While every illumination-grade LED is a bright point source of light, the demands of certain applications for high light density have not been perfectly satisfied by previous generations of LED technology.
Street lighting and area lighting, in particular, require bright illumination of large spaces, with a long distance between the light source and the illuminated plane. This presents multiple challenges to the luminaire manufacturer:
• To keep the size and weight of the luminaire to a minimum while producing high light intensity
• To provide good optical performance, delivering a controlled beam of light of the required shape, while maintaining uniformity of light intensity and colour across the beam
• To achieve good system efficiency
• To manage thermal performance and avoid thermal stress at the LEDs
The first challenge – minimising size and weight – tends to push designers towards the use of LEDs offering high light density (lumens/ mm2). In response, LED manufacturers developed Chip-Scale Package (CSP) high-power LEDs: the package of these LEDs is hardly larger than the die it houses, and so the board footprint is smaller than that of a conventional high-power LED. In theory, then, more CSP LEDs can packed into a given board area than conventional high-power LEDs, giving a higher lumen density.
CSP LEDs also offer other benefits when compared to a conventional high-power LED package:
• The chip-scale package is cheaper as well as smaller, allowing a reduction in unit cost of the LED
• The small optical source entails the use of a smaller secondary optic, again providing for higher lumen density
• Low junction-to-PCB thermal resistance
• The absence of bonding wires eliminates a common point of failure in conventional LEDs. A broken bond wire causes an open-circuit failure.
Attractive as these benefits seem, however, the early CSP products have suffered from various drawbacks. In particular, optical performance is compromised. A primary lens – the large dome seen on conventional high-power LEDs – aids light extraction; extraction losses are higher in CSP LEDs. In addition, CSP LEDs emit light from all four sides as well as from the top surface. When assembled in dense clusters, these side emissions lead to further optical loss through shadowing. Worse, these side emissions can cause optical crosstalk, when neighbouring LEDs absorb each other’s light output, leading to colour shifts.
A final disadvantage is that early CSP LEDs are not compatible with most existing secondary optics.
A new type of LED: Nichia DMC
Now LED manufacturer Nichia has introduced a new generation of DMC LEDs which overcomes the limitations of the early CSP products.
Nichia’s Direct Mountable Chip (DMC) LEDs feature a special reflective ‘wall’ which blocks the light emitted at the sides of the die, and redirects it towards the front surface, as shown in Figure 1. As a result, DMC products offer much improved light extraction. Optical losses are just 7% compared to domed high-power LEDs. The absence of side emissions means that the DMC LEDs can be assembled in dense clusters, with no risk of optical shadowing or cross-talk. They produce a tightly controlled beam with a quasi-Lambertian radiation pattern.
This offers manufacturers of streetlights and area lights a breakthrough in lumen density: Nichia’s DMC products have both a small mechanical and optical footprint, as shown in Figure 2. The small size of the optical source, with no side emissions, allows them to be packed closely together on the board while achieving a sharp-edged, controllable beam.
Now the unique features of the DMC LEDs have been implemented in a family of light modules for area and street lighting from Rena, as shown in Figure 3. Offered in Zhaga-compatible form factors with standard or double lumen output, these modules may be used with a wide range of secondary optics from Carclo’s Mini-Hubble series and LEDiL’s Strada and Stradella series.
The Rena modules benefit from excellent thermal design, an important feature of any DMC-based light engine that clusters multiple high-power LEDs in a small board area, as well as optional active thermal feedback to the LED driver.
The Nichia DMC series of LEDs offers important benefits in high-density lighting applications, but their implementation in new luminaires requires new thinking about the system’s mechanical, optical and thermal design. By using the ready-made Rena implementations, lighting OEMs can take advantage of the DMC LED products from Nichia today, and enjoy faster time to market and reduced design risk.