Led Lights: let there be LIGHT

achieving performance

An important characteristic of any white light is its color rendering, a metric that essentially describes how a source will affect the apparent color of objects. Phosphor-based white LEDs typically have a color rendering index (CRI) of about 74 to 82, a fair value similar to that of standard linear fluorescent lamps; the CRI for incandescent lamps is 95+. Fluorescent CRIs are considered sufficient for most general illumination applications, although certain applications such as specific types of retail lighting may need CRIs above 95. Developing new phosphors will let manufacturers obtain higher values.

We can define an overall figure of merit for white LED

efficiency as

FOM = Nwp Nlum Nss

where Nwp is the wall-plug efficiency, defined as the fraction of input electrical power extracted as optical output power; Nlum is the luminous efficacy, defined as the photopically weighted output divided by the radiometric optical output power; and NSS is the Stokes-shift efficiency, which corresponds to the energy efficiency for the conversion of light from the chip to longer wavelengths.

The conversion of a 450-nm photon generated by the LED chip to a 600-nm photon via the phosphor, for example, implies a Stokes-shift efficiency of 450/600 = 0.75, a 25% loss of its initial energy.

It turns out that the white LEDs are most efficient around 405 nm, due primarily to the peak in wall-plug efficiency observed near that wavelength. For appropriately chosen phosphors, luminous efficacy does not change considerably with wavelength, and the Stokes shift improves monotonically as the chip wavelength increases. In relative terms, the 405-nm system is capable of 40% higher light output than the 470-nm system, which in turn is capable of 20% higher output than the 380-nm system.



see figure. Although LED efficiencies are still below or on a par with existing lighting technologies, solid-state lighting is comparatively early in its development. Projections show that eventual efficiencies could reach those of fluorescent lighting with appropriate development.1

Srinath Aanegola is program manager, Jim Petroski is thermal and mechanical design engineer, and Emil Radkov is manager for phosphors at GELcore LLC, Valley View, OH.

For questions, contact
Aanegola at 216-606-6579,
216-606-6599 (fax), or srinath.aanegola@lighting.ge.com.
or visit http://www.lighting.ge.com

References:

“A Market Diffusion and Energy Impact Model for Solid-State
Lighting,” Sandia National Laboratories Internal Report
SAND2001-2830J, August (2001).

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