How to calculate Beam Angle, Field Angle and Nema class

In floodlight VH (large area) photometries the field angle is the opening in degrees at 10% of the maximum intensity. The beam angle is the opening in degrees width at 50% of the maximum intensity.

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So the field angle is wider than the beam angle, it requires less intensity. As a mnemonic remember that a field (of corn for example) is much bigger than a beam of wood (in a barn on the same farm). Well. Hey. That is how I remember it!

Here is an example (from OxyTech's PhotoView program): 

Here's a graphical explanation of where those numbers come from (screenshots taken from OxyTech's photometric program PhotoView):

In the above image the calculation for beam and field angles for H (=horizontal) has been made explicit (blue arrowed lines). See if you can roughly confirm the beam and field angles for V (in red) from the photometric polar diagram.

The NEMA classification for floodlights is simply some text with two numbers which specify the field angle. It is a simple lookup into a table to find out the numbers;

Field Angle NEMA Type
10° to 18° 1
> 18° to 29° 2
> 29° to 46° 3
> 46° to 70° 4
> 70° to 100° 5
> 100° to 130° 6
> 130° 7

You can see from the table that the bigger the NEMA Type the wider is the field angle. The types are found for both V and H angles. First the H Type is listed then the V Type. For example take the image at the top of this page. The H field angle is 113, so from the table the H Type is 6. The V field angle is 97.7, and from the table this is type 5. So the whole NEMA classification is:

as shown at the bottom of the first image.

Here is a 3D view of the photometric solid of a flood-light photometry with a much wider beam horizontally than vertically...

 ...not surprisingly it is a 7x4 (7 horizontal 4 vertical):


  1. I have a question

    Is it right to say that NEMA classificates the floodlights according to the field angle? Is that only for outdoors? Does Nema have any classification according to the beam angle? I was checking in the internet that Nema uses the beam angle, not the field angle to classificate luminaires.

  2. "Is it right to say that NEMA classificates the floodlights according to the field angle?"

    According to my understanding YES. The field angle in horizontal and vertical "direction".

    I suppose you could use this NEMA indoors, but it is designed for "VH" photometries which usually means floodlights which usually means outdoors.

  3. Hi

    What is the formula for calculating the beam spread of a light fixture given a beam angle and distance?


  4. Hello Unknown (is that your real name? what were your parents thinking?)

    What do you mean by beam spread? The linear on the floor distance?

    1. Hi Owen,
      I am In the process of learning and understanding. I think that person may refer to spot diameter.
      Becoz I was also confused in my assignment while calculating the spacing b/w luminaries.
      They have instructed to use beam angle nd find spot dia at particular distance and calculate the spacing. Let’s say for downlight having 36 degree beam angle, what would be the spot dia at a distance of 2m( horizontal plane)
      Kindly correct me if I understood any theory in wrong way.

    2. I think you should start at gamma=0 (straight down) and move upwards till you get to 50% intensity of the g=0° direction. I assume that g=0° direction is the maximum intensity angle. I also assume that the photometry is symmetrical. So draw an isosceles triangle from the height of the luminaire to the road. The base of that triangle is the inter-distance.


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