up entrepreneurs. Low cost, Internet of Things computers such as the Nucleo, GeekCreit, BeagleBone, Seeed Studio, AdaFruit, DFRobot, Elegoo, Raspberry-Pi, Arduino, SparkFun, and SoraCom can serve data loggers using a wide variety of sensors. They can communicate via cellular services, WiFi, or hard-wired home scaled networks. They can be polled by a more robust system capable of doing the required analysis and effecting some action such as running artificial lighting systems for remediation or compensation for any shortfalls in the natural lighting.
Sensors have been the most cost-prohibitive components since the cost of networks and the polling computer are typically shared with that of other applications. When one’s approach to gardening technique is characterized by a pinch of this and a dash of that, light balancing has long been relegated to the category of mystic art. Control has been determined by a simple clock.
Photosynthetically Active Radiation (PAR) plays an active role in your cultivation efforts while also controlling forest carbon and water quality. Research quality PAR sensors, that are calibrated to National Institute of Standards and Technology (NIST) specifications, are expensive. There are however, ways to beat the $300 – $1200 cost of a laboratory quality sensor. The focus for this little monoscussion is on just how the most expensive and the cheapest sensors can be calibrated to some standard using a simple setup.
When that standard is informed by your experience as a grower together with the natural light that enters the growing chamber, your analysis will more likely be based upon your observations and supported by the logged data. Your decisions will be along the lines of “I want more of this and less of that.” When you start measuring the relative progress of plants adjacent to the North, East, South, and West windows, your standard will evolve as you try to give your plants the agricultural equivalent of personalized medicine.
Standardizing your own PAR sensors can be achieved by simply measuring the light reflected off of a white surface from a light source where consistency can be assured from one test to the next. You have a choice between balancing the output of sensors subjected to this reflected white light without filters, or measuring through color filters. The essential point is that whatever you do, do it the same way next time. Measuring at multiple windows within the same greenhouse, or from a few windows across a widely distributed group of greenhouses, calibrating the measurement systems prior to deployment will give you reliable, actionable intelligence.
As for the sensors themselves, they may be the kind that output a variable current or a variable frequency. Simple LEDs can convert light energy to electrical energy just as they can convert electrical energy to light. All of the sensors can be placed behind precision photographic filters to divide the whole white light into bands for measuring either the red-green-blue or cyan-magenta-yellow components of whole light. These additive and subtractive primaries are used extensively in scientific, photographic, and video-graphic applications so the consistency within a particular brand of filter is reliable.
Photosynthetically active radiation (PAR) is defined as light with wavelengths between 400–700 nanometers. As one might expect, PAR has high spatial variability, especially in rugged terrain that features some combination of forested, cleared, and developed areas. Other variables include the accuracy and stability of the sensor array. The less expensive sensors tend to exhibit less consistency at low PAR levels.
As a grower, the low light levels may not be of serious concern as you are less likely to be troubled about the plants getting more than enough light. Then again, the cost of running artificial light to compensate, where no compensation is required, may justify a closer look or more expensive sensors in a large scale system.
A simple and inexpensive device for logging photosynthetically active radiation is well within reach for most growers. Complementing that measurement effort with a system capable of running artificial lighting at just the right color and for just the right time is also doable. Reverse engineering comprehensive systems to achieve modularity will make it possible to swap out components such as sensors, signal converters and amplifiers, micro-computers, and code as needed.
Evolving such a system can be rewarding personally and financially, as well as gastronomically.