Difference betweenλd  (main wavelength) and λp (peak wavelength)

1, WLP (peak wavelength): spectral emission intensity or radiation power of the highest frequency corresponding to its part is λp.It is a kind of physical quantity,

2, WLD (main wavelength): the eye can see the main color of the light source  that this main wavelength corresponds to.in the CIE1931 horseshoe coordinates, from the E point (0.33, 0.33) as the object to do an extension cord with a horseshoe shaped curve in the formed picture.

In usually you see a beam of light, it is not a single wavelength of light, it is composed of many wavelengths of light. Among them,Some of the energy of light wavelength is greater that others.the wavelength is the peak wavelength of the beam(λp).
But the light wavelength distribution of the beam is not necessarily  as λpunits to Symmetric distribution. The human sense wavelength is a result of comprehensive effect of each wavelength. corresponds to a single wavelength of light . the wavelength value is λd . dividing an curve area into right and left halves. The Vertical line would be correlated toλd .You want to know light color ofuv led strip，you’ll have to see the lambda D value(λd), because the λd would  more accurately reflects the true colours ofuv led strip.