In illumination engineering it is very important to see the total shape of the light distribution curve. A light distribution curve is a 2D- or polar diagram -characterization of the performance and it tells for an experienced eye what in detail to expect of the component, e.g., how narrow the light distribution is, are there any discontinuation points to be expected (shadows) or what the relative intensity is in HV 0 degree vs. 30 degrees.
A Full Width Half Maximum (FWHM) angle has been defined, in relative terms, for a symmetrical optics with its maximum intensity in the middle of its light distribution (horizontal and vertical 0 degree), to be the angle, where the intensity of illumination has dropped to 50% from its maximum peak value. Furthermore, many advanced companies define a further so-called 10% value, which is the angle, where the intensity of the illumination has dropped to 10% of its maximum peak value. This is a very useful parameter, e.g., when specifying optical components with an extremely narrow light distribution. The closer the 10% value is to the FWHM value the more light is really focused in the important narrow beam and the less stray light you have outside of the main beam. Now, one may wonder, why to use two values for a component, FWHM and 10% value, why is not FWHM itself sufficient? The reason is that FWHM value is not unambiguous, and it can even be misused to mislead a person specifying his system.
Let’s take a simple example with imaginative lenses A and B.
FWHM value does not give unambiguous information of an optical component.
These two lenses have the same FWHM value, but they perform in a very different way
Lens A is a lens with relatively bad optical efficiency and additionally, a proportionally big share of light falls outside of the centre beam area, i.e., its 10% value is a wide-angle value. Due to the shape of its light distribution curve – a flat curve with no really high peak in the middle, but more or less a “hill” type of a shape – it still has a FWHM value of +/- 5 degrees.
The other lens, Lens B, is a lens with high optical efficiency, with very concentrated beam and a narrow-angle 10% value. Its curve shape reminds of a Himalayan mountain instead the hill for lens B. The surprising fact is that this lens has the same FWHM value of +/- 5 degrees, as lens A. How can it be possible? Putting the absolute (not relative) curves of these 2 lenses on top of each other in the same diagram, shows that lens B gives 5x the light than lens A, but still the FWHM values are identical! The conclusion of this simple example is that different lenses cannot be compared against each other just using FWHM values. FWHM does not give the answer to the question how much absolute light is distributed in the specified angle or area. More facts are needed, 10% value already gives a good hint of how an optical component performs.