Resources | Standardization | CIE | Horticulture | Sep 17, 2019

Horticultural Lighting - The Root to International Standards

Plants have evolved to make the most of the conditions in which they naturally grow. For some plants, this environment will be fully supportive; it may be less so for others. In general, the plant with the fully supportive conditions will predominate in the absence of external intervention. Humans have changed plants over the centuries, selectively growing the plants that provide maximum perceived benefits in terms of color, smell, yield and, perhaps more recently, nutritional value. For most of our evolution, we have had to make use of the environmental conditions available. The desire for cheap food, all year around, and attempts to minimize "food miles" has increased the move towards providing totally constructed growing environments.

Division 6 of the International Commission on Illumination (CIE) is responsible for photo-biology and photo-chemistry. Most of the Division's work has concentrated on the effects of optical radiation (this being radiation from the ultraviolet through to the infrared) on people. However, the CIE has been encouraging international work on the impact of optical radiation on plants and specifically for horticulture since at least the 1980s. CIE Technical Committee (TC) 6-42 covered "Lighting Aspects for Plant Growth in Controlled Environments" and TC 6-61 "Measurement of Radiation Using the Phytometric System for Plant Applications". Considerable development work was carried out by these TCs, but the decision was taken to defer their activities in 2015, pending discussions with other organizations.

The CIE International Lighting Vocabulary [1] already includes terms for some aspects of horticultural lighting and these are used by a number of national voluntary standards, such as those published by the American Society of Agricultural and Biological Engineers (ASABE) [2].

A lot of effort has been put into understanding the complexities of growing plants under electric lighting. Kniep and Minder [3] are generally credited with the first attempts at defining an action spectrum for the absorbance and quantum yield for plants. Hoover [4 ]and then McCree [5] built on this early work. From a scientific perspective, this certainly seems to be an ideal starting point, particularly as one of the fundamental laws of photo-chemistry is that a photon of optical radiation has to be absorbed before a photo-chemical reaction can occur. What McCree discovered is that the action spectrum varied between plant species, indeed also between plants grown outdoors and those grown indoors, but that there was a general envelope of response over the wavelength range 350 nm to 750 nm.

Of course, the optimum growing conditions for plants depend on many more factors than the spectral composition of the light and there may be some synergy between these other factors and specific wavelengths of exposure. It is also likely that the response of the plant may be dependent on multiple wavelengths from different parts of the optical spectrum, either working together or against each other. Therefore, it may be important to consider the effects of not including some wavelengths, as well as ensuring that the key wavelengths are included.

The dosimetry required to assess the exposure of plants is complex in practice. The key part of the work of CIE TC 6-61 had been an attempt to address this. During stages of growth, during the season and even during the day, the spectral power distribution of optical radiation falling on a given leaf will vary. As plants grow, they will tend to influence the spectral power distribution received by neighbouring plants and indeed by their own leaves. This will be true for plants growing naturally outdoors, for those growing in greenhouses under sunlight and those grown completely under electric lighting.

Clearly, plants respond differently to light than people, which means that familiar quantities such as the lumen, based on the V(λ) spectral luminous efficiency function, are not useful to horticulture. The American Society of Agricultural and Biological Engineers, in ANSI/ASABE S6402, has suggested that a single action spectrum is not appropriate because it is highly variable between plant species. Therefore, it recommends that photosynthetically active radiation (from 400 nm to 700 nm) should not be weighted for most horticultural quantities. This effectively means they are recommending an action spectrum with a weighting of one from 400 nm to 700 nm and zero outside those wavelengths.

In addition to growth, different wavelengths of optical radiation are responsible for particular aspects of a plant's development, including the development of any required flowers and fruit. LEDs have provided the opportunity to vary the light spectrum throughout the stages of a plant's life, taking into account the varying spectral needs, length of time of light exposure in a given day and the number of photons incident on the plant. However, the science is still maturing and there is an element of "try it and see". In time, it may be possible to standardize a series of action spectra to assist manufacturers of LED products and growers with optimizing the light exposure conditions.

The CIE is a technical, scientific and cultural non-profit organization whose first objectives is: to provide an international forum for the discussion of all matters relating to the science, technology and art in the fields of light and lighting and for the interchange of information in these fields between countries. Until recently, most of the work of the CIE has focused on lighting for people, but of late, the CIE has initiated discussions with a number of professional and trade bodies involved with horticultural lighting.  An outcome of these discussions is a workshop on Horticultural Lighting, which will be held as part of the conference program of the CIE's 29th Quadrennial Session in Washington DC in June 2019. Various aspects of what is known and what is not known about providing evidence-based horticultural lighting will be discussed [6].

Another matter to consider in conjunction with horticultural lightning is the need of people who work in areas that are set up primarily for horticultural benefits and the impact of the lit environment on those people. National legislation is likely to restrict exposures to levels below exposure limit values. However, consideration also needs to be given to any impact on both the ability to undertake visual tasks and any non-visual effects of the lighting on the workers.

Starting from this basis of the CIE's scope and activities, and the CIE's standing as an international standardization body recognized by ISO, now is the time for the CIE to bring to fruition standards, reports and other publications concerned with horticultural lighting – starting with the roots (the basic science).

Anyone interested in supporting this work, as an organization or as an individual, can contact the CIE to show their interest by sending an email to

[1]    CIE S 017/E:2011. "International Lighting Vocabulary"
[2]    ANSI/ASABE S640 Jul2017. "Quantities and Units of Electromagnetic
        Radiation for Plants (Photosynthetic Organisms)