What is Fixture’s Beam Angle & Beam Diameter (Part-2)

How to Measure Beam Diameter at Floor:

  • If we install lights at a certain height then how much light will be on the surface will be calculated by following equation.
  • Diameter of light Speared on Floor = 0.018 × Beam angle × The distance
  • For example if we need to calculate the diameter of light for a spotlight of 14° at 3 meter distance.
  • Diameter of Light Spread on Floor=0.018×14×3=0.756
  • As light moves away from a light source, it spreads out and becomes less intense.
  • The beam spread chart below gives a quick reference for common light angles and distances.

Beam Spread at various Beam angle and distance

Beam Angle At 5 Feet At 10 Feet At 15 Feet At 20 Feet
10° 0.9 feet 1.8 feet 2.7 feet 3.6 feet
15° 1.35 feet 2.7 feet 4.05 feet 5.4 feet
20° 1.8 feet 3.6 feet 5.4 feet 7.2 feet
25° 2.25 feet 4.5 feet 6.75 feet 9 feet
40° 3.6 feet 7.2 feet 10.8 feet 14.4 feet
60° 5.4 feet 10.8 feet 16.2 feet 21.6 feet
90° 8.1 feet 16.2 feet 24.3 feet 32.4 feet
120° 10.8 feet 21.6 feet 32.4 feet 43.2 feet

Lamp has Same Lumen but Different Lux due to change in Beam Angle:

  • Amount of Lux at Floor is depending upon Distance between lamp and working floor and Beam Angle of Lamp.

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  • If the Lumens and distance between working plan and lamp is the same for all the four lights having beam angle of 10°,28°,38° and 60°.
  • The amount of Lux at working plan is different. At narrow beam angle 10° it is more Lux at the center of Light (1390 Lux) and it will be reduce as we move from the center. While for wide angle 60° it is less Lux at the Center (39 Lux).

Narrow Beam Angle have Good Light (Lux) at Central

  • A LED light bulb with a narrower beam angle may also seem brighter but the overall total luminous flux (Lumen) will be the same as the same LED light bulb with a lens which produces a wider beam angle. The brighter light is created by focusing the light within a more localized area, much like a magnifying glass can be used to focus the light of the sun. This is sometimes referred to the angular intensity of the light. 
  • If we use a narrower beam angle, we will increase light intensity but reduce the size of the area being illuminated for the same height.
  • The 10 degree beam will be brightest in the center; however, the lux drops very fast away from the center. Thus, it totally is wrong to conclude that 10 degree beam is brighter than the 60 degree beam and hence10 degree beam is a better light.
  • The 60 degree beam has low center lux because it has more light spread over a larger area. The 10 degree beam is good to provide spot lighting. The 60 degree beam may be good for different lighting ambiance.

Illumination as per Distance (Inverse Square Law of Illumination):

  • Only natural light provides even illumination on earth even though it pass from clouds, environment and shadows.
  • But all artificial light are affected from various factor and when the distance increases from the light source then the illuminance reduces according to distance.
  • This is phenomena is called the inverse square law of illumination where the illuminance falls to a quarter of its value if the distance is doubled.

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  • As the luminous flux (Lumen) travels away from the light source the area over which it spreads increases, therefore the illuminance (lux) must decrease. The relationship is called as the inverse square law.
  • Illumination (E) = Lighting Intensity (Lumen) / (Distance)2
  • The inverse square law describes how the intensity of a light is inversely proportional to the square of the distance from the light source (the illuminator).
  • As light travels away from the point source it spreads both horizontally and vertically and therefore intensity decreases. In practice this means that if an object is moved from a given point, to a point double the distance from the light source it will receive only a ¼ of the light (2 times the distance squared = 4).
  • Taking this theory further, if an object at 10m from a light source receives 100 LUX, moving the object to 40m, it will receive only 1/16th of the light (4 times the distance, squared = 16) resulting in the object receiving only 6.25 LUX.

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

3 Responses to What is Fixture’s Beam Angle & Beam Diameter (Part-2)

  1. Vivekanand Pandey says:

    Thank you sir
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