Middle position of an optical bandpass.

The nominal operating wavelength(s).

(CWL): The arithmetic center of the passband of a bandpass filter. It is not necessarily the same as the peak wavelength.

In a laser, the nominal value central operating wavelength. It is the wavelength defined by a peak mode measurement where the effective optical power resides (see illustration). In an LED, the average of the two wavelengths measured at the half amplitude points of the power spectrum.