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Understanding Beam Angle

The beam angle of a flood style light bulb lets you know how much area it will illuminate at a given distance (beam spot size). The lighting industry has developed a uniform formula to determine the beam angle.  The light output (lumens) is measured at the center of the beam and then light strength (intensity) is measure out from the center until it is 50% of the intensity level (as measured in the center).  The  beam angle is then determined in degrees (see below).  The most common designations for flood light beams are Spot (7 to 15 degree), Flood (20 to 30 degrees) and Wide Flood (35 plus degrees).  

The beam angle does not tell all the story.  It is impotant to know that the light continues to spread out into what is called the Beam Field.  The Beam Field is the area the light covers that has at least 10% of the maximun beam intensity (see below diagram).  The lighting industry does not test for the Beam Field but a good rule of thumb is to double the beam angle.
Flood Light Bulb Spot Size Calculator

The Beam Angle that the manufacturers provide as a degree does not help most people.  What you want to know is the area that a flood light will illuminate.  The spot size of a flood light bulb is the diameter (at any given distance) that the bulb illuminates to 50% of the maximum light output.  The below calculator will help you to figure spot size.  You need to know the distance to the area or object being lit and beam angle of the light bulb.  Fill in the distance and the beam angle boxes.  The calculator can be used with any standard of measurement (inches, feet, meters, etc).  Click on the calculate Spot Size button to get your answer.

Distance to Object Being Lit . Units
Light Bulb Beam Angle Degrees
Spot Size Units
Spot size in the lighting industry is where the light strength (intensity) goes below 50%.  I most case you can double the measurement to get the area covered by light. (Note: As the distance is increased to the area being lit the density of the light will decrease)

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