Illumination: LEDs 101

by all | 13 February 2015 2:02 pm

By Pierre Carreau
Light-emitting diodes (LEDs) are everywhere today, from traffic lights to medical equipment and, in the sign industry, from small-format tickers to massive outdoor billboards. In the past five years, the prices of LEDs have dropped by nearly half, their efficiency has more or less doubled and new technologies have been introduced.

Despite these trends, however, many professionals in the sign industry do not understand all of the fundamentals of LEDs that are necessary for them to recognize the best opportunities to use them in their work.

Historic milestones
The history of LEDs goes back to 1906, when Henry Joseph Round, personal assistant to the famous Italian inventor Guglielmo Marconi, became the first person to observe electroluminescence by applying 10 V of electricity to a silicon carbide (SiC) crystal. This was followed in 1927 by Russian scientist Oleg Vladimirovich Losev observing light emission from a zinc oxide and SiC crystal diode.

In 1962, General Electric (GE) researcher Dr. Nick Holonyak, Jr., invented the first practical visible-spectrum LED, which shone red. This led to the common sight of red LEDs in elevator panels and other mainstream applications.

In an LED, an electrical current passes through a semiconductor material from a positive layer to a negative one. As it does so, it pushes both of these layers into an ‘active’ layer, the collision of which creates light in the form of a photon.

“The current itself is the light,” said Holynak in 2012, reflecting on the 50-year anniversary of his discovery.

In 1990, Shuji Nakamura, an employee at Nichia, a Japanese chemical engineering and manufacturing company, invented the first high-brightness (HB) gallium nitride (GaN) LED, which shone blue. When partially converted to yellow by adding a phosphor coating, this finally enabled LEDs to mimic standard white light.

Different LEDs
As exemplified by the aforementioned historic milestones, different minerals can be used to create different colours. Today, LEDs can be not only red or blue, but also yellow, orange or green. By mixing the appropriate phosphor with the encapsulate compound, the right colour for a given application can be achieved.

There are also different types of LEDs. Conventional radial wired LEDs offer only very low lumen output and are mostly used as light sources for status indicators on control panels and similar applications.

Surface-mounted device (SMD) LEDs are brighter and have become common in the sign industry today. They have helped ‘miniaturize’ lighting systems and, from their manufacturers’ perspective, one of their primary advantages is enabling automated assembly.

Chip-on-board (CoB) LEDs offer even higher lumen output, along with improved thermal management, longer life in the field and less need for maintenance. Miniaturization is maximized and they allow high-density packaging.

One of the latest technologies is organic LEDs (OLEDs), which could potentially be printed on flexible substrates, allowing new capabilities in sign lighting design. While it is unclear as yet how they will be used, they will certainly represent a push away from polymer electroluminescent light (PEL).

From chip to module
LED chip manufacturers like Nichia, Cree, Philips and Samsung, which produce wafers containing thousands of individual chips, do not supply directly to the sign industry. Rather, sign shops buy from LED module manufacturers—but it is important to note there are very different types of companies that identify themselves as module manufacturers.

A select few apply their own phosphors, for example, while many others strictly buy SMD chips and are not equipped for handling wafers and phosphors in-house.

Some outsource all fabrication processes to other companies, but to their own specifications, developed in-house. And finally, some so-called manufacturers simply buy finished products from a third party and sell them under their own private-label brands.

One of the biggest concerns facing LED module manufacturers is binning them by colour. As LEDs are mass-produced in wafers and then separated into individual chips, sorting them with precise binning is a difficult, time-consuming and costly process, comparable to sorting grapes to make a fine wine.

If an LED module manufacturer accepts a wide range of LEDs, the price is greatly reduced because they have not been as carefully binned. The downside is a higher probability of negative appearance, because the variations in the LEDs’ performance will be substantially increased.

It can be particularly challenging to match colours between LEDs from different manufacturers. White LEDs, for example, can vary because, as mentioned, they start out as blue LEDs and then a phosphor is added to produce white light. Different manufacturers use different qualities of phosphors, so even when the resulting LEDs share the same colour temperature, they can end up with easily visible variations in the colour of their light. Fortunately, this problem has been tackled in recent years and most manufacturers are now closer to a shared standard.

Another way to achieve consistent white light output without the binning process is to calibrate each chip one-by-one, by applying the right quantity and colour of phosphor on an individual basis. All blue chips on a wafer may appear different, for example, but a light sensor can analyze each of them and then apply phosphor appropriately.

Only a few manufacturers so far have the capability to deploy such a process, but it is highly advantageous, as it ensures colours are always precise and identical from batch to batch.

Measuring performance
The aforementioned colour temperature is measured in degrees Kelvin (K). The white light spectrum used in the LED industry ranges from 2,800 K (amber) to 13,000 K (blue-white), with the most common being 6,500 K (daylight). In a major sign program, it is very important to take a closer look at the LEDs to ensure they represent their specified colour temperature accurately.

Another attribute mentioned earlier is lumen output, which indicates the intensity of an LED module. This becomes a major factor when populating a channel letter with multiple LEDs.

One lumen is equal to the amount of light visible from a single candle from 0.3 m (1 ft) away. Typically, LED modules offer three levels of brightness to fit most applications’ needs: low output (10 to 30 lumens per module), medium output (30 to 75 lumens per module) and high output 
(75 to 150 lumens per module).

When determining which modules to use for a particular signage application, however, it is also important to check the beam angle, measured in degrees. Industry standards include 120, 140 and 155 degrees. The wider the beam angle, the lower the intensity, as the light is more greatly diffused, but the greater the coverage across a sign face. With a narrower beam angle, the light is less diffused and more intense, but more LEDs will be needed to cover the same area on
a sign face.

Brightness in lumens is not necessarily dictated by the LED module’s required wattage, which merely indicates energy consumption. Rather, the most important measurement is an LED module’s lumens-per-watt ratio, which indicates illumination efficiency.

In Europe, an LED’s ingress protection (IP) rating measures resistance against dust and water. The higher the rating, the more dustproof and waterproof the LED will be. The Canadian Electrical Code (CEC) does not account for IP ratings; they are strictly a ‘comfort zone’ consideration for signmakers here.

It is also important to select the right power supply, given signs are often installed in harsh environments. Different power supplies are available for indoor and outdoor installations in dry or damp, hot or cold locations and to meet local rules and regulations.

The power supply also needs to be reliable. Some 90 per cent of service calls relating to LED-illuminated signs are related to problems with their power supplies. Unfortunately, pricing often dictates selection. For a few dollars less, some signmakers will choose a power supply that will, in the end, negatively affect the performance of the sign.

Pierre Carreau is a Montreal-based senior account manager for Allanson International, which manufactures power supplies for LED signage. For more information, contact him via e-mail at pcarreau@allanson.com.

Source URL: https://www.signmedia.ca/illumination-leds-101/

---