These filters possess level spectral transmittance characteristics in the visible region, and are numbered with the average value of transmittances at 10nm steps in the 400nm to 700nm range. ND-50, for instance, signifies an average transmittance of 50%. Since the filters control light intensity without selectively absorbing energies at specific wavelengths, they are ideal for exposure control in color photography, and are also used for light control in various other measuring instruments. Sine neutral density filters exhibit variations in spectral characteristics depending on melting conditions, they are designed to uphold designated performance capabilities through control of the thickness. Polishing tolerances are normally ±0.1 mm, and the lower the transmittance of glass types, the more significant will be the transmittance variations caused by the tolerances. Employment of adequate types of neutral-density filters and control of their thickness enable formation of combinations of filters with any transmittances that may be desired.