Fixed wavelength detector:

It uses a light source that emits maximum light intensity at one or more discrete wavelengths that are isolated by appropriate filters. These have minimum noise but no free choice of wavelength. A medium pressure mercury lamp can be selected for wavelengths of 254, 280, 313, 334 and 365 nm by the use of narrow band pass interference filters. Visible region wavelengths can be accomplished by using a quartz iodine lamp and appropriate filters. These have short terms noise levels commonly <0.0001 absorbance unit.