LED Types

LEDs come in two basic categories

Low power LEDs come in 3, 5 and 8 mm sizes. 5 mm is the most common size. Due to heat dissipation limitations set by epoxy encapsulation, maximum driving current is limited to ~20 mA. Typical forward voltage is 3.2V DC. These characteristics limit LED power to 0.1 W range and light output rarely exceeds 2-3 lumen.

High power LEDs come in 1-5 Watt packages with single LED chip and up to 40 Watt  for multi-chip lamps. The HB LEDs are driven at much higher currents, typically 350, 700, or 1000 mA. As of today, commercially available 1W packages can produce up to 120 lumen of light. (March, 2009)

Conventional 5-mm LED
High power LED structure (source: Lumileds)
5mm-led high-power-led-structure