Light Distribution

Beam angle

The beam angle of a luminaire will be expressed in degrees and sometimes with the letters FWHM (full width, half maximum) e.g. 5° FWHM.

The beam is measured at its peak intensity and points established where the intensity is half of that. If the intensity reached half of the peak 10° either side of the peak then the beam angle would be 20°.

The beam angle is a useful quick way of gauging whether a luminaire and its selected optic might fit the requirements of an application. Generally, Stoane Lighting luminaires will come with a range of beam angle options covering very narrow to wide flood.

Optical design has some limits and there is often a balance to strike between the amount of light, the beam angle that is needed and the size of luminaire that can be tolerated. Generally, bigger products can accommodate larger optics that can deliver tighter beams from smaller light sources, though will produce less output. We're very well used to finding this balance and proposing light source, luminaire and optic combinations to meet the desired results.

Field angle

Whilst the beam angle can be a good indication on the performance of a luminaire and optic, it cannot tell us the whole story. For example, whilst we can deduce at what angle the intensity reaches half of the peak we cannot tell from that number what the distribution is like outside of that point, whether there is a lot of spill and perhaps whether there is a soft transition from the edge of the ‘beam’ to no light. The field angle tells us where the intensity reaches 10% of the peak intensity. In the case where 10% is reached 40° either side of the peak the field angle would be 80°.  

In a product with a beam angle of 20° and a field angle of 80°, it might be deduced that there would be a softer beam edge or more spill than for example a product with a beam angle of 20° and a field angle of 40°.

Cutoff angle

The cutoff angle is defined as the point where the intensity reaches 2.5% of the peak intensity, the point at which we might consider that outwith there is no light. If the peak reaches 2.5% of peak intensity, 60° either side of peak then the cutoff angle would be 120°.

Stoane Lighting photometry reports will include beam, field and cutoff angles.