Which lighting application was one of the first to embrace a mass adoption of LEDs? Signage.
Red, orange, yellow, blue, and green LED’s were the first to reach performance levels capable of providing a compelling reason for sign manufacturers to switch from traditional light sources to LED.
Their unique characteristics—including compact size, ease of installation, resistance to breakage and vibration, good performance in cold temperatures, reduced maintenance costs, long life, and potential for energy savings as compared to the traditional products—drove the high rate of adoption. A few years later, the steep climb in efficiency and overall performance of white LEDs allowed them to enter the mass signage market.
The high efficacy (lumens per watt) performance of white LEDs lead to their rapid adoption into signage and other early adopting mainstream applications such as refrigeration case and roadway lighting. Most of the white light technologies that were developed over the past 130 years, such as high intensity discharge (HID) lamps, have shown an initial rapid increase in efficacy and then many years of very slow improvements.
Often referred to as the “hockey stick curve" due to its shape, the performance curve of white LED light technology is steep, showing an exponential increase over a very short period of time. White LEDs of today have hit lumen per watt (lm/W) levels that were unheard of only a couple years ago.
Some types of discrete phosphor converted white LEDs have surpassed 200 lm/W and have an estimated quantum limit of 255 lm/W with long-term research and development goals to reach 350 lm/W with new methods. The overall efficiency of the complete LED system is, however, less due to other system component losses.
有些类型的磷光体转换白光LED能达到每瓦200流明的光度，但量子的限制为每瓦255流明的光度255lm / W，经过长期研发，目标将达到每瓦350流明的光度。尽管能达到这样的光度，LED系统的总体功效还是会因为其它组件的损耗而降低。
Losses internal to the LED package, electrical, thermal, and optical losses in an LED module, and power supply losses all reduce overall LED system efficiency, but LED systems still have much higher system efficacy than traditional light sources. Additionally, unlike fluorescent or neon, the directional nature of the light from LED sources also contributes to higher overall system efficiencies due to a more efficient use of the light produced.
But what really constitutes a good quality LED that can be effectively integrated into an LED signage module or an LED light engine for a general lighting luminaire? To start, we need some basic understanding of what a light emitting diode (LED) is and how it is manufactured. LEDs are semiconductor diode components that convert electrical current into light (photons).
How Does an LED emit light?
Every LED chip consists of two semiconductor material types:
N-Type material—extra electrons (negative charge carriers)
N型材料 - 额外电子（负电荷载体）
P-Type material—holes (positive charge carriers)
When voltage is applied, electrons combine with holes to release energy in the form of light and heat.
LEDs are grown on substrates on which layers are grown atom by atom. After various processing steps, the entire wafer is diced into individual chips. Each wafer can yield thousands of chips. Those chips are placed into a discrete LED package that can then be manufactured into a lighting module. The chip design, fabrication process, and package all contribute to and determine an LED’s performance. New advanced LED designs with improved light extraction and beam-shaping lenses have helped drive better performance.
Achieving White Light
The light that is produced from an LED is generated in a very narrow wavelength band, meaning that the light is typically a saturated color. The semiconducting material used in an LED determines its wavelength or color of light. Because of this, there is really no such thing as an LED that by itself emits white light.
White light from LEDs is most widely achieved through the use of a blue LED chip combined with a phosphor, similar to the way a fluorescent lamp operates. The phosphor layer absorbs a portion of the blue light and emits light at longer wavelengths. The phosphor concentration defines how much of the blue light is converted. The phosphor can be applied over the LED die, such as an encapsulation or coating process, or placed at some distance from the blue source, such as a remote phosphor approach.
Again, the LED chip design, package design, and phosphor conversion all contribute to the overall efficacy (lm/W) performance of the LED. In signage applications there are other performance characteristics of LEDs that are important, such as thermal resistance, robustness to the environment, color over angle, chromaticity shift and lumen depreciation over time, to name a few.
LM-80 Test Data
LED package manufacturers provide long term lumen depreciation and color stability data at various operating currents and temperatures in a test report called LM-80. This information can be used to extrapolate estimated life performance of an LED in real world applications. It is from this information, along with the thermal properties of the LED in the sign module into which it is built, that a sign module manufacturer can estimate long term life performance of a sign module.
The biggest enemy affecting long-term performance of the LED is heat. Heat can affect multiple components of an LED package resulting in more rapid lumen depreciation and chromaticity shift over time. The faster the lumen depreciation, the shorter the effective life of the LED as depicted in the figure below.
In choosing a discrete LED package supplier for making LED sign modules, what does a reputable manufacturer look for in relation to LED quality? Here are a few of the more important properties:
Color over Angle (color distribution)
What does this all mean to the sign maker, and how do you know what you are getting in your LED sign module? Well, ask questions. Know what LED is used in the module you buy. Understand the thermal limitations of the module. The difference between a sign module made with a quality LED and one made with one of suspect quality performance can be the difference between a happy, loyal, long-term customer and the other kind.