I was going through a 5mm blue LED data sheet and found this diagram in it. Couldn't figure out what it is.
It is a plot of the amount of light for each direction. Imagine the LED is in the bottom middle, where the 0 is and it is shining up.
Close to the LED, the beam is around 40 degrees wide (20 degrees to each side)
If you let the LED shine flat over a piece of white paper, the marked area would be the brightest.
This is a plot of the relative output over the angle of view.
'relative' because you can see the angle it shines brightest is marked "1.0" so all the other values are scaled from there.
Imagine if you had the LED from the datasheet and a photosensor, and you moved the photo sensor around the edge of a protractor with the LED at the center of the protractor's base. As you move the photosensor around the angles from -90 (from the side) to 0 (pointed straight at each other) to 90 (from the other side) the output of the photosensor would draw out this plot.
The main takeaway from this is most of the light is contained in a 20 degree arc straight ahead, and ALL of the light is contained in a 60 degree arc (the plot goes to zero around the 30 degree mark). Almost none of the light is being projected in the angles from 40 to 90.