Wavelength accuracy is an important parameter of every spectrophotometer. Here you can learn more.
Wavelength accuracy is the ability of the device to operate at the selected wavelength.
If the wavelength accuracy of the instrument is ± 1 nm it means that for the selected center wavelength -500 nm, the real center wavelength may be in the range 499 - 501 nm.
Similar to photometric accuracy, wavelength accuracy can vary for different instruments.
Wavelength accuracy is determined by scanning a wavelength filter with sharp, clearly defined transmission peaks and comparing the results to those on the certificate of calibration provided.
Ideally, any reference materials used to determine wavelength accuracy should have narrow, well-defined peaks at a variety of wavelengths in the UV and visible range.
A holmium oxide solution is often used for this purpose.
Reference