Derivative Scanning for Wavelength Identification

With spectrographs of low dispersion, wavelength identification can become difficult even under high magnification. With prism instruments, such as the Bausch and Lomb Medium Quartz spectrograph used here, the problem is further compounded by nonlinear display on the photograph. However, if a small region of the spectrum is recorded, it is possible to calculate a dispersion that will be approximately linear. The problem is then reduced to measuring the distance between lines of known wavelengths on the strip chart recording. Such distances can be measured with good accuracy if a first derivative tracing is made from the densitometer output, rather than the conventional transmission recording.