The variable spectral filter changes wavelength along the length of the filter. These filters have the same transmission characteristic as the standard fixed narrow bandpass filters. The undesirable secondary transmission ranges are blocked by graduated color glasses.

When the linear variable spectral filters are used with a slit on the entrance side, broadening the slit leads to a wider transmission curve and a reduction of the maximum transmittance. Slit widths of up to 1 mm have practically no influence on the spectral characteristics. The method of manufacturing these filters leads to slight deviations in the dispersion and linearity from filter to filter.