Beam angle

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For the concept in physics and engineering, see 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.[1][2][3] It is known to be an imperfect gauge of perceived beam width.[4][5][6] 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

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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.[7] By this definition, which is commonly used,[8][9][10][11] the beam angle is the angular full width at half maximum (FWHM).[12] 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.[13] Beam angle was defined this way by the National Electrical Manufacturers Association (NEMA) in ANSI C78.379-2006,[14] and this definition was adopted by Energy Star.[15] Although the DesignLights Consortium (DLC) uses center beam intensity to define beam angle in its glossary,[16] it cites the IES definition when specifying technical requirements.[17]

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,[18] such as with "batwing" distributions having center beam intensity less than maximum intensity and reflection symmetry or rotational symmetry.[19][20][21]

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Field angle

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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.[7][16]

The floodlight classification system introduced in NEMA publication FL 1-1964 uses Type designations that are defined in terms of field angle ranges.[22][23][10]

Beam spread and field spread

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The related term beam spread is often treated as a synonym for field angle.[24][25][26][27][28] 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.[29] 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,[7] thereby treating beam angle and field angle as types of beam spread.[12][10]
  • 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.[13]

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.[12][30] 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[31] and floodlights[32] but neither document provides a definition.

Beam axis or centerline

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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.[7]

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.[13] 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.[33]

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

Symmetry

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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.[7] 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.[34]
  • 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).[15]

See also

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References

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  1. ^ LRC (Oct 1994). "Reflector Lamps" (PDF). National Lighting Product Information Program (NLPIP) Specifier Reports. 3 (1). Troy, NY: Rensselaer Polytechnic Institute Lighting Research Center. Archived from the original (PDF) on 12 Dec 2003.
  2. ^ DOE (Sep 2014). CALiPER Application Summary Report 22: LED MR16 Lamps. Washington, D.C.: United States Department of Energy. pp. 4–5. OSTI 1377013.
  3. ^ "Lighting Tips & Intuition #72: Beam Angle and Field Angle". RenfroDesign.com. New York: Renfro Design Group, Inc. Retrieved 3 Dec 2025.
  4. ^ van Kemenade, Ir. Johan; Reker, Jan (1988). "Beam Characteristics for Accent Lighting". Journal of the Illuminating Engineering Society. 17 (2): 128. doi:10.1080/00994480.1988.10748732.
  5. ^ Wang, Q.; Davis, R.G. (1996). "A Study on New Beam Parameters". Journal of the Illuminating Engineering Society. 25 (1): 100. doi:10.1080/00994480.1996.10748138.
  6. ^ "IEE Whitepaper: Evaluation of Best-in-Class LED Reflector Lamps" (PDF). EdisonFoundation.net. Washington, D.C.: Innovation Electricity Efficiency, An Institute of The Edison Foundation. Jan 2013. p. A-1. Retrieved 26 Nov 2025.
  7. ^ a b c d e IES (2022). "ANSI/IES LS-1-22, Lighting Science: Nomenclature And Definitions For Illuminating Engineering". ies.org. New York: Illuminating Engineering Society. Retrieved 24 Nov 2025.
  8. ^ Lamp and Ballast Catalog (PDF). Danvers, MA: Osram Sylvania. 2008. p. ix. Archived from the original (PDF) on 24 Dec 2012.
  9. ^ 2025 GE Lamps Catalog: LED & Traditional (PDF). Mayfield Heights, OH: Current Lighting Solutions. 2025. p. 172. Retrieved 12 Dec 2025.
  10. ^ a b c "IES Flood Report Summary". AGi32.com. Lighting Analysts, Inc. Retrieved 29 Nov 2025.
  11. ^ "Spotlight Angles Reference". autodesk.com. Autodesk. Retrieved 4 Dec 2025.
  12. ^ a b c "Photometric Glossary". LTIoptics.com. LTI Optics, LLC. Retrieved 28 Nov 2025.
  13. ^ a b c CIE (2020). "e-ILV, online version of CIE S 017:2020, International Lighting Vocabulary". cie.co.at (2 ed.). Vienna: International Commission on Illumination. Retrieved 24 Nov 2025.
  14. ^ NEMA (2020) [2006]. ANSI C78.379-2006 (R2015, S2020), American National Standard for Electric Lamps—Classification of the Beam Patterns of Reflector Lamps. Arlington, VA: National Electrical Manufacturers Association. Retrieved 5 Dec 2025.
  15. ^ a b EPA (Apr 2024). "ENERGY STAR Program Requirements, Product Specification for Downlights, Eligibility Criteria, Version 1.0, Rev. April 2024" (PDF). EnergyStar.gov. Washington, D.C.: United States Environmental Protection Agency. Retrieved 24 Nov 2025.
  16. ^ a b "Glossary". DesignLights.org. Medford, MA: DesignLights Consortium. Retrieved 24 Nov 2025.
  17. ^ "DLC Technical Requirements for LED Lighting: SSL V6.0 & LUNA V2.0" (PDF). DesignLights.org. Medford, MA: DesignLights Consortium. 3 Nov 2025. p. 25. Retrieved 30 Nov 2025.
  18. ^ Tuenge, Jason; Arnold, Gabe; Axelson, Margaret; Rodriguez-Feo Bermudez, Eduardo (Oct 2024). Radiometric Testing of Germicidal UV Products, Round 2: Upper-Room Luminaires (CALiPER Report). Washington, D.C.: United States Department of Energy. p. 12. OSTI 2476236.
  19. ^ Bush, Steve (28 Oct 2021). "Batwing distribution for horticultural leds". ElectronicsWeekly.com. Electronics Weekly. Retrieved 25 Nov 2025.
  20. ^ Lai, Po-Yen; et al. (2021). "Investigation of SARS-CoV-2 inactivation using UV-C LEDs in public environments via ray-tracing simulation". Scientific Reports. 11: 22612. doi:10.1038/s41598-021-02156-8.
  21. ^ Ashdown, Ian (4 Apr 2022). "Horticultural Lighting Design and Déjà Vu". FIRES. Illuminating Engineering Society. Retrieved 28 Nov 2025.
  22. ^ NEMA FL 1-1964, Floodlights. New York: National Electrical Manufacturers Association. 1964. OCLC 38826781.
  23. ^ Kaufman, John E. (1966). IES Lighting Handbook: The Standard Lighting Guide (4 ed.). New York: Illuminating Engineering Society. p. 9-35. LCCN 66-19818.
  24. ^ Rea, Mark S. (2000). IESNA Lighting Handbook: Reference & Application (9 ed.). New York: Illuminating Engineering Society. p. 20-12. ISBN 0-87995-150-8.
  25. ^ Williams, Bill. "Theatrical Lighting Mechanics". FineDesignAssociates.com. Fine Design Associates. Retrieved 11 Dec 2025.
  26. ^ "DLC SSL Technical Requirements V4.4" (PDF). DesignLights.org. DesignLights Consortium. 2018. p. 16. Retrieved 27 Nov 2025.
  27. ^ "Understanding Beam Spread and NEMA Types". ProLighting.com. ProLighting. 29 Sep 2020. Retrieved 26 Nov 2025.
  28. ^ "Factor floodlight brochure (R01)" (PDF). CurrentLighting.com. Current Lighting Solutions. 18 Oct 2022. Retrieved 26 Nov 2025.
  29. ^ Kaufman, John E. (1959). IES Lighting Handbook: The Standard Lighting Guide (3 ed.). New York: Illuminating Engineering Society. p. 3-10.
  30. ^ Jacques, David M. "Top 50 Lighting Terms Defined". ChurchProduction.com. Church Production & EH Media, LLC. Retrieved 29 Nov 2025.
  31. ^ IES (2023a) [2020]. ANSI/IES LM-11-20 (R2023), Approved Method: Photometric Measurement Of Searchlights. Illuminating Engineering Society. Retrieved 29 Nov 2025.
  32. ^ IES (2023b) [2020]. ANSI/IES LM-35-20 (R2023), Approved Method: Photometric Testing Of Floodlighting For HID Or Incandescent Lamps. Illuminating Engineering Society. Retrieved 29 Nov 2025.
  33. ^ CIE (2019). CIE TN 010:2019, Technical Note: Determination of the Optical Beam Axis, Centre Beam Intensity, and Beam Angle of Directional Light Sources (PDF). International Commission on Illumination. p. 4. doi:10.25039/TN.010.2019. Retrieved 14 Dec 2025.
  34. ^ "FXLED® 500 datasheet" (PDF). RABlighting.com. RAB Lighting, Inc. Retrieved 29 Nov 2025.