Horticulture lighting standard ANSI/ASABE S640

The American Society of Agricultural and Biological Engineers (ASABE) published the first of three standards for the horticulture lighting industry on August 8, 2017, bringing much needed codification to horticulture lighting technology. ANSI/ASABE Standard S640 titled "Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms)" establishes quantities and units used to describe light in relation to plants. Standards are important to the industry because they help everyone get on the same page with regard to the language used to describe the technology, the units of measure for lighting, metrics used to market products, and methods of bench-marking performance. 

The first of three standards, ANSI/ASABE S640, covers units of measure used to describe horticulture lighting. We took a look a the standard and summarized some key points:

1. PAR (photosynthetic active radiation) is a unit of measure of radiation relevant to plant growth, falls in the wavelength range of 400-700nm, and is expressed in two terms:

  • PPF - Photosynthetic Photon Flux - PAR emitted by a source, measured in units of micromoles

  • PPFD - Photosynthetic Photon Flux Density - PAR that falls on a unit of surface area

2. There are several ways to describe the wavelength portion of a PAR measure; these include Photosynthetic (400-700nm), UV (100-400nm), Far Red (700-800), and spectral (100-800nm). This results in terms such as Far Red Photon Flux Density, UV Photon Flux Density, or Spectral Photon Flux Density - in addition to Photosynthetic Photon Flux Density (PPFD). 

3. A measure may use two high level types of units to describe radiation: Radiate units (a quantity of energy) or Quantum units (a quantity of photons). This means Photosynthetic Photon Flux and Photosynthetic Radiant Flux both describe the same thing, however the first is expressed in micromoles (or µmol, a quantity of photons) and the latter is expressed in watts (W, a unit of energy). 

4. Far red light falls between 700 and 800nm

5. UV light is divided into three bands:

  • UVA - 315-400nm

  • UVB - 280-315nm

  • UVC - 100-280nm

6. There are two distinct ways to plot a PAR spectrum:

  • SPD, Spectral Power Distribution, a plot of PAR against wavelength, expressed in the units of radiant watts

  • SQD, Spectral Quantum Distribution, a plot of PAR against wavelength, expressed in the units of micromoles

7. Daily Light Interval is a measure of PPFD over a 24 hour period


The standard discusses rationale behind several decisions, and notes that there is currently no accepted interpolation of bands across the PAR spectrum (as is the case with the UV spectrum). We have provided a brief summary of key components of the standard; we suggest readers purchase and read the full standard for a comprehensive overview of the units used to describe PAR. 

Whats next?

  • The Design Lights Consortium (DLC) will publish draft policy for energy efficiency in horticultural lighting in September 2018. This will create uniform requirements for energy rebates and incentives among utility providers

  • ASABE will publish the next horticulture lighting standard some time in 2018

Further reading:


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