Surface Mount LEDs – Part 3: Optical Properties
This is Part 3 of a 4 Part Series
- Surface Mount LEDs – Part 1: The Basics
- Surface Mount LEDs – Part 2: How They are Made
- Surface Mount LEDs – Part 3: Optical Properties (this article)
- Surface Mount LEDs – Part 4: Applications
Light Emitting Diodes (LEDs) have become the “go to” source for lighting as they are tiny, long lasting, cost competitive and efficient. Further, the optical properties of LEDs are highly tunable, offering design latitude for nearly any application. It is important to understand the optical properties of the LED(s) that you consider for your application. Characterization of the optical properties of an LED is typically done with in an integrating sphere (see above), and the data from those measurements enable critical understanding of the LED device of interest.
It is important to understand the color of the emitted light. The emission spectrum shows illumination intensity as a function of wavelength of the electromagnetic radiation (EMR) emitted. An example for LEDs of different colors is given below. The emission spectrum shows important information, such as peak emission wavelength, as well as the width of the emission peak. For applications requiring a light which is more monochromatic, a relatively sharp emission curve is pursued (e.g., the 365 nm violet LED below). A small FWHM or full-width-at-half-max value of emission bandwidth indicates a relatively high level of monochromaticity.
For certain applications, such as LEDs for ambient lighting, a broader emission spectrum is desirable. This may be achieved using an LED having a shorter (higher energy) characteristic emission peak, such as a GaN-based LED, combined with one or more carefully selected and applied phosphor(s), such as Cerium mixed with Yttrium Aluminum Garnet. The phosphor(s) absorb the higher energy light emitted from the LED, and then re-emit light across a broader, lower energy spectrum as shown on the left below. Proper engineering can result in emission spectra that more closely resemble natural light, which is desirable for applications requiring “natural” lighting. The spectrum of natural sunlight at sea level is shown to the right below.
Alternatively, multiple LEDs of different color (e.g., Red, Green and Blue), may be operated in concert together so as to provide light of mixed multiple wavelengths. This approach is used in displays, and is more complicated and expensive, but is also easily varied such that almost any color may be generated.
It is also important to understand the emission pattern of the LEDs of interest. Example emission patterns are shown below, indicating the relative intensity of emitted light with respect to projection or viewing angle. Lenses are typically used to engineer the emission pattern. Parabolic lenses confine the pattern to a smaller range of viewing angles, while planar lenses broaden the viewing angle somewhat, and hemispherical lenses maximize viewing angle as shown below left. Lenses that diffuse light increase the viewing angle as shown below right.
Emission efficiency is typically expressed in lumens per watt (W) of power consumed. The lumen is the SI unit of luminous flux, and is equal to the amount of light that is emitted per second over a unit solid angle of one steradian from a light source of one candela intensity. A LUX is equal to one lumen per square meter at a radial distance of 1 meter from said light source as shown below.
Color rendering index (CRI) is important in situations where it is critical that objects of interest show the same color as they would in natural light.
Applications like photography, microscopy, cinematography, automotive painting, dentistry, etc., require lighting with a high CRI. CRI scales from 0 to 100 percent. The higher the CRI, the better the color rendering ability. CRI values of 85 to 90 are considered to be good, and a CRI of 90 or higher is considered excellent for color rendering.
Color temperature is the temperature value at which an ideal black-body radiator emits light of a color that is the same as that of the LED light tested. An example color temperature chart is shown below.
Other information may also be valuable in selecting an LED light source for your application, such as the chromaticity space diagram (see example below), or if more detail is needed, the tristimulus coordinates (e.g., the mathematical combination of brightness, hue and saturation). These are explained elsewhere.
So in order to select the best LED for your application it is highly important to understand the optical properties of said LEDs. Additionally, new LEDs which emit new colors (or wavelengths outside the visible spectrum), or have different viewing angles or emission angles, or higher efficiencies, etc., are introduced almost daily. It pays for designers to keep up with this fast paced technology. Won’t you join the “LED Revolution”?