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Light, light, light.

English is full of words with multiple meanings. We can sit at a table while studying a table of data. We cast a fishing line, put a cast on a broken leg, and applaud the cast of a play. Rulers can lead a country or measure a piece of furniture. We have decks of cards and decks of ships. These and many other multiple-meaning words are part of what makes English fun.

Consider light, the word. We can turn off the lights. See things in a different light Have light hair. Travel light. Eat a light meal. Drive a light-duty truck. Light a fire. Have a light touch. Make light of something, or shed light on it. We can even say, "hey, buddy, got a light?" In normal conversation these many uses of light cause no problems.

In matters of science, however, we have more rigid needs for specificity. Light is a basic term in a multidisciplinary field of science, technology, and application that includes physicists, chemists, psychologists, physicians, engineers, designers, sleep specialists, and many others. During our lives, light, as a concept, has always had a strange duality. Is light radiant energy, a human response to that energy, or both? Consider this definition from ANSI/IES RP-16-10 Nomenclature and Definitions for Illuminating Engineering:
  Light: Radiant energy that is capable of exciting the retina and
  producing a visual sensation. The visible portion of the
  electromagnetic spectrum extends from about 380 to 770
  nanometers. Note: The subjective impression produced by
  stimulating the retina is sometimes designated as light. Visual
  sensations are sometimes arbitrarily defined as sensations of
  light, and in line with this concept it is sometimes said that
  light cannot exist until an eye has been stimulated. Electrical
  stimulation of the retina of the visual cortex is described as
  producing flashes of light. In illuminating engineering, however,
  light is a physical entity radiant illuminating engineering,
  however, light is a physical entity radiant


The definition makes it clear that light is radiant energy, that is, something rooted in physics. But the definition qualifies and confines light to that which produces a visual sensation. Light, by the IES definition, is radiant energy weighted by the luminous efficiency function, V([lambda]). Which means that--even as a physical stimulus--light cannot be defined without reference to a human response, or, more precisely, without reference to a mathematical representation of the human visual response. This duality is awkward, and sometimes confusing. Still, we've learned to live with it and context is usually enough to infer the intended meaning.

But, we now also know that these same wavelengths cause circadian, neuroendocrine, and neurobehavioral responses. Is it correct to use "light" to describe radiant energy that produces nonvisual responses? If we consider the complete IES definition, the answer must be no. The definition relates light to the visual cortex and the luminous efficacy function, and neither represents nonvisual responses.

Ambiguous definitions hamper clear communication, especially across disciplines. When different people use the same word to mean different things, miscommunication is inevitable. In scientific circles and in modern lighting practice, light--as a word and concept--is now employed in three very different ways:

Meaning must be inferred from context, but context is often not enough. Or, more problematic, not all people that work with light are aware of its triality. Errors result when concepts are mixed. Take light as both visual and biological responses. The peak wavelength for lumens and lux is 555 nm, whereas circadian phototransduction is more strongly influenced by shorter wavelengths. The spectral power distributions (SPD) for the orange-red and blue-green LEDs illustrated in Fig. 1 produce equivalent lumens. Yet, according to the hypothesized model of Rea and his colleagues [Rea et al. 2005, 2007], which considers the neuroanatomy and physiology of the human visual and circadian systems, the blue-green SPD will be 250 times more potent to the circadian system.

In medical studies that endeavor to show causal relationships between light and human health, light is often characterized with lux. This is a fundamental error that slows scientific progress. Light, the photometric concept, is not interchangeable with light, the biological concept, and language is partly to blame.

As a way forward, the communities of lighting might consider retaining light for casual use, while employing precise language when demanded for clarity. When considering light as a physical quantity, "optical radiation" has greater specificity. It is defined as:
  Optical Radiation: Electromagnetic radiations having wavelengths
  between approximately 100 nanometers and one millimeter.
  [IES 2010].
  * Physical quantity: Light is radiant energy in the wavelength
  region of 380 - 780 nm. In this sense, light is entirely separate
  from a human observer.
  * Visual response: Light is the perceptual
  response to radiant energy in the wavelength region of 380 - 780
  nm. In this sense, light is related to both the nature of the
  radiant energy and the nature of the human visual system.
  * Biological response: Light is the biological response to
  radiant energy in the wavelength region of 380 - 780 nm. In
  this sense, light is related to both the nature of the radiant
  energy and the nature of the nonvisual pathways that


[FIGURE 1 OMITTED]

Terms do not exist for the visual and biological responses, so new terms would need to be adopted. Perhaps pharos for the visual response and helios for the biological. These words were first suggested by Parry Moon and Domina Eberle Spencer in their proposal to form a new international lighting vocabulary after World War II, albeit, with different usages than proposed here [Moon and Spencer 1946]. In Greek mythology, helios was the personification of the sun. Pharos was a small island off the coast of Alexandria upon which a lighthouse was built, known as the Pharos of Alexandria and one of the Seven Wonders of the Ancient World. Both terms share the advantages of being similar across many languages and having etymological roots in lighting.

New terms would inevitably meet with resistance. After all, it would take time to get used to calling light by any other name. But English is a living language. The Oxford English Dictionary now recognizes sexting, retweet, woot, trolleyed, and buttload among many other dubious entries. Might helios and pharos be more useful additions to our lexicon?

[IES] Illuminating Engineering Society. Nomenclature Committee. 2010. ANSI/IES RP-16-10 Nomenclature and definitions for illuminating engineering. New York (NY): Illuminating Engineering Society. 119 p.

Rea MS, Figueiro MG, Bullough JD, Bierman A. 2005. A model of phototransduction by the human circadian system. Brain Res Rev. 50:213-228.

Rea MS, Figueiro MG, Bullough JD, Bierman A. 2007. Corrigendum to "A model of phototransduction by the human circadian system". Brain Res Rev. 56:270.

Moon P, Spencer DE. 1946. A study of photometric nomenclature. J Opt Soc Am. 36(11):666-675.

With the previous issue, Prof. David DiLaura retired from his role as Editorin-Chief, a post he held for 8 years. Under his leadership and toil, the journal of the Illuminating Engineering Society became LEUKOS. He has my admiration and thanks.

KEVIN W. HOUSER, PHD, PE
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Author:Houser, Kevin W.
Publication:Leukos
Article Type:Editorial
Geographic Code:1USA
Date:Jul 1, 2011
Words:1169
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