LEDs are highly monochromatic, emitting a pure color in a narrow frequency range. The color emitted from an LED is identified by peak wavelength (lpk) and measured in nanometers (nm). Peak wavelength is a function of the LED chip material.
 Although process variations are 0 NM, the 565 to 600 NM wavelength spectral region is where the sensitivity level of the human eye is highest.
Therefore, it is easier to perceive color variations in yellow and amber LEDs than other colors.
LEDs are made from gallium-based crystals that contain one or more additional materials such as phosphorous to produce a distinct color.
Different LED chip technologies emit light in specific regions of the visible light spectrum and produce different intensity levels.

Luminous intensity (or candlepower)

It is the light density within a very small solid angle, in a specified direction. In other words, this is the total number of lumens from a surface emitted in a given direction. The unit of measure is candela. In modern standards, the candela is the basic of all measurements of light and all other units are derived from it. Candlepower measurements are often taken at various angles around the source and the results plotted to give a candlepower distribution curve. Such a curve shows luminous intensity (how “bright” the source look) in any direction.

Luminous flux

It is the time rate of flow of light. The unit of measure is the Lumen. One lumen may be defined as the light flux emitted in one unit solid angle by a one-candela uniform-point source. The lumen differs from the candela in that it is a measure of light flux irrespective of direction. The lumen is used to express a quantity of light flux: total output of a source, output within a specific angular zone, amount of absorbed light, etc.
However, if you need to calculate something which is not related to the human eye, for example temperature increase due to absorbed light, do not use luminous flux, instead we need to use the correct unit of power, the Watt (see below).