1 The Optima XL-I integrates both absorbance and interference optics on a single instrument; the Optima XL-A has absorbance optics only. 2 This optical system is called by a variety of names (Rayleigh optics, interference optics, and refractive optics). The names can be used interchangeably but it will be referred to here as Rayleigh interference optics. Choosing Which Optical System of the Opt...