What does it mean to have a good directional lighting product? Beyond simply brightness, other aspects of a light source design and performance are important for various directional-lighting applications. Multiple criteria are considered for high-luminance directional lighting devices (HIDL) that enable designers to achieve their optimal end result. These criteria include:
Luminance.
High luminance levels are important especially for outdoor safety uses such as flashlights and roadway lighting and in task lighting for performance of demanding activities.
Beam Characteristics.
Unlike diffuse lighting applications, directional lighting design is about a single beam emitted from the light source in the desired direction. The size/spread of the beam, its distance, and its relative brightness across its entire area are critical considerations.
- Beam angle / spread. For some applications, users desire a bright beam spot, i.e., high on-axis intensity. This requires a narrower beam angle/smaller spread.
- Throw distance.* For some applications a long beam distance (throw distance, also called lux distance) is also a key criterion when light needs to reach far from the source. An LED that maximizes illuminance (lm/m2, aka lux) requires a narrow beam angle combined with high luminance. For example, a high-performance flashlight with a narrow beam provides equivalent lux at a greater distance than a flashlight with a broader beam, as shown below in Figure 1.
Figure 1 – The lumens output of both flashlights is the same, but lux level and distance are increased with a narrower beam angle. At distance A, the light intensity of the concentrated-beam flashlight (bottom) is much greater than the intensity of the wide-angle beam flashlight (bottom). At distance B, the concentrated beam appears similar to the light intensity of the wide-angle at distance A.
- Beam spot quality. Beyond high luminance in a beam spot, many applications also require a high quality of light. This means that the beam spot provides uniform brightness and color across its entire area.
- Center Beam Candle Power (CBCP). Beam pattern is essential in directional lighting to ensure that light is shining precisely on the desired area, and with uniform brightness. Typically, a narrow beam pattern provides the intensity, distance, and high lumen output demanded for HLDL applications. An important measure of an effective directional lighting device is its Center Beam Candle Power (CBCP). Also called center-beam intensity or maximum candlepower, CBCP is a measure of the absolute output (lumen intensity) at the center of a beam.
CBCP is a better metric to determine and LED emitter’s performance for HLDL than simply the number of lumens output. Lumens measure luminous flux, which is the total amount of light emitted by a source in all angles. For directional lighting such as spotlights or downlights, any peripheral or diffuse light isn’t relevant, only the light within a narrow angle of the beam. In fact, two different LEDs can have the same lumens, but different CBCP; the higher CBCP is the right choice for a directional application.
For example, Luminus Pico COB LEDs provide very high CBCP within a very narrow beam angle for superior HLDL performance (Figure 2). In fact, a higher CBCP is available from the smaller (4 mm) LES—double the CBCP of the 6 mm LES.
Table 1 – High Center Beam Candle Power (CBCP) Comparison of Pico COB CXM Series
Using 12W PAR 20 with a 62 mm diameter lens and the same driver
Figure 2 – Beam spots produced by Luminus Pico COB CXM-series LEDs. The smaller the source size (CXM-4, 4 mm), the smaller the beam angle and the higher the CBCP (brighter spot center).
The relative CBCP of these chips demonstrates an important concept. Customers who need high CBCP (or longer throw) from their lights may think they need a higher number of lumens from a COB. But in fact, there can be an inverse relationship between lumens and CBCP, as shown in Figure 2: the highest-lumen source (CXM-9) has the lowest CBCP. For HLDL, a smaller source size LED, such as the Pico COB CXM-4, is a more effective solution.
Additionally, various optical characteristics of LEDs can be used to optimize different aspects of HLDL device designs to achieve different application goals. Optical characteristics to consider include LED profile, energy efficiency, lifetime performance and reliability, and emitter shape. These topics are discussed in the Help Center article “What LED Optical Characteristics Are Best for Directional Lighting?”.
Color. Different HLDL lighting applications can require a wide range of color characteristics, for example operating room light must illuminate human tissues as accurately as possible, while streetlights need to limit the quantity of blue wavelengths so they don’t interfere with human and animal circadian cycles. The color rendering index (CRI) is one important measure; see Table 1 below for recommended CRI for different HLDL applications.
To learn more about the science of color rendition, the CRI, CCT measurement, TM-30 and others systems used to characterize all facets of lighting color, and how to apply these systems effectively for different applications, refer to the White Paper: “Achieving Optimal Color Rendition with LEDs.”
Table 2 - Recommended CRI Values for Directional Lighting Applications
Learn more about how to optimize lighting device design and components to achieve the best HLDL performance in the white paper High-Luminance LEDs for Directional Lighting Applications.
Guidance on selecting the right LED for your HLDL application can be found in the Help Center article, “How Do I Select the Right LED for my Directional Lighting Product?”.
*“Throw” is commonly used in the flashlight field. In the ANSI standard, throw distance is defined as the distance where the illuminance is 0.25 lux; in this document the lux value of 1 is used for simplicity. For more information refer to https://www.slideshare.net/canfang/led-optics-in-flashlight
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