Luminous flux (lm, Lumen)

Brightness is not equal to brightness:

Our human color perception is not easy to seize. Here a simply demonstration: One takes a green LED and a red LED, which deliver both the same radiating power. With a measuring instrument one would measure the same radiation with both light emitting diodes. Instrumentation the two LEDS would be equivalent. Humans see however differently. Our eye appears the green LED much brighter than the red LED. We determine, the relative eye sensitive depend on the wavelength of the light.

This dependence of the light sensitivity, of the wavelength is held in so-called light response curves of the CIE (Commission international de l'Eclairage) for a standard observer.
There are several of such curves for different lighting conditions (twilight, midday sun, etc.). The accompanying illustration shows the light response curve for daylight.

We see, that the maximum is about 550 nm, therefore with green light. With red light the sensitivity is more than the factor 10! smaller.
Therefore we take the red LED (wavelength approx.. 650 Nm) much darker truely than the green (wavelength approx.. 550 Nm), although they deliver physically the same radiating power.

The luminous flux of a colored LED is the product = of its theoretically maximally luminous efficiency (the maximum value lies with a wavelength 555nm and is named km). Km has a value of 683 lumen/Watts), it is a fixer value,
multiplied by the spectral light sensitivity degree associated for the respective wavelength
(Green = 555nm = 1, red = 670nm = 0,1) and the radiating power in Watts.

Example with 1 Watt LED:

Red LED 670nm = 683(lm/W) x 0,1 (Km) x 1 (W) = 68,3 lm
Green LED 555nm = 683 (lm/W) x 1 (Km) x 1 (W) = 683 lm
With white LEDS the light-current strength (lm) of the individual wavelengths is added.