Signal-to-noise (S/N) ratio theory has proven to be useful in understanding, designing, and optimizing spectrophotometric measurement systems. Although a narrower spectral bandwidth does improve the resolution of closely spaced peaks, it also decreases the Signal-to-noise ratio. The narrowest slit width should be used that will yield an acceptable Signal-to-noise ratio. In this research, the wavelengths of peak absorbance of the holmium glass filter were determined to attain the optimum Signal-to-noise ratio accompanied with spectral bandwidths of 0.1 nm, 0.2 nm, 0.5 nm, 0.8 nm, 1 nm, 2 nm, 3 nm and 5 nm. The influence of spectral bandwidth on the Signal-to-noise ratio was by far the most important parameter affecting the location of the measured wavelengths of absorbance or transmittance of the sample.