The Beckman DK2A Ratio Recording Spectroreflectometer is a
      spectroreflectometer using an integrating sphere with                 
      three ports: one for the detector; one for the sample;                  
      and one for the standard. Detectors are interchangable.                 
      A photomultiplier is used for visible wavelengths; a                    
      lead sulfide cell for infrared wavelengths. The detectors               
      measure relative reflectance of the sample with respect to              
      the standard by alternately illuminating both (via a                    
      mirror-chopper) and measuring the AC voltage resulting                  
      in the detector.                                                        
                                                                              
      The output is the relative reflectivity of the sample                   
      presented in two forms: as a plot on a stationary chart                 
      recorder; and as a sequence of values on punched paper                  
      tape spaced at 50 Angstrom intervals. The instrument                    
      also has an 'expanded scale' output option.                             
                                                                              
      The Beckman DK2A is described in detail by Adams and                    
      McCord (1970) [ADAMS&MCCORD1970].