Directional Lighting​

LED spotlights and floodlights are directional luminaires that focus most of their light in a particular direction. Directional lighting creates a controlled beam of light that provides task illumination, focuses attention, creates contrast, defines spaces, and reveals form and texture. In retail, commercial, museum, hospitality, or upscale residential applications, multiple layers of light of often employed to add atmosphere, depth, drama and functionality to a space. Spotlighting and floodlighting are designed to create the accent and task layers of lighting. A deliberate contrast created by using a concentrated beam of light creates visual hierarchies within the space. Focused, localized, and high intensity illumination provides visibility to carry out a particular job. Spotlights and floodlights are architecturally integrated to illuminate a space or an object without creating visual noise or affecting space appearance.

Optical Characteristics of Directional Beams​

A directional luminaire produces a concentrated beam that travels uninterrupted from the luminaire to the target area. The directional beam is used to throw flattering light on a specific design element. The light pattern and lumen distribution of a beam spread can dramatically affect how effective the important features of a design element are visually prioritized. The beam quality of directional luminaires is typically evaluated by these components: center beam candlepower (CBCP), beam angle, field angle, field-to-beam ratio, and spill light.

Every light distribution has a point of maximum intensity. The light intensity at this point is known as CBCP. For most symmetrical beams CBCP refers to the candela value at nadir (directly below the luminaire). High CBCP is often desired in narrow-beam applications for creating visual excitement and enhancing a design element for maximum impression.

Beam angle, otherwise known as the full beam width at half-maximum (FWHM), is the angle at which the half-maximum intensity occurs. Field angle is the angle between the two directions for which the intensity is 10% of the CBCP. The light that lands within the field angle is considered to be useful. Lumens that falls outside the field angle are called spill lumens which must be eliminated or minimized. The field-to-beam ratio gives information whether the beam is hard or soft. A hard beam has a low field-to-beam ratio and appears clean and crisp. A soft beam has a large field-to-beam ratio and fades gradually out toward the edges.

Spotlighting and Floodlighting​

Directional luminaires are classified by beam distribution. Luminaires with a beam spread (field angle) of less 20° are called spotlights. Spotlight beams can be further divided into spot (10°-19°), narrow spot (6°-9°), or very narrow spot (5° or smaller). Floodlights have a minimum beam spread of 20° and can be classified as "narrow flood" (20°-25°), "flood" (30°-40°) and "wide flood" (55°-60°). Spotlights deliver a punch of high CBCP light to create points of interest. Spotlighting directs concentrated beams of light on objects to create a focal point. Spotlighting may be used in conjunction with ambient or perimeter lighting to provide the accent layer of lighting. In order to achieve the full effect of accentuation, spotlighting should be five times brighter than the ambient light surrounding the object of focus. Floodlighting is used to accentuate larger objects and provide task illumination for a specific zone. Flood lights that deliver a broad spread of light in an asymmetrical pattern can be used as wall washers. In interior lighting applications, the boundary between spotlights and floodlights is very ambiguous. Most directional luminaires are called spotlights, including those having a flood beam.

Accent Lighting​

Directional luminaires are a crucial tool for delivering accent lighting. Composing the lighting itself in layers gives visual interest, depth and dimension to a space. Accent lighting creates points of focus, enhances the impact of architectural and decor features, attracts attention and generate energy, against the background of the ambient layer of lighting. The focal pool of light accentuates brilliance, prominence, or attractiveness of features and displays of artwork, merchandise, and architecture. Attention created by illuminance contrast is an important consideration in the application of merchandising lighting. Most retail lighting designs incorporate the accenting layer to emphasize the texture, shape, finish and color of clothing, jewelry, and foodstuff. The contrast ratio of accent lighting to its surroundings not only causes the focal point to shine out, but also establishes a hierarchy of importance on the selling floor. In showrooms and exhibitions, accent lighting is an effective marketing tool that rounds out a presentation concept and adds value to products by modelling them perfectly. In museums and art galleries, accent lighting creates drama for sculptures and paintings. Narrow beams of light from directional spotlights cut through subdued ambience, bringing out the natural resonance and brilliance of artifacts and drawing the visitors' gaze to the pieces on display. The atmosphere in a restaurant, bar, night club, or other intimate space will be informed by the lighting design. Accent illumination focused on the table or sitting area creates a private environment to enjoy. An interplay of diffuse and directional light triggers emotions and inspires conversations.

Task Lighting​

Directional luminaires are also used to provide another layer of lighting - task lighting. This type of luminaires cleans up the look of ceilings for a more finished look while eliminating the messy look of exposed cables and the feeling of crowding that come with the use of desktop and floor standing task lights. Task lighting from ceiling mounted spotlights focuses on a specific area to provide targeted illumination for accomplishing tasks in offices, conference rooms, and the functional areas of retail and hospitality facilities. A typical 3:1 ratio of task lighting to general illumination provides a nice contrast and a focused beam for evaluating merchandise and artwork, carrying out a wide range of visually demanding computer based or paper tasks, facilitating the orientation and direction within a space, and removing unflattering shadows over faces so that verbal communication is enhanced. Directional luminaires are oftentimes designed to be adjustable so that light fixtures can aim light in a specific direction. Tightly controlled narrow beams eliminate uncomfortable luminance from fields of view. The absence of high angle glare enhances visual comfort, aesthetic perceptions, social interactions, environmental and job satisfaction, and task performance.

Lighting Technology​

The apparent advantages of LEDs over halogen, fluorescent and metal halide lamps have been fueling a massive migration to LED lighting. An LED emits light through recombination of electrons and holes when an electrical current flows through a forward biased p-n junction. The semiconductor diode itself is a narrow-band emitter having typical bandwidths of a few tens of nanometers. To produce a light spectrum that is perceived as white light by the human eye, an LED is packaged in way that allows the monochromatic emitter to pump phosphors within the device package. The highest efficiency LEDs today are InGaN-based blue LED chips which have an external quantum efficiency (EQE) as great as 60%. In blue pump LEDs, a portion of short wavelength light is down-converted by the phosphors into longer wavelength light. The longer wavelength light then mixes with the portion of blue light that does not undergo down-conversion to generate white light. White LED light can also be produced by violet pump LEDs. The resulting white light exhibits high color fidelity, but LEDs of this type are less efficient than blue pump LEDs.

Aside from the benefits of high luminous efficacy, solid state durability, and long operational life, LED technology offers a number of critical advantages against legacy technologies when it comes to directional lighting. LEDs, with their directional nature, are well suited to spotlighting. LED luminaires can use small secondary optics to obtain greater control over the light distribution. The spectral power distribution (SPD) of LEDs, which dictates the color appearance and color rendering performance of the light source, can be tailored to specific applications. The color quality of a light source is a critical element in interior lighting design. It affects how humans interact with the environment or the object of interest. Another spectral advantage of LEDs is that the radiant energy of LEDs falls only within the visible range of the spectrum and LEDs do not develop ultraviolet (UV) and infrared (IR) radiation. The photochemical damage of UV radiation has to be taken into account in retail display and museum applications. Thermal IR energy radiated by traditional light source also poses a significant concern as it can deteriorate cosmetic products, dry out vegetables, discolor fabrics, melt chocolate and confectionery, etc. Absence of UV and IR radiation makes LEDs an extremely safe light source.

Types of Directional Luminaires​

Track lights​

Track lighting provides the flexibility to deliver light where it is needed most. In commercial, hospitality and museum applications, lighting must adapt to a constantly changing environment. Track lighting can indeed adjust to a multitude of changes and easily accomplish all of the lighting needs. It can be responsive the flexibility of variable atmospheres in restaurants, bars, night clubs, and cafeterias. Track lighting has become the method of choice in retail stores and merchandise showrooms for its ability to support the flexible store concepts. The contrast between light and shadow can be managed for maximum impact with track lighting in display environments such as museums and galleries. The key infrastructure of track lighting is the track system, which serves to power luminaires and to support and position luminaires mechanically for the best possible aiming angles. Track systems come in single-circuit and multiple-circuit configurations. The luminaire must be equipped with a corresponding track adapter for mating to the track.

Recessed downlights​

Recessed lighting with downlights provides a premium ceiling appearance for its inconspicuous aesthetic and minimal aperture brightness. Recessed downlights are designed to be minimally visible from below a ceiling while spreading light as a broad-beam floodlight or concentrating the light as a narrow-beam spotlight in a downward direction. The housing is recessed into the ceiling. The trim is the only visible part of the luminaire. Recessed downlights designed for accent and task lighting often come with adjustable trims such as eyeball trims, gimbal trims, retractable trims, and slot apertures. In addition to round and square trims in traditional form factors, recessed LED downlights may be designed as slim linear modules such as the iGuzzini Laser Blade LED luminaires, which produce beams of utmost precision in a miniaturized footprint.

Ceiling mount luminaires​

These directional luminaires are very similar to track luminaires except that the luminaire junction box or base attaches directly to the ceiling surface. The light head of a ceiling mount spotlight can be tilted and is oftentimes designed to be horizontally rotatable. Ceiling mount luminaires may also come in a multi-lamp design allowing for several adjustable sources in one fixture assembly.

Design and Construction​

LED luminaires for direct lighting can be categorized into lamp-based luminaires and integrated luminaires. Lamp-based luminaires come with accompanying LED lamps that have the form factor of the legacy light source, e.g. MR16, PAR20, PAR30 or PAR38. The beam distribution, light quality and service life is dependent on the LED bulbs. Unfortunately, LED bulbs are mass-produced products that most often offer an entry-level light quality and their service life can be limited due to under-engineering and the use of low cost components. These products most often do not have the compatibility with advanced dimming controls. The bulky lamp size makes it impossible to design a compact luminaire. As a result, lamp-based LED spotlights are being phased out in favor of integrated performance LED luminaires.

Integrated performance luminaires use LED modules in conjunction with other optical components to deliver light in a specific pattern or distribution. This design allows the light emitted by the LEDs to be efficiently extracted out of the system and directed in a desired way. The typical fixture-as-heat-sink thermal design of integrated LED luminaires and direct mounting of LEDs onto the heat sink maximize thermal distribution across the available surface area and thermal conduction efficiency due to low thermal resistance along the entire thermal path from the LED module to the luminaire housing. LED bulbs leave a tight space for the driver circuitry, making it challenging to design a full-featured driver. In contrast, integrated performance luminaires incorporate either co-located drivers or external drivers. In whichever case, there's a sufficient space to accommodate a driver circuit that provides excellent current regulation, supports sophisticated lighting controls, and operates with high efficiency. In LED bulbs the LED driver is in close proximity with the thermal mass of LEDs and some reactive components can be thermally stressed. The co-located drivers in integrated systems are thermally isolated from the LEDs to ensure optimal driver operation throughout the rated product life.

LED Module​

The LED module is one of the key design and engineering points of an LED luminaire. It is where the light source electrically, thermally, and mechanically interfaces with the system. An LED assembly is composed of a metal core printed circuit board (MCPCB) or an FR-4 PCB with metalized vias, onto which one or more packaged LEDs are mounted in a series string or in a mix of parallel and series for constant current designs. The packaged LEDs can be ceramic-based high power packages, chip-on-board (COB) packages, mid-power plastic leaded chip carrier (PLCC) packages, or chip scale packages (CSPs). These LED packages have different thermal behaviors and optical performance. High power, COB and CSP LEDs are designed with a robust thermal path that allows the LEDs to perform in high drive conditions and high junction temperatures without irreversible degradation. Mid-power LEDs are generally not recommended for directional lighting applications. A spotlight or narrow-beam floodlight is required to pack enough of a punch to reach the target. Obviously it is unlikely for an array of discrete mid-power LEDs to deliver high CBCP in a small LES (light emitting surface). Furthermore, these plastic packages are prone to accelerated lumen depreciation and color shift at high drive currents and operating temperatures. This complication makes mid-power LEDs the least desirable light source for accent lighting which requires the tightest control of the color characteristics among all lighting applications.

The LEDs are usually reflow soldered to a PCB to provide electrical and mechanical connections to additional components such as the driver and heat sink. The reliability of the solder joint between the LED package and PCB is one of the key determinants of overall reliability of an LED luminaire. The formation of a reliable solder joint depends on the solder joint geometry, solder alloy, interface chemistry, and the reflow profile which influences the wetting behavior and microstructure of the solder joint. In addition to creating a strong metallurgical bond after resolidification of the solder, creep resistance of the solder alloys is another critical determinant of interconnect reliability. Solder joints are made of creep resistant alloys which can limit the amount of strain energy build-up in the solder joints and maintain excellent shear strength even after repeated thermal cycling under high coefficient of thermal expansion (CTE) mismatch conditions. To simply light source integration, COB LEDs are often used in combination with solderless COB holders. Zhaga-compliant LED holders provide a modular ability for solder-free electrical connection and mechanical engagement while facilitating optical alignment of secondary optics.

Thermal Management​

LEDs, while being more efficient than gas discharge lamps and fluorescent lamps, convert in excess of 50% of the electrical energy into heat. The thermal energy that is localized in the LED package will result in heat flux concentration and may raise junction temperature if it is not dissipated. Operating LEDs beyond their maximum rated junction temperature will lead to significant acceleration in the evolution of failure mechanisms such as growth of dislocations in the active region of the diode and phosphor thermal degradation. The rate at which the luminous efficiency and color stability of an LED will decline is highly dependent on the temperature at the p-n junction. Maintaining performance of the LEDs therefore creates a need for a high-performing thermal management solution.

Thermal management of LED luminaires aims to reduce thermal resistance of all the elements that make up the thermal path so that the thermal transfer rate (conduction and convection) will outpace the load rate at which thermal energy is introduced to the junction. As a rule, thermal engineering of an LED system can be broken down into three components: the package (junction-to-substrate), the module (substrate-to-PCB), and the system (PCB-to-heat sink). As noted above, different packaging platforms result in varied thermal performances and heat extraction efficiencies. Ceramic-based high power LEDs, for example, are not only capable of surviving a high operating temperature, but also designed with an adequately sized thermal conduction path that allows better substrate-to-PCB heat dissipation, when compared with plastic PLCC packages. The module level thermal management works with creating high reliability, high operating temperature capable interconnects between the LED package substrate and the PCB. The MCPCB is the most widely used type of insulated metal substrate (IMS) boards for its low thermal resistance and high electrical insulation performance.

The heat sink is the last and most important part of the thermal management system. It first conducts heat away from the LEDs via the PCB and then convects heat to the ambient air. To facilitate interfacial thermal conduction, a thermal interface material (TIM) is often placed between the PCB and heat sink. Heat sinks are made of a high thermal conductivity material such as aluminum. Die casting is a popular manufacturing process which produces aluminum heat sinks that are durable and dimensionally stable even for complex shapes, while maintaining close tolerances and smooth cast surfaces. The performance of a heat sink is dependent on many variables, and the key performance parameters include (1) a flat contact area for thermal interfacing with the entire rear surface of the PCB; (2) effective thermal transfer within the heat sink (thermal conductivity, effective thickness of the heat sink); (3) a large surface area of the boundary to maximize convective heat transfer; (4) aerodynamic geometry that promotes effective air circulation, thus increasing the heat transfer coefficient.

Secondary Optics​

Directional LED luminaires produce tight, clean and punch beams through secondary optics that fall into one of two types: specular reflectors or total internal reflection (TIR) lenses. Specular reflectors have a reflective surface that has irregularities smaller than the wavelength of the incident optical radiation. These optics are made from either metalized and optical coated plastic such as polycarbonate, or processed anodized and coated aluminum. Metalized plastic reflectors have a reflectance of up to 0.97, and aluminum reflectors have a surface reflectance of up to 0.95. For better distribution of luminous flux to the desired direction, specular reflectors are often designed with an array of small surfaces (facets) with different angles of rotation.

To produce a precise beam pattern with a narrow FWHM divergence angle (10° - 35°), total internal reflection (TIR) lenses are most effective. A TIR lens is a combination of a refractive lens and a reflector with a common axis. The refractive lens nestles inside the reflector to capture virtually every ray of light emitted by the LED and directs light to the reflector which then shapes the beam in a desired angle. The use of multiple lens elements allows for two-step regulation of luminous flux from the light source in a single lens assembly, resulting in a high optical efficiency, tight beam control, and uniform light distribution. This type of compound lenses is injected molded from polycarbonate (PC) or polymethylmethacrylate (PMMA).

Color Reproduction​

Correct perception of the colors of objects under artificial illumination is of paramount importance in retail stores, showrooms, museums, art galleries, and residential spaces. The ability of a light source to faithfully reproduce object colors depends on the interaction of the light source's SPD and the spectral reflectance function of objects. White light is a mix of various wavelengths between about 380 nm and about 780 nm. The specific wavelengths present within the light spectrum and their intensities can have a significant impact on how accurate the colors of objects are rendered under the given illumination. Depending on the light source, different wavelengths of varying intensities, may be present in the light spectrum. Daylight provides a spectrum of light that has wavelengths uniformly spreading across the visible spectrum, allowing the human eye to perceive bright and vivid colors and to distinguish subtle differences in color appearance. Accordingly, it is generally recognized that daylight is best "white light" for color reproduction. Daylight is therefore used as the reference light source to evaluate the color quality of artificial light sources.

LEDs by nature are not white light source, but produce narrow-band emissions. To produce white light with blue LEDs, a varying part of the narrow-band, short wavelength light emitted by the diode must be phosphor-converted into longer wavelength light within the device package. Unfortunately, the more the short wavelength light is down-converted, the lower the luminous efficacy of radiation (LER) of the LED has and the higher the amount of waste heat is generated. To simultaneously obtain the efficacy and cost targets with blue pump LEDs, the emission spectrum of the red phosphor is significantly narrowed in most LEDs available on the market. While these LEDs are labeled with a reasonable color rendering index (CRI), they fail to bring out saturated colors and human skin tones, paintings, clothing, and foods may appear dull or undersaturated. The general color rendering index Ra is calculated as the average of only the first eight color rendering index values, R1 - R8, and ignores the highly saturated color samples, R9 - R14. When evaluating the color rendering performance of an LED luminaire for museum and retail applications, the saturated colors should not be left out. R9, a saturated deep red color, is the most-often cited of the extra CRI colors. A minimum CRI Ra of 90 and R9 of 75 is usually required for color-critical applications. Professional evaluation of the color rendition of a light source may use TM-30, color quality scale (CQS), gamut area index (GAI), etc. TM-30 is an evaluation framework for characterizing color fidelity and gamut area while allowing for a multi-faceted comparison of color rendition across hues.

Violet pump LEDs are gaining increasing popularity in color-critical applications. White light emitted by these LEDs are produced by down-converting the violet wavelengths with red, blue, and green phosphors. This method produces a broad emission profile with impressive coverage and intensities of long wavelengths for rendition of saturated colors.

cct" data-toc="1" >Correlated Color Temperature (CCT)​

In general, the correlated color temperatures (CCT) of a light source is adapted to the specific environment. In residential and hospitality applications, the desired mood, ambience or atmosphere guide the selection of color temperature. The preferred CCT in these setting is in the range of 2700 K to 3500 K, which is referred to as having a "warm white" appearance. Warm white light creates a feeling of intimacy and helps people relax. It is also flattering to skin tones and thus promotes the communication and interaction of people. However what most people do not realize is that warm white light makes them less prone to disruption of circadian rhythms because of the relatively low percentage of blue contained in the light spectrum. Light with a high CCT, e.g. cool white light that has a CCT above 4100 K, has a significant portion of blue in the spectral composition. Lighting with a strong blue component should be avoided in the evening and at night because it suppresses the production of melatonin which is secreted into the blood by the pineal gland. Suppression of melatonin release with cool white light during nighttime adversely affects sleeping patterns and disturb the day/night rhythm.

In some applications, the selection of color temperature is governed by the visual tasks, room color palette, and aesthetic goals. The retail environments may use lighting with a higher CCT, e.g. in the range of 3500 K to 4100 K (neutral white) to complement the store theme, and to create a sense of excitement and inspire people to reach for their wallets. The selection of color temperature has another dimension in museum applications. It is often driven by factors such as curator and designer preference and preservation of light-sensitive materials. Light sources with lower CCT are usually preferable in artwork display environments because they contain relatively less high energy, short-wavelength radiation.

LED Binning​

LEDs are sorted into chromaticity, luminous flux, and sometimes forward voltage categories (bins) after production so as to keep variations of color and brightness of a particular group of LEDs within the specified limits. The tighter the binning, the closer the color and brightness of each LED in that bin will appear to another. Color consistency, or the uniformity of color appearance across multiple luminaires, is a critical detail in accent lighting. The magnitude of visually acceptable chromaticity differences within a bin is defined as a MacAdam ellipse in the Standard Deviation Color Matching (SDCM) method or as a parallelogram in the American National Standards Institute (ANSI) method. Directional lighting for retail and hospitality applications typically requires the LEDs to be binned to a 2- or 3-step MacAdam ellipse. Color bins of LEDs for museum and gallery lighting may fall within a 1-step MacAdam ellipse tolerance (approximately 0.007 ∆u'v' or ±30 K @ 3000 K CCT).

LED Driver​

The LED driver provides a constant current to the load regardless of changes in the incoming AC voltage or LED forward voltage. Multiple variables affect the efficiency, performance and reliability of an LED driver. The efficiency of the LED driver not only has a major impact on operating costs over the luminaire's lifespan, but also plays a key role in determining the thermal stresses of co-located systems. A high efficiency driver dissipates less heat and thus produces less thermal stresses to neighboring components. The constant current driver is designed as a switching mode power supply (SMPS) which operates by switching energy storage elements at a high frequency. Switching regulation enables LED drivers to operate a high circuit efficiency, but requires additional circuits to suppress electromagnetic interference (EMI) generated due to high frequency switching. Power factor correction (PFC) is required for LED drivers with more than 25W output power to improve the ratio of apparent power to real power and reduce harmonics.

Large output current ripple delivered to the LED load must be smoothed out to ensure flicker-free, optimal operation of the LEDs. However, the use of electrolytic capacitors to reduce the output ripple makes the LED driver often the first component of a luminaire to fail because the electrolyte inside the capacitor evaporates over time. The evaporation rate depends upon the temperature inside the driver which, in turn, calls for effective thermal management of the driver. In co-located systems, the LED driver should be thermally isolated from the self-heating LEDs such that an optimal operating temperature is maintained and the electrolyte evaporation process is not accelerated.

Dimming​

Dimming operation of LED luminaires can be implemented through voltage modulation, pulse width modulation (PWM), or constant current reduction (CCR). Voltage modulation or phase control is a very ineffective method of dimming LEDs, but this feature is often indispensable because of the widespread use of legacy dimming circuits in many established facilities. PWM dimming changes LED light output by directly adjusting the duty cycle of a pulsed current waveform. CCR or analog dimming simply controls the drive current fed to the LEDs. PWM dimming is often preferred over CCR dimming because this method keeps the current flowing through the LED unchanged at different dimming levels, thus preventing color shifts that occurs with changes in drive current. CCR dimming can be simple to implement, but can cause color shifts at lower drive currents. In PWM dimming, the duty cycle of the pulses may vary from 0% to 100%, and the apparent brightness of the LED can be regulated as a result. The problem with PWM dimming is that PWM operation may generate EMI from the high frequency switching. Analog dimming circuitry allows users to control through a variety of protocols, including 0-10V, DMX, DALI, and ZigBee.