Good sources which approximate such light include single mode lasers and very low pressure, cooled spectral calibration lamps. Sometimes a continuous spectral segment may be only a few parts of a nanometer wide and resemble a line spectrum. In most monochromators the location of the entrance and exit slits are fixed and the grating rotates around a plane through the center of its face. 1 and 2 and Section 2.6 . It is easy to imagine that fine spectral detail would be more easily identified in the first instrument than the second.
Linear dispersion is associated with an instrument's ability to resolve fine spectral detail.
In a monochromator, L B is the arm length from the focusing mirror to the exit slit or if the grating is concave, from the grating to the exit slit. Linear dispersion, therefore, varies directly with cos beta, and inversely with the exit path length, L B , order, k, and groove density, n. Lambda n . In order to monitor only light at 800 nm, filters must be used to eliminate the higher orders. First order wavelengths between 200 and 380 nm may be monitored without filters because wavelengths below 190 nm are absorbed by air.