Wavelength accuracy in spectrophotometry is assessed using which materials?

• A. Didymium glass or holmium oxide
• B. Neutral density filters
• C. Dichromate solution
• D. Sharp cutoff filters

Answer: (A)

Wavelength accuracy is about how closely the instrument’s reported wavelengths match the true wavelengths of light. To test this reliably, you need a standard that provides many sharp, well-defined spectral features at known wavelengths. Holmium oxide and didymium glass meet this need because they show a dense set of narrow absorption lines at precise wavelengths across the visible region. By measuring where the instrument places these lines, you can verify and adjust the wavelength scale to ensure it matches the true values over the spectrum.

Neutral density filters don’t provide spectral features to compare against; they just reduce light intensity. Dichromate solution is used more for checking photometric accuracy and instrument response at a given wavelength, not for calibrating the wavelength scale. Sharp cutoff filters alter transmission with wavelength but don’t present fixed reference lines for wavelength calibration. So the materials that best assess wavelength accuracy are didymium glass or holmium oxide because they offer precise, known spectral lines across the spectrum.