Beam angle

{{Short description|Width of the beam of light emitted by a light}} {{For|the concept in physics and engineering|Beam width}}

'''Beam angle''' is a measure of angular width or diameter of the beam of light emitted from products used in architectural lighting, landscape lighting, stage lighting, and other lighting applications.<ref name="NLPIP">{{cite journal |author=LRC |date=Oct 1994 |title=Reflector Lamps |journal=National Lighting Product Information Program (NLPIP) Specifier Reports |volume=3 |issue=1 |location=Troy, NY |publisher=Rensselaer Polytechnic Institute Lighting Research Center |url=http://www.lrc.rpi.edu/programs/NLPIP/PDF/VIEW/SRReflect.pdf |archive-url=https://web.archive.org/web/20031212194156/http://www.lrc.rpi.edu/programs/NLPIP/PDF/VIEW/SRReflect.pdf |archive-date=12 Dec 2003}}</ref><ref>{{cite book |author=DOE |title=CALiPER Application Summary Report 22: LED MR16 Lamps |location=Washington, D.C. |publisher=United States Department of Energy |pages=4–5 |date=Sep 2014 |doi=10.2172/1377013 |osti=1377013}}</ref><ref>{{cite web |title=Lighting Tips & Intuition #72: Beam Angle and Field Angle |url=https://www.renfrodesign.com/page/beam-angle-field-angle |website=RenfroDesign.com |location=New York |publisher=Renfro Design Group, Inc. |access-date=3 Dec 2025}}</ref><ref>{{cite web |last=Primrose |first=Jon |title=Glossary of Terms - Lighting (Advanced) |url=https://theatrecrafts.com/pages/home/topics/lighting/glossary-advanced/ |website=TheatreCrafts.com |access-date=29 Dec 2025}}</ref> It is known to be an imperfect gauge of perceived beam width.<ref>{{cite journal |last1=van Kemenade |first1=Ir. Johan |last2=Reker |first2= Jan |title=Beam Characteristics for Accent Lighting |journal=Journal of the Illuminating Engineering Society |volume=17 |issue=2 |page=128 |date=1988 |doi=10.1080/00994480.1988.10748732}}</ref><ref>{{cite journal |last1=Wang |first1=Q. |last2=Davis |first2= R.G. |title=A Study on New Beam Parameters |journal=Journal of the Illuminating Engineering Society |volume=25 |issue=1 |page=100 |date=1996 |doi=10.1080/00994480.1996.10748138}}</ref><ref name="IEE">{{cite web |date=Jan 2013 |title=IEE Whitepaper: Evaluation of Best-in-Class LED Reflector Lamps |url=https://www.edisonfoundation.net/-/media/Files/IEI/publications/IEE_Evaluation-of-Best-in-Class-LED_Final.pdf |website=EdisonFoundation.net |location=Washington, D.C. |publisher=Innovation Electricity Efficiency, An Institute of The Edison Foundation |page=A-1 |access-date=26 Nov 2025}}</ref> Several definitions of ''beam angle'' are in common use. This article compares them and illustrates how they relate to each other and to related concepts.

==Definitions== [[File:Batwing-intensity-extreme-v0.png|thumb|right|alt=Polar plot of luminous intensity (extreme batwing)|Polar plot of the luminous intensity distribution (yellow) for a luminaire with center-beam intensity much smaller than maximum intensity: beam angle of 94° based on maximum intensity (blue) versus 120° based on center-beam intensity (green)]] [[File:Batwing-intensity-mild-v0.png|thumb|right|alt=Polar plot of luminous intensity (mild batwing)|Polar plot of the luminous intensity distribution (yellow) for a luminaire with center-beam intensity smaller than maximum intensity: beam angle of 144° based on maximum intensity (blue) versus 148° based on center-beam intensity (green)]] The Illuminating Engineering Society (IES) defines ''beam angle'' as the angle between the two directions for which the intensity is 50% of the maximum intensity as measured in a plane containing the nominal beam centerline.<ref name="LS-1">{{cite web |author=IES |date=2022 |title=ANSI/IES LS-1-22, Lighting Science: Nomenclature And Definitions For Illuminating Engineering |url=https://ies.org/standards/definitions/ |website=ies.org |location=New York |publisher=Illuminating Engineering Society |access-date=24 Nov 2025}}</ref> By this definition, which is commonly used,<ref>{{cite book |title=Lamp and Ballast Catalog |location=Danvers, MA |publisher=Osram Sylvania |page=ix |date=2008 |url=http://assets.sylvania.com/assets/documents/Complete-Catalog.b176dbb1-d6e0-40f0-ab92-e768e58f5dc1.pdf |archive-url=https://web.archive.org/web/20121224070808/http://assets.sylvania.com/assets/documents/Complete-Catalog.b176dbb1-d6e0-40f0-ab92-e768e58f5dc1.pdf |archive-date=24 Dec 2012}}</ref><ref name="IEA">{{cite book |last1=Ohno |first1=Yoshi |last2=Gaudemer |first2=Jean |last3=Oh |first3=Jun Seok |last4=Dubard |first4=Jimmy |last5=Jeon |first5=Sangkyoo |last6=Scholand |first6=Michael |title=IEA Energy Efficient End-Use Equipment (4E) Solid State Lighting Annex: Interlaboratory Comparison 2017 Final Report |location=Paris |publisher=International Energy Agency |page=125 |date=20 Dec 2021 |url=https://www.iea-4e.org/wp-content/uploads/2021/12/SSL-Annex-IC-2017-Goniophotometers-Final-Report-2021.12.20.pdf |access-date=14 Jan 2026}}</ref><ref>{{cite book |title=2025 GE Lamps Catalog: LED & Traditional |location=Mayfield Heights, OH |publisher=Current Lighting Solutions |page=172 |date=2025 |url=https://cdn.currentlighting.com/site/brochure/GEL207-GE-Lamps-Catalog.pdf |access-date=12 Dec 2025}}</ref><ref name="LAI">{{cite web |title=IES Flood Report Summary |url=https://docs.agi32.com/PhotometricToolbox/Content/Flood_Report_Tool/ies_flood_report_summary.htm |website=AGi32.com |publisher=Lighting Analysts, Inc. |access-date=29 Nov 2025}}</ref><ref>{{cite web |title=Spotlight Angles Reference |url=https://help.autodesk.com/view/RVT/2025/ENU/?guid=GUID-8601EE39-481B-4CD0-B673-D9BFFA096FE6 |website=autodesk.com |publisher=Autodesk |access-date=4 Dec 2025}}</ref> the beam angle is the angular full width at half maximum (FWHM).<ref name="LTIO">{{cite web |title=Photometric Glossary |url=https://www.ltioptics.com/en/support-references.html |website=LTIoptics.com |publisher=LTI Optics, LLC |access-date=28 Nov 2025}}</ref> Notably, the term ''intensity'' is used here as a shorthand for luminous intensity (not to be confused with illuminance); beam angle can also be defined in terms of radiant intensity. The intensity distribution data used to determine beam angle is typically measured using a goniophotometer.

The International Commission on Illumination (CIE) uses ''centre beam intensity'', rather than maximum intensity, to define beam angle.<ref name="e-ILV">{{cite web |author=CIE |title=e-ILV, online version of CIE S 017:2020, International Lighting Vocabulary |edition=2 |url=https://cie.co.at/e-ilv |website=cie.co.at |location=Vienna |publisher=International Commission on Illumination |year=2020 |access-date=24 Nov 2025}}</ref> Beam angle was defined this way by the National Electrical Manufacturers Association (NEMA) in ANSI C78.379-2006,<ref>{{cite book |author=NEMA |date=2020 |orig-date=2006 |title=ANSI C78.379-2006 (R2015, S2020), American National Standard for Electric Lamps—Classification of the Beam Patterns of Reflector Lamps |url=https://www.nema.org/standards/view/american-national-standard-for-electric-lamps-classification-of-the-beam-patterns-of-reflector-lamps |location=Arlington, VA |publisher=National Electrical Manufacturers Association |access-date=5 Dec 2025}}</ref> and this definition was adopted by Energy Star.<ref name="Energy Star">{{cite web |author=EPA |title=ENERGY STAR Program Requirements, Product Specification for Downlights, Eligibility Criteria, Version 1.0, Rev. April 2024 |url=https://www.energystar.gov/sites/default/files/2024-05/Downlights%20V1.0%20Final%20Specification%20--%20Revised%20April%202024.pdf |website=EnergyStar.gov |location=Washington, D.C. |publisher=United States Environmental Protection Agency |date=Apr 2024 |access-date=24 Nov 2025}}</ref> Although the DesignLights Consortium (DLC) uses center beam intensity to define beam angle in its glossary,<ref name="DLC">{{cite web |title=Glossary |url=https://designlights.org/glossary/ |website=DesignLights.org |location=Medford, MA |publisher=DesignLights Consortium |access-date=24 Nov 2025}}</ref> it cites the IES definition when specifying technical requirements.<ref>{{cite web |title=DLC Technical Requirements for LED Lighting: SSL V6.0 & LUNA V2.0 |url=https://designlights.org/wp-content/uploads/2025/11/SSL-V6-LUNA-V2-TR_final_11032025.pdf |website=DesignLights.org |page=25 |date=3 Nov 2025 |location=Medford, MA |publisher=DesignLights Consortium |access-date=30 Nov 2025}}</ref>

If the direction of maximum intensity is in the center of the beam, there is no difference between ''beam angle'' definitions based on maximum intensity or center beam intensity. However, differences can arise if the direction of maximum intensity is not the center of the beam,<ref>{{cite book |last1=Tuenge |first1=Jason |last2=Arnold |first2=Gabe |last3=Axelson |first3=Margaret |last4=Rodriguez-Feo Bermudez |first4=Eduardo |title=Radiometric Testing of Germicidal UV Products, Round 2: Upper-Room Luminaires (CALiPER Report) |location=Washington, D.C. |publisher=United States Department of Energy |page=12 |date=Oct 2024 |doi=10.2172/2476236 |osti=2476236}}</ref> such as with "batwing" distributions having center beam intensity less than maximum intensity and reflection symmetry or rotational symmetry.<ref>{{cite web |last=Bush |first=Steve |title=Batwing distribution for horticultural leds |url=https://www.electronicsweekly.com/blogs/led-luminaries/782458-2021-10/ |website=ElectronicsWeekly.com |publisher=Electronics Weekly |date=28 Oct 2021 |access-date=25 Nov 2025}}</ref><ref>{{cite journal |last=Lai |first=Po-Yen |display-authors=etal |title=Investigation of SARS-CoV-2 inactivation using UV-C LEDs in public environments via ray-tracing simulation |journal=Scientific Reports |volume=11 |article-number=22612 |date=2021 |doi=10.1038/s41598-021-02156-8|pmid=34799671 |pmc=8605003 |bibcode=2021NatSR..1122612L }}</ref><ref>{{cite journal |last=Ashdown |first=Ian |title=Horticultural Lighting Design and Déjà Vu |url=https://ies.org/fires/horticultural-lighting-design-and-deja-vu/ |journal=FIRES |publisher=Illuminating Engineering Society |date=4 Apr 2022 |access-date=28 Nov 2025}}</ref> If the beam edge is not clearly defined (i.e., no large gradient) and the center-beam intensity is much smaller than the maximum intensity, a beam angle based on maximum intensity can be substantially smaller than a beam angle based on center-beam intensity (see polar plots on this page).

==Related terms and concepts== ===Field angle=== {{see also|Stage lighting instrument#Field angle}} According to some older definitions, beam angle was measured to 10% of maximum intensity; this wider angle is now termed the ''field angle''.<ref name="LS-1" /><ref name="DLC" />

The floodlight classification system introduced in NEMA publication FL 1-1964 uses ''Type'' designations that are defined in terms of field angle ranges.<ref>{{cite book |title=NEMA FL 1-1964, Floodlights |publisher=National Electrical Manufacturers Association |location=New York |date=1964 |oclc=38826781}}</ref><ref name="HB-4">{{cite book |last=Kaufman |first=John E. |title=IES Lighting Handbook: The Standard Lighting Guide |publisher=Illuminating Engineering Society |edition=4 |location=New York |date=1966 |page=9-35 |lccn=66-19818 |url=https://archive.org/details/ieslightinghandb00illurich}}</ref><ref name="LAI" />

===Beam spread and field spread=== The related term ''beam spread'' is often treated as a synonym for field angle.<ref name="HB-9">{{cite book |last=Rea |first=Mark S. |title=IESNA Lighting Handbook: Reference & Application |publisher=Illuminating Engineering Society |edition=9 |location=New York |date=2000 |page=20-12 |isbn=0-87995-150-8 |url=https://archive.org/details/iesnalightinghan00ream}}</ref><ref>{{cite web |title=Theatrical Lighting Mechanics |last=Williams |first=Bill |url=https://finedesignassociates.com/resources/theatrical-lighting-mechanics/ |website=FineDesignAssociates.com |publisher=Fine Design Associates |access-date=11 Dec 2025}}</ref><ref>{{cite web |title=DLC SSL Technical Requirements V4.4 |url=https://designlights.org/wp-content/uploads/2021/01/DLC_Technical-Requirements-Table-V4-4.pdf |website=DesignLights.org |publisher=DesignLights Consortium |page=16 |date=2018 |access-date=27 Nov 2025}}</ref><ref>{{cite web |title=Understanding Beam Spread and NEMA Types |url=https://www.prolighting.com/blog/2020/09/29/understanding-beam-spread/ |website=ProLighting.com |publisher=ProLighting |date=29 Sep 2020 |access-date=26 Nov 2025}}</ref><ref>{{cite web |title=Factor floodlight brochure (R01) |url=https://cdn.currentlighting.com/site/brochure/FACTOR_Floodlight_Brochure.pdf |website=CurrentLighting.com |publisher=Current Lighting Solutions |date=18 Oct 2022 |access-date=26 Nov 2025}}</ref> However, it has been defined with different meanings: * As recently as 1959, IES did not use or define the term field angle, and instead termed this beam spread.<ref name="HB-3">{{cite book |last=Kaufman |first=John E. |title=IES Lighting Handbook: The Standard Lighting Guide |publisher=Illuminating Engineering Society |edition=3 |location=New York |date=1959 |page=3-10 |url=https://archive.org/details/ieslightinghandb00illu}}</ref> It currently defines beam spread as the angle between the two directions in a plane in which the ''intensity'' is equal to "a stated percentage" of the maximum intensity,<ref name="LS-1" /> thereby treating beam angle and field angle as types of beam spread.<ref name="LTIO" /><ref name="LAI" /> * CIE defines beam spread as the total angle within which the ''illuminance on a plane normal to the axis of a beam'' exceeds 10% of the maximum illuminance, but notes that sometimes the beam spread is calculated at 50% of the maximum illuminance.<ref name="e-ILV" />

The American Lighting Association (ALA) Product Data Standard has a field for beam spread but does not define the term.<ref>{{cite web |title=Product Data Standard |date=Sep 2018 |website=alalighting.com |publisher=American Lighting Association |url=https://alalighting.com/product-data-standard |access-date=28 Dec 2025}}</ref>

The term ''field spread'' has been used as a counterpart to beam spread, where the former is based on field angle and the latter is based on beam angle.<ref name="LTIO" /><ref>{{cite web |last=Jacques |first=David M. |title=Top 50 Lighting Terms Defined |url=https://www.churchproduction.com/education/top-50-lighting-terms-defined/ |website=ChurchProduction.com |date=March 2022 |publisher=Church Production & EH Media, LLC. |access-date=29 Nov 2025}}</ref> The CIE and IES glossaries do not contain an entry for field spread. The term is used to define scope in the IES test methods for searchlights<ref name="LM-11">{{cite book |author=IES |title=ANSI/IES LM-11-20 (R2023), Approved Method: Photometric Measurement Of Searchlights |date=2023a |orig-date=2020 |url=https://webstore.ansi.org/standards/iesna/ansiieslm1120r2023 |publisher=Illuminating Engineering Society |access-date=29 Nov 2025}}</ref> and floodlights<ref name="LM-35">{{cite book |author=IES |title=ANSI/IES LM-35-20 (R2023), Approved Method: Photometric Testing Of Floodlighting For HID Or Incandescent Lamps |date=2023b |orig-date=2020 |url=https://webstore.ansi.org/standards/iesna/ansiieslm3520r2023 |publisher=Illuminating Engineering Society |access-date=29 Nov 2025}}</ref> but neither document provides a definition.

===Beam axis or centerline=== IES does not define the term ''beam centerline'', but it does define ''center-beam candlepower'' (CBCP)—more properly termed ''center-beam intensity''—as the luminous intensity along the geometric centerline of a directional light source, and notes that this definition assumes the centerline intensity is also the maximum intensity.<ref name="LS-1" />

CIE defines ''centre beam intensity'' as the value of the luminous intensity measured on the optical beam axis, and defines ''optical beam axis'' as the axis about which the luminous intensity distribution is substantially symmetrical, noting that the optical beam axis does not necessarily correspond to a physical lamp axis.<ref name="e-ILV" /> The optical beam axis is along the direction of the centroid of the luminous intensity distribution (analogous to a centroid wavelength), which is determined as a weighted-average direction of emitted luminous flux.<ref>{{cite book |author=CIE |date=2019 |title=CIE TN 010:2019, Technical Note: Determination of the Optical Beam Axis, Centre Beam Intensity, and Beam Angle of Directional Light Sources |page=4 |publisher=International Commission on Illumination |doi=10.25039/TN.010.2019 |url=https://files.cie.co.at/CIE_TN_010_2019.pdf |access-date=14 Dec 2025}}</ref>

Definitions do not distinguish between the beam axis and the field axis.

===Symmetry=== Some intensity distributions do not possess full rotational (circular) symmetry; these may for example have an elliptical cross section. * IES notes that in such cases, the beam angle (or field angle) is generally given for two planes at 90 degrees, typically the maximum and minimum angles.<ref name="LS-1" /> Each angle value in the pair would correspond to a plane of reflectional symmetry. NEMA Types are often presented in pairs; with the floodlight positioned upright and aimed at the horizon, the first number is for the angle in a horizontal plane and the second number is for the angle in a vertical plane, where the intersection of planes is the nominal beam centerline.<ref>{{cite web |title=FXLED® 500 datasheet |url=https://www.rablighting.com/sites/default/files/downloads/fxled500w_datasheet.pdf |website=RABlighting.com |publisher=RAB Lighting, Inc. |access-date=29 Nov 2025}}</ref> * Energy Star, citing ANSI C78.379-2006, defines the beam angle as the angle between the two opposite directions in which the average intensity is 50% of the center beam intensity as measured in at least two perpendicular planes (resulting in a single angle value).<ref name="Energy Star" />

== See also == *{{Annotated link|Photometry (optics)}} *{{Annotated link|Radiometry}}

== References == {{reflist}}

Category:Optical quantities Category:Architectural lighting design Category:Angle