The laser wavelength can be determined by measuring the angles for several orders. In theory, it is also possible to calculate the wavelength separation between laser modes or the laser tuning range in this fashion. Unfortunately, it is often difficult to measure the angles hi and hm precisely. Instead, the change in wavelength can be determined in terms of the change in angle of the grating. By comparing the position of the grating when hiผhm (zeroth order aligned through the spectrometer) to the angle of the grating when other orders are aligned, the change in the grating angle hg can be determined. It should be noted that hi can become a negative number, as would happen for the system described here for the
second order.