Miniature battery-powered devices have become commonplace in everyday life. These products are trending towards increased computing power, greater wireless range, and more polished user interaction all while extending battery life and reducing cost. One particularly important design consideration that must be taken into account is illumination. This article discusses the need for advanced illumination before examining trends in LED performance and customizable LED colors.
The Need For Advanced Illumination
An example of some of the design considerations required for LED performance is shown below in Figure 1. Where a luminaire is utilized in the user interface (UI) of an Internet-of-Things (IoT) sensor node to provide a range of information.
Figure 1. Multifunction IOT sensor with LED indicator. Image used courtesy of ROHM.
These lights may change colors, blink at different intervals, and vary in intensity to convey connectivity status, measurement progress, battery life, and any number of other data.
In a similar fashion, individual lights may be arranged into a shaped array to add an additional dimensionality to the display. Figure 2 shows an example of a minimalist wearable, in which multi-color LEDs are used behind a dead-front surface to create characters, animations, sparklines, and the like.
Figure 2. A wearable device using a multicolor LED array. Image used courtesy of Adobe Stock.
In both of these examples, the illuminated indicators are critical to the product’s usability and limited by the battery’s energy capacity. In response to these opposing constraints, LED manufacturers have made enormous strides in the last several decades toward creating customizable, highly efficient devices.
LED Performance Trends
It has been theorized that a perfectly ideal light source can produce 251 lumens for every watt of applied power. As manufacturers chase this idealized goal, it has been observed that every ten years, the luminous output of LEDs has increased by twenty while cost has decreased by ten. This trend, also known as “Haitz’s Law,” is responsible for the proliferation of LED illumination seen today in every type of device ranging from small battery-powered wearables to industrial warehouse fixtures. Figure 3 demonstrates the magnitude of the progress made in the last twenty years, where the luminous intensity is shown for surface mount (SMD) LED’s in a 0603 package.
Figure 3. Progress of luminous intensity for 0603 package LEDs from 2000 to 2020.
Interestingly, this increase in luminous intensity has introduced an unusual problem at the other end of the brightness curve. To create light at less than maximum intensity, the LED bias current must be reduced since once this bias current approaches the knee of the IV curve, device-to-device variation in intensity can become an issue. As shown in Figure 4, the spread of luminous output exceeds the acceptable range once the bias current is reduced to single-digit milliamperes.
Figure 4. Luminous intensity variation when traditional LEDs are biased at low currents.
Such variation becomes especially problematic in battery-powered devices making use of LED arrays. As these devices tend to operate at the lower end of the bias current range, the side-by-side appearance of LEDs in an array may appear unacceptably variegated.
Low Bias Current LEDs
Through advancements in material science, construction techniques, and packaging topologies, manufacturers have developed dedicated lines of LEDs for low current applications. As shown in Figure 5, these LEDs can maintain an acceptable spread of luminous variation even at one or two milliamperes of bias.
Figure 5. Luminous intensity variation for LEDs designed to operate at low bias currents.
ROHM has introduced two such lines of low-power LEDs: PicoLED and SecoLED. As shown in Figure 6 below, these LEDs are characterized at one and two milliamperes of forward bias respectively and exhibit a luminous intensity between 2.0 and 13.0 mcd, depending on wavelength.
Figure 6. ROHM’s dedicated lines of low-power LEDs: PicoLED and SecoLED.
The development and introduction of LEDs specifically designed for low-current applications is an excellent example of today’s general advancement in illumination technology. Such devices have become additional tools and levers available to product designers that impact every aspect of performance, from user interaction to battery life.
Customizable LED Colors
Most design engineers are now familiar with the limited array of LED colors for illuminating products. The typical palette consists of red, amber, yellow, green, and blue, greatly limiting the possibilities for interesting LED interfaces. However, in recent years, the addition of specialized phosphors to the LED fabrication process have opened up a new world of color customization.
Phosphors absorb light in the deep-blue and ultraviolet spectrum and then emit visible light of a different wavelength. In Figure 7, the integration of phosphors into the epoxy resin of a blue LED can yield a pastel color that was previously unattainable.
Figure 7. Combining a blue LED with various phosphors to alter the final color.
Almost any color can be realized by carefully selecting the appropriate phosphors and base LED die (Figure 8). In particular, advancements in red phosphor have been instrumental in the generation of warm white LEDs that compete with traditional incandescent bulbs.
Figure 8. Chromaticity diagram for phosphor tuned LED.
The recent advancement of LED technology concerning efficiency and luminous output has given birth to a new category of low-power LEDs with bias currents in the single-digit milliamperes. These devices are critical in battery-powered electronics, such as wearables, where energy constraints are the tightest and the look and feel of the UI is paramount. When combined with customized color tuning, modern LED solutions can enable high-quality LED arrays with limitless design freedom. To learn more, visit ROHM’s website using the links below:
All other images were used courtesy of ROHM.
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