Most lighting designers are well versed in beam angle. Narrow beam angles are spots, wide beam angles are floods, elliptical beams are for grazing luminaires. Field angle is just as important, but may get overlooked when comparing luminaires.

Ever wonder why two products have similar wattage and beam angle on paper, but perform so differently in a mock up? A large part of that difference may be because of their field angle. 

Let’s start with the technical definitions. A beam angle measures the full-width at half maximum of the beam. In other words, where the intensity drops to 50% of the center beam intensity. The field angle is wherethat intensity drops off to 10%. 

Anything beyond that 10% is call spill light and considered to be wasted – as that light will not be used to illuminate your intended object or wall. As a designer, excessive spill light is often why you need louvers, baffles, and other corrective optics to minimize spill light’s impact on your design. This just adds cost and complexity to your design.

Optics can be manufactured to maximize the usable light within the beam angle, while minimizing the spill light beyond the field angle. High quality optical design can minimize wasted spill light, and the need for costly baffles and louvers. 

How do you identify field angle and the percentage of the spill light from a luminaire? The IES files can provide this information by looking through the lumen per zone. If you break the zones down to 10 degree increments or smaller you can usually start to see stark differences between different products with the same beam angle. When you see a long tail of spill light like the red optic in the image to the right, you know that you’re using more power, more fixtures, and more budget, to get the job done. 

Mock-ups aren’t the only way to evaluate optic quality – you can also dig into the numbers in the measurements (not estimates!) provided in IES files from certified laboratories. It can be difficult to read between the lines of a manufacturer's cut sheet. But digging into the data provided, will provide you with a more accurate picture of the actual usable light being delivered from each luminaire.
Reposted from Linkedin.
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