There are three common backlight technologies for the LCD, the EL (Electroluminiscent Lamp), LED (Light Emitting Diode), and the CCFL (Cold Cathode Fluorescent Lamp).

EL (Electroluminiscent Lamp)

This type of backlight is very thin and offers a uniform light source. Some manufacturers offer various colors of EL with their products, but the most common color is the blue-green. Other colors available are white, amber, and blue. The EL is popular due to its relative low cost, as compared to the other backlight sources. In addition, the light emitted is cold and cool to the touch, it is uniform, and thin.

Although the EL has some great benefits, there are some draw backs to its technology. For starters, the EL does not perform very well under high humidity conditions. When the display is subject to high humidity, the EL begins to delaminate and starts to malfunction. Other problems with the EL include the need to have a DC-AC converter (known as an inverter), relative low brightness, and short life span (about 3000~5000hrs to half initial brightness).

Though these may be drawbacks, some designers do not worry about these things because they can be overcome relatively easy. Example, to extend the life of the EL, they can install an automatic 1 min shut off. So, if you use the backlight only 60 minutes a day, you technically can have your product last between 8~13 years. As for the brightness, if the product is rarely used at night, then the EL is more than sufficient.

Click hereto see a sample of an EL lamp.

LED (Light Emitting Diode)

The LED backlight is the most commonly used backlight for Cellular phones. The LED backlight offers some good benefits over the EL backlight. For starters, the LED backlight does not require an inverter, just a DC source of +3VDC~+5VDC, secondly, depending on the backlight configuration, the brightness can be very bright, and thirdly, the life of the LED exceeds 50K hours.

Some display vendors also offer color choices for the LED, the typical color is yellow-green, but more recently, blue, and white LED backlights have been offered.

The LED backlight can be configured in two ways, direct and edgelit (see Diag. 1.0 below). In the edgelit configuration, the backlight relies on a light guide to ensure that the light is distributed evenly behind the LCD. Otherwise hot spots will appear. The benefits of this type of configuration are low cost, low heat, thin profile, and long life. However, this configuration has a limitation with the number of LEDs allowed and the brightness. Since the LEDs are being lit from the side, the light is relatively low.

Diag. 1.0

The array configuration, on the other hand, is much brighter than the edgelit. This is due to two reasons; 1. The light source is directly behind the display and does not travel through a light guide, and 2. the number of LEDs is much higher (see Diag. 2.0)

Diag. 2.0

Despite its benefits, of brighter, longer life, and simple +3VDC~+5VDC operation, the array has its own drawbacks. One of them is the cost. Since there are more LEDs, the cost goes up. Another drawback is the heat generated from the LED. If the backlight is not properly current limited, the heat generated from the LEDs can cause a problem for the LCD panel, by causing the pixels/display to become darker. The other drawback is the thickness, since the LEDs are directly behind the display, a minimum distance is required to ensure proper distribution of the light. Otherwise you will be able to hot spots.

Click here to see pictures of the LED backlight display.

CCFL (Cold Cathode Fluorescent Lamp)

This type of light source is most common in graphics and color displays. It provides a uniform and bright white light. A common application that would use a CCFL backlight is a Laptop computer. Some other applications that use this type of backlight include, gas pumps, medical instruments, industrial PCs, POS, etc.

The CCFL has a drawback that is similar to the EL backlight...it needs a DC-AC Inverter. Though not the same type as the EL, the CCFL inverter needs to generate more voltage than those used for the EL. A typical EL inverter outputs 120VAC @400Hz, whereas the CCFL Inverter needs to output 1000VAC @ 30kHz~40kHz.

The typical life of the CCFL is about 20~25k Hrs. And there are newer CCFL technologies that are extending the life of these tubes to 50k Hrs.

Although the CCFL is the brightest and whitest light source, it is also the most expensive of the three. On average, the CCFL with DC-AC inverter is about 5~6 times the cost of an equivalent direct lit white LED backlight.

Click here to see pictures of the CFL backlight display.