Engineering Reference
LED Optical, Package and Current Terminology Reference
Lookup LED lumen, candela, lux, viewing-angle, package, current-rating and thermal terms with the conditions needed for valid comparisons.
Optical Terms
| Quantity | Unit | Meaning | Boundary |
|---|---|---|---|
| Luminous flux | lm | Total visible-light output weighted by human visual response | Does not describe beam direction or irradiance |
| Luminous intensity | cd | Flux per unit solid angle in a specified direction | Depends on angular distribution and measurement geometry |
| Illuminance | lx = lm/m² | Luminous flux incident on an area | Depends on distance, angle, beam and surface geometry |
| Viewing angle | degrees | Angular width defined at a stated relative-intensity criterion | Criterion such as half-intensity must be checked |
| Radiant flux | W | Total optical power without photopic weighting | Not interchangeable with lumens across wavelength |
| Luminous efficacy | lm/W | Luminous flux per electrical or radiant watt as explicitly defined | Confirm denominator and operating conditions |
Package Families
| Package | Typical role | Thermal and assembly boundary |
|---|---|---|
| 3 mm / 5 mm through-hole | Indicator and low-power signaling | Lead frame to ambient; board spacing and lead length affect heat |
| 0603 / 0805 / 1206 SMD | Indicators and compact illumination | PCB lands carry heat; optical orientation and polarity mark vary |
| PLCC / side-view | Displays, backlights and edge lighting | Beam direction and footprint orientation are package-specific |
| Ceramic high-power LED | Illumination and high-flux emitters | Thermal pad, junction-to-solder-point resistance and MCPCB matter |
| COB module | Dense area lighting | Module thermal resistance, substrate, drive current and optic must be designed together |
Current and Thermal Terms
| Term | Meaning | Boundary |
|---|---|---|
| IF | Continuous forward current under stated thermal conditions | Derate with junction/ambient temperature and thermal path |
| IFP | Peak pulsed forward current | Valid only for specified pulse width, duty cycle and repetition rate |
| Test current | Current at which VF, flux or color is characterized | Not automatically the recommended operating current |
| Current matching | Channel-to-channel or string-current consistency | Resistors alone may not equalize strings with different VF |
| Thermal droop | Light-output reduction as junction temperature rises | Electrical input power is not equal to emitted optical power |
Comparison Checklist
Compare LEDs at matched current and junction-temperature conditions; distinguish total flux from directional intensity; use the specified angular criterion; include driver loss and thermal resistance; and confirm bin, package, polarity, footprint and lifetime conditions.
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FAQ
What is the difference between lumen and candela?
Lumens describe total photopically weighted output; candela describes intensity per solid angle in a direction.
How are lux and lumens related?
One lux is one lumen per square meter, but actual illuminance depends on beam distribution, distance, angle and area.
Does a narrower viewing angle increase lumens?
Not necessarily. It can concentrate similar flux into higher on-axis intensity without increasing total flux.
Is peak LED current safe continuously?
No. Peak pulsed current applies only under specified pulse width, duty cycle, repetition and thermal conditions.
Does an LED test current equal its recommended current?
No. It is the characterization condition for a stated parameter.
Why does LED output fall when hot?
Junction temperature changes efficiency, forward voltage, wavelength and light output.
Can package size determine LED power?
No. Die, thermal pad, PCB, ambient, drive waveform and junction-temperature limit all matter.
Are electrical watts equal to optical watts?
No. Some electrical power becomes light and the remainder becomes heat; luminous flux also applies human-eye weighting.