EVERYTHING YOU WANTED TO KNOW ABOUT THE ELECTROMAGNETIC SPECTRUM

... but were afraid to ask!

A generalized composition of our Sun's radiation.
The vast majority of the electromagnetic radiation emitted by our Sun is in the form of infrared, visible-light, and ultraviolet. Click diagram to expand.

Greetings! This is the first in a four-part series on the use of cutting-edge research, digital imaging technology, autonomous aeronautics,  geographic information systems, and positioning technology as a force-multiplier for the talents of the modern vineyard manager.

This week’s installment is a synopsis of the nature of the electromagnetic spectrum – a knowledge primer to use as a springboard to better understand the phenomena making reliable observation of plant health that beyond the capability of the unaided human eye a reality. 
 
Many vineyard management and other wine industry professionals have encountered references to the electromagnetic spectrum, or ES. 
Within the viticulture space, you may be aware of the use of specialized cameras to analyze light reflected off foliage to determine plant health. In winemaking, many of you are familiar with spectrophotometry, involving light of certain color ranges being shone through a sample of wine, indicating the presence and concentration of certain compounds.  
 
Both of these techniques make use of the electromagnetic spectrum’s interaction with matter that we want to study and gain insight into. Much like the microscope and telescope before them, multispectral cameras and spectrophotometers extend our ability to understand parts of our world that are invisible to the naked eye – creating a kind of extrasensory insight. 
 
The electromagnetic spectrum, as a topic of discussion, is infinitely complex. Minds no lesser than that of Albert Einstein's grappled with its finer points. However, this blog post will bypass any treatment of arcane particle physics – instead focusing on the practical dimensions of the subject.  
 
The electromagnetic spectrum is all around us - without it, life as we know it could not exist; to wit, the plants at the base of the food chain rely directly on the ES as the energy source for photosynthesis. 
Electromagnetic radiation is generated by stars like our sun, yet we also create it with our technology – radio stations, toasters, light bulbs, X-ray machines, and the detonation of atomic bombs all emit electromagnetic waves that are part of the ES. Even our bodies contribute, in the form of radiant heat. 
 
Electromagnetic radiation takes the form of a continuous stream of tiny, energy-containing particles called photons, that move at the speed of light. These photons oscillate, or move up and down vertically, along their horizontal path. This results in the stream of photons developing a wave pattern. The more energy these photons contain, the more rapid their oscillation, resulting in greater frequency and a shorter wavelength.   Here is a useful simplification, in the form of a pseudo-equation:

Higher Frequency = Shorter Wavelength = More Energy.

The portions of the electromagnetic spectrum are classified by wavelength – with very long waves used in radio communications, mid-length waves to run our microwave ovens and infrared heaters, and very short waves running our CAT-scan machines. 
 
What does this all have to do with the vineyard, you ask? Lots! But first, we must put the vineyard in context. All portions of the ES are either helpful or harmful to life on earth, but the most fundamental portion of the spectrum to life as we know it is a tiny slice in the range with wavelengths between ½ millimeter and 1 / 2,500th of a millimeter. Not all of this range is visible to the human eye, which has evolved to detect only a “Goldilocks zone” of wavelengths carrying an amount of energy that is “just right”. We perceive this range of wavelengths as the familiar ROYGBIV color spectrum – also known as visible light. We have evolved to perceive this range of electromagnetic radiation due to the heat of the surface of our Sun - about 9,980 degrees Fahrenheit - that emits 95% of radiation that strikes the surface of the earth in wavelengths that we see as visible light, and feel as infrared radiation. 
Infrared radiation, which occupies the ½ millimeter to 7/10,000th of a millimeter range, is sometimes referred to as “light”. This is technically incorrect, as it is invisible to our eyesight (see diagram) - we sense this wavelength range as radiant heat. 
 
So, on to the vineyard! The photosynthetic foliage of plants has selectively evolved pigments that absorb wavelengths of solar energy in the range we perceive as red and blue, due to their greater suitability for driving the photosynthetic process - and reflect the energy waves of lengths we perceive as green. These reflected “green-length” waves make their way to our eyes and

 
 A generalization of photosynthetic foliage's interaction with solar electromagnetic radiation. Wavelengths in the violet, blue, orange, and red ranges are strongly absorbed by pigments in plant tissue, where they are a fundamental innput to photosynthesis, flowering, and leaf orientation. Wavelengths in the green, yellow, and infrared ranges is reflected and/or transmitted, thus detectable to sensors such as a multispectral camera (or the human eye, in the case of green and yellow only.)

 A generalization of photosynthetic foliage's interaction with solar electromagnetic radiation.
Wavelengths in the violet, blueorange, and red ranges are strongly absorbed by pigments in plant tissue, where they are a fundamental innput to photosynthesis, flowering, and leaf orientation.
Wavelengths in the greenyellow, and infrared ranges is reflected and/or transmitted, thus detectable to sensors such as a multispectral camera (or the human eye, in the case of green and yellow only.)

cause us to perceive the familiar color of healthy foliage.  
Infrared radiation that is carried by waves just longer than that of red light – yet not short enough to be visible -  are known as near-infrared. Like green-length waves, they are not absorbed by any pigments present in plant foliage, so they are reflected or transmitted (pass through the mesophyll cells to the ground below). The healthier, more turgid, and denser the plant foliage, the greater the proportion of near-infrared radiation that is reflected rather than transmitted. Thus, to the observer above the plant (read: us) reflection of more infrared and green, and less red and yellow, morereflected indicates more vigorous foliage.  
 
But there are two problems here. The first is obvious: our eyes cannot detect this near-infrared radiation. The second is more complex: what is the reference point constituting strong or weak reflectance? Fortunately, scientific research and technological advancement has been directed at these problems over the past century or so, and we are currently refining and researching the discoveries advanced by full-spectrum photography to drive incredible insights into plant health.  

A general depiction of how photosynthetic foliage interacts with wavelengths in the red, yellow, green, and infrared ranges. - A - Foliage of plants under stress transmits more IR, reflects less green & IR and more red & yellow. - B - Under relatively balanced stress conditions, foliage absorbs red, reflects green, transmits less and reflects more IR, and transmits most yellow. - C - Under healthy, vigorous conditions, foliage strongly absorbs red, and strongly reflects green and IR. Yellow is almost completely transmitted .

A general depiction of how photosynthetic foliage interacts with wavelengths in the red, yellow, green, and infrared ranges.
- A - Foliage of plants under stress transmits more IR, reflects less green & IR and more red & yellow.
- B - Under relatively balanced stress conditions, foliage absorbs redreflects green, transmits less and reflects more IR, and transmits most yellow.
- C - Under healthy, vigorous conditions, foliage strongly absorbs red, and strongly reflects green and IR. Yellow is almost completely transmitted .

The first problem began to be solved sometime in the first half of the 20th century with the development of film sensitive to infrared radiation. The second problem involves the development of algorithms. Healthy plants of a given species will reflect green light in a very tight and predicable wavelength range – visible in the very homogenous green color of a vineyard, for example. As a plant’s health deteriorates, the range of wavelengths reflected by its foliage begins to shift from green to yellow – a familiar sight. In addition, the red light that would be absorbed by the plant under healthy conditions begins to be reflected. This is the cause of the familiar yellow/brown appearance of an unhealthy plant.  But before this becomes visible to the human eye, the level of reflectance of infrared radiation begins to drop. Thus the relationship between infrared, red, and green reflectance can be expressed as a ratio. Changes in this ratio are a signal that serves as a very reliable early warning to changes in plant health before they become visible. 
 
Stay tuned for next week’s installment – how specialized multispectral cameras can be put to use in reliably recording this change in ratio!