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.
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|
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):