1. Bad Color Rendering

Currently, a white LED produced from a blue LED covered with yellow phosphor generally has bad color rendering (below 80%), as the emitted light lacks red and does not restore this color correctly. But a TL tube or a compact fluorescent lamp may have color rendering indices from 80% to more than 90%.

When the color rendering index exceeds 80%, it is specified in the technical characteristics of the LED. Today, we find white LEDs with a color rendering index greater than 80, yet the number of them is still quite small. Most of the white LEDs fail to meet with the requirements of the general lighting application.

2. Low Luminous Efficiency and High Price

The efficiency of a light source is determined by the luminous flux [lm] emitted per unit of power consumed by the light source [W]: lumen/Watt [lm/W].

Let’s make a comparison. The luminous efficiency of a common 60W incandescent bulb is higher than 700lm, while the luminous efficiency of a single LED is around 25lm. More LEDs are needed to replace the traditional lighting devices, which results in high cost.

3. Luminous Decay of LED Itself

In order to meet the requirement for brightness, power white LEDs are usually designed with high power. Due to the luminous decay of LED itself and the aging of phosphors and packaging materials, when LEDs working in the condition with long time and high current, the luminous flux of LEDs will be reduced.

When the luminous flux descends badly, failing to meet the requirement for lighting, the LED is no good any more. The worse the luminous decay is, the shorter the life-span of power white LEDs.

4. LED Optical System

Generally speaking, an LED bulb is composed of several LEDs. Thus the light LED bulbs emit is neither area light nor authentic point light. It’s the combination of many lighting sources. So, it is hard to achieve the same lighting effect when replacing the traditional bulbs.

5. LED Thermal Management

The thermal management of an incandescent filament or tungsten halogen lamp is designed on the fact, that the thermal radiation is the way to remove the heat from the lamp. Thus the thermal conductivity of the socket is very low. A replacement process has to ensure that the LED is protected from high temperature.

Though there are other problems concerning LEDs remain to be tackled, the state of the art LEDs show clear advantages for all applications, where colored light sources are necessary. The direct replacement of conventional lamps is an interesting way to increase the reliability of systems and to reduce the real costs of ownership. In the not too distant future we will see white LED light sources with further cost benefits and increased external efficiency, which will again show clear advantages over conventional light sources.