The light output of LEDs is rated in either candelas (also candle) or lux, with the candela being the more common. Both are a measure of luminous intensity. The higher the candelas/lux, the brighter the light.
Standard LEDs emit only a modest amount of light, not even 1 candela. They’re rated in millicandelas, or thousandths of a candela. Millicandela is typically abbreviated as mcd. Typical intensity of a T1-3/4 (5mm) red LED is about 50 to 75 mcd.
Conversely, so-called superbright LEDs might have an intensity of 500 mcd to over 5 candelas.
Last, LEDs are rated by their viewing angle. Many high-brightness LEDs have a limited viewing angle—you need to be staring directly at them to see most of the light. As the axis of the LED is turned, the brightness falls off dramatically. Viewing angle is expressed in degrees.
You can use viewing angle to your advantage: