When measuring pressures in the diamond anvil cell, undersirable line broadening and wavelength shifts result if small ruby crystals are used in a calibration system designed for large ones. These effects can be eliminated through the proper choice of objective lens, condensing lens, and slit system. Using such a system, we obtained excellent results with samples as small as 30 &mgr;m. In addition, we describe the use of the spectrum from a neon light to provide an internal wavelength standard during the measurement of the ruby spectrum. This eliminates the stripchart ‐ wavelength drive synchronization error, a potential source of large errors. A calculation of the estimated error associated with a typical pressure measurement indicates that, for small ruby samples, the pressure can be determined with a precision of ±0.5 kbar.