Home >> Science & Nature >> Science

Relationship of Brightness & Time in a Power LED

LEDs are diodes constructed to behave as light sources. They are found in remotes, lamps and toys. Light bulbs will burn out and abruptly cease to work, but LEDs do not. They will gradually lose their brightness over a period of several years. The time period is determined by their quality and usage. A power LED can handle more current than a regular LED, but it is just as susceptible these factors.

High-Power LED Features
- A diode is made with two different types of semiconductors. The place where the semiconductors are joined is called a junction. A normal diode conducts electricity in one direction. It is called forward-biased when it in this state. A regular LED has two leads and will produce light in only when it is forward-biased. A power LED, also called a high-power LED, can handle currents of 100 milliamps or more when it is forward-biased. A normal LED does not have problems with heat dissipation, but the power LED does, because its junction temperature will increase very quickly.
Brightness
- LED brightness is subjective. It depends on the amount of current supplied to the LED; the higher the current, the brighter the device. Normal LEDs must be wired in series with a resistor, to control the amount of current it receives. Brightness is also referred to as intensity. It is measured with millicandela, called MCD for short. The MCD is defined as how bright the light is at its source, which is on its axis. A typical LED is 5 to 10 MCD, while ultra-bright ones can be 30,000 MCD or much higher. The total lifetime for an LED based on its ability to produce light may be from 11 to 13 years. It is usually presented in hours. To measure how light is affected by time, lumen maintenance is used.
Lumen Maintenance
- A lumen measures the amount of light that falls on a particular portion of a surface, or area. Lumen maintenance compares the brightness of a new LED with its brightness over a period of time. The two standards most often used are L70 and L50, where the L means lumen maintenance. L70 is when the LED̵7;s performance has degraded 70 percent, while L50 is when the performance has degraded 50 percent. If an LED had 500 lumens when it was brand new and dropped to 250 after 50,000 hours, then its lumen maintenance for that time period is L50.
LED Degradation
- Factors that affect lumen maintenance include materials, LED construction, damage from too much current and environment. The junction temperature also affects it. If the temperature is too high, the LED's lifetime is decreased. One ultra-bright LED can be constructed so that it degrades if the junction is above 20 degrees Celsius, while another can be made to withstand junction temperatures up to 40 degrees Celsius. The human eye can tell when an LED has lost 70 percent of its light, but this level is still acceptable for lighting. The L70 is therefore commonly used as a standard in the lighting industry.

Science