A new HPLC absorbance detector, the Waters Model 996 is described. It covers the spectral range 190 to 800 nm with a nominal resolution of 1.2 nm, equivalent to one diode element. It is based on a deuterium arc source, a 512 element, self-scanned, photodiode array detector and a flat-field, aberration-corrected, concave holographic grating. A design approach is described which establishes the best cell geometry to minimize the concentration limit of detection through fully exploiting the performance potential of the key components. Sensitivity to flow stream artifacts, such as refractive index changes during gradient elution and pump- induced compositional ripple or thermal pulsations, are minimized in the optical design through the use of a 'reversed taper beam'. Source output is stabilized using a separate photodetector. After outlining the original objectives and the restrictions which they place on the design, we describe the optical system. Next we explain the logic behind the design, and end with representative performance data.