Finding the perfect spot – The art and science of narrowing beam angles

Exploring the role of optics, physics, and aesthetics in achieving tighter spot beams for retail and architectural applications.

 

Over the years, our team has received numerous requests from customers for spot beams with tighter angles. While achieving even a 1-degree beam angle is relatively straightforward without size limitations, creating a more focused beam with a larger light source presents a significant challenge. We also understand that using a bigger optic for the desired narrow beam is not always feasible, as products are optimised for their standard sizes and wider beam range. Additionally, miniaturisation is a strong trend in the industry. While LEDiL is passionate about pushing boundaries, it is important to understand the physical constraints to arrive at the best possible solution.

 

Basics

In illumination engineering, understanding the Full Width Half Maximum (FWHM) and Full Width Tenth Maximum (FWTM) angles is crucial to evaluate optical component performance. FWHM is the width of light distribution at half the peak level, while FWTM is the width at one-tenth the peak level. FWHM alone doesn’t indicate the total light distribution, so FWTM is needed for a better understanding of performance.

 

The FWHM value alone is insufficient to evaluate the performance of an optical component.

 

For example, the two lenses shown in the illustration above have the same FWHM angle, but lens A (grey line) has much higher optical efficiency (cd/lm) and a more concentrated beam with a narrow-angle FWTM value, while lens B (blue line) has relatively poor efficiency and a larger portion of light falls outside the centre beam area, resulting in a wider-angle FWTM value. It’s important to consider both FWHM and FWTM values when comparing different optical components.

 

Physics

Most lighting tasks can be accomplished with narrow beams ranging from 15–24 degrees for which typical lighting fixtures and LED sizes are optimised. However, for more specialized tasks, a narrower beam of 6–10 degrees may be necessary. For example, in a museum with a high ceiling, a 15-degree beam may spill over the intended area, while an 8-degree beam will do the task with sacrifice on efficiency. In the past halogen lights were used to achieve narrow beams, but there are better alternatives available nowadays. Optics, such as lenses and reflectors, work by reflecting or refracting light in a fixed way. While collimators work best with a theoretical point source, LEDs like COBs do not emit light from a single point. Instead, the light comes from different angles and deviates from the desired direction. The degree of deviation angle is affected by the accuracy of the optic as well as the size of the COB.

 

Deviation angles of optics with LES 6 mm and 14.5 mm COB.

 

If the physical limits are reached, adjustments can be made to the size of the optic or LED. Making the optic larger or the LED smaller can reduce the deviation angle and create a closer-to-point-source effect. If increasing the optic size is not possible, a smaller LED with higher lumen density can be used. Narrow beams have a higher candela peak, so extra-narrow beams can achieve enough intensity with fewer lumens than wider beams.

 

The bigger the LES, the wider the beam.

 

Sometimes, using multiple smaller sources with an array of lenses can result in better accuracy, but it may also require compromises compared to a typical solution.

 

Track light example with multiple LEILA-Y optics for museum lighting.

 

Aesthetics

The spots are not all the same, as explained in our article. A theoretical collimator produces an image of the LED with colour uniformity issues, which the lens needs to compensate for. The beam angle also plays a significant role, as the beam needs a sharp cutoff without much stray light after the main beam. Achieving a tight beam is not enough in many applications; it must also be aesthetically pleasing for retail and architectural lighting. Colour mixing features in the lens are crucial for high-quality lighting. A lens with colour mixing features produces beautiful, uniform, state-of-the-art lighting, while a lens without such features is inadequate for high-quality spot lighting despite having the narrowest and most intensive beam.

 

Example of a spot lens with and without colour mixing features.

 

Conclusions

The size of the light source (LES size) and the optics used play a significant role in determining the beam angle of spotlights. A smaller light source results in a narrower beam, while bigger optics can also help achieve a narrower beam. However, a bigger light source produces more light but can result in some light going outside the target area, making the lighting less intense. Therefore, a spotlight with a smaller light source can actually achieve a more desirable result with better contrast and a more powerful candela peak. The narrowest beam can be achieved with track lights that have multiple single lenses where the light source is relatively small compared to the size of the lens.

 

Comparing the characteristics of spot optics with varying sizes and LES configurations.

 

Our team is constantly striving to innovate and push the limits of spot lighting solutions. While we sometimes encounter requests that may not be feasible due to physical constraints, we are always happy to collaborate with our clients to find alternative concepts or compromises to meet their needs.