Solid state devices such as Lumex LEDs are very reliable and will outlive the intended life of any equipment they are designed into. On the other hand, if they are misused or powered the wrong way they last but only a few seconds. The limits are:
- Power Dissipation: This is the maximum power into the life that can be supported by the device without heating up the pn junction. Once the rating is exceeded, the luminous efficiency will drop and the device will eventually fail if this rating is seriously violated.
- Power Derating over +25˚C: LEDs are sensitive to ambient heat. If the ambient temperature (around and on the case of the device) exceeds +25˚C, then the ability of the device to support the rated power will decline by the specified mW or mA per each degree centigrade.
- Continuous Forward Current: This value states the maximum continuous forward current with which the device should be driven. Typically life deterioration is not as severe if overrating is not more than 20% of stated value. Any overrating past 20% will reduce operating life and in time light efficiency and output.
- Peak Forward Pulsed Current: Most devices are rated under 10µS pulse 10% duty cycle pulsing. If values for other pulsing rates are designed please contact Lumex directly.
- Light Color: The emitted color when a device is powered is stated in terms of commonly known colors. This should not be confused with epoxy lens color. The light colors of LED devices are dictated by the tiny chip (.010″ x .010″) used inside the lens. These chips generate a very narrow band of light. They are labeled as monochromatic light sources as compared to incandescent sources that emit light covering the full spectrum of visible radiation. Bi-color devices utilize two distinct chips of the desired colors. The typical wavelength of the light color is also stated. Lumex LEDs utilize chips that are proven quality and value. New colors and higher intensity chips are constantly developed but are incorporated only after timely and demanding testing and evaluation.
- Epoxy Color: This is the color of the epoxy lens when the device is not powered. The epoxy color must be neutral or complimentary to the light source color to allow light to come through. Combinations of coloring and diffusing chemicals are added to a basically crystal clear epoxy reins to produce four types of lenses.
- Colored and diffused
- Colored transparent
- Non-colored diffused
- Crystal clear
Diffused lenses offer wide viewing angles at the expense of on-axis light intensity while non-diffused types have narrow viewing angles but much higher on-axis intensities. These two lens variables along with the addition of chip location variations (relative to the lens top) can create astronomical variations in published light intensity values. Only you can be the judge of how bright a LED is in your product. The correct selection of colors and light intensities is still very subjective.
- Operating Current: The recommended steady continuous forward current values are stated. At this current level, the device will function at its best giving you the right color and light intensity. LEDs are current driven devices and must be operated with constant and limited current. Current limiting resistors can be used to safeguard from current variations.
- Operating Voltage: The forward voltage drops when the voltage is operated at the specified forward current is stated in typical and maximum values. For higher operating voltages, one must incorporate current limiting resistors. The practically maximum voltages one can operate LEDs is 49 volts. Higher voltages require large high wattage resistors. A.C. operations are possible using Bi-polar devices (two chips sharing anodes and cathodes in reverse) or by the use of rectifying diodes in single chip devices. In both cases, current limiting resistors must still be used.
- Average Brightness: The average visible light-output in millicandellas of foot lamberts is specified under a given value or forward current (typically 20mA If). All Lumex LEDs are 100% inspected and graded so that each batch and lot (typically 1000 pieces) contain units of uniform intensity. Foot lambert units are used when the device has large reflective and diffusing lenses and reflectors and a central light spot cannot be located.