What's the difference between peak wavelength and dominant wavelength?

Simple answer:  Peak wavelength is the peak value measured by a spectrometer and dominant wavelength is calculated using eye response to give a single wavelength that describes what the LED looks like.

Peak Wavelength

We will use the SST-10-G as an example. The SPD below shows a typical set of normalized values measured using a spectrometer. There is a distinct peak value at 535 nm.  This is the peak wavelength for this example SPD.  The peak wavelength and associated SPD in W/nm are used for light calculations and lens/filter design. 

Luminus green LEDs are binned by peak wavelength and radiometric power as shown below.  Since the eye response is a maximum at 555 nm, the same radiometric power produces more lumens as the peak wavelength gets closer to 555 nm.

Peak wavelength and radiometric units are used in most light calculations.  The SST-10-G is most commonly used in instrumentation applications so this datasheet has mostly radiometric units.

Dominant Wavelength

Dominant wavelength is more about what the light source look like. Light and color perception are pretty complicated topics.  Here we will introduce the basic concepts needed to understand dominant wavelengths and cover more topics in future articles.

This calculator helps to illustrate these concepts: calculator link.

When the calculator starts, it is set on Illuminant E, the most common reference point used for dominant wavelength calculations.  Luminus uses Illuminant E as a reference point for all of the dominant wavelengths cited in our datasheets. 

The 1931 CIE "horseshoe" is constructed by using single wavelength inputs in the mathematical operation used to calculate CIEx and CIEy coordinates.  The horseshoe does not have a bottom; there are no monochromatic wavelengths on what is called the line of purples.  Generating a color near the line of purples with real light sources requires a combination of red and blue light sources. The complementary wavelength is used when the dominant wavelength of such a source intersects the line of purples. The convention is to use a negative number to indicate a complementary wavelength.

In the figures below, the horseshoe is constructed using 5 nm increments from 400 nm to 700 nm.  The red and blue points tend to pile up at the ends of the horseshoe which was one of the motivations in developing the 1976 CIE diagram (u', v'). All of the colors that can be perceived by human vision are defined inside the horseshoe + the line of purples.

The dominant wavelength of an LED is the intersection between a line projected from the white reference point, through the CIEx, CIEy coordinate of the LED to the horseshoe.  There is no simple formula for dominant wavelength, it is determined using algorithms combined with geometry. 

If the LED CIEx, CIEy point is close to the horseshoe, the color has higher purity.  Red and blue LEDs tend to be close to the horseshoe and have high purity.  Green LED sources tend to be farther from the horseshoe and have lower purity. Visible lasers are nearly monochromatic and lie on the horseshoe.

Using dominant wavelength for color LEDs is useful for designers that are interested in the perceptual aspects of their system.  Dominant wavelength is a best measure of what a color LED looks like. Dominant wavelength should not be used for white LEDs.  They should be characterized using CCT and Duv. 

Calculations:

Color-Science-Calculate-a-MacAdam-Ellipse-Table

Color-Science-Calculate-CCT-and-Dominant-Wavelength-from-CIE-x-y-point

Color-Science-Calculate-CIEx-and-CIEy-from-an-SPD

Color-Science-Calculate-Dominant-Centroid-Peak-Wavelengths-from-an-Excel-File-Datalore-Notebook-

Color-Science-CIE-1931-Reference-Structures-Excel-

Data-Analysis-Python-Luxpy-Package

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