Simple inexpensive high performance optical absorption detectors are possible using light emitting diodes (LEDs) as light sources. The designs presented in the literature are reviewed. Designs used by the investigators are described in detail with respect to construction, electronic design, performance and cost; these have not previously been described in the literature. Characteristics of commercially available LEDs are tabulated. At the simple end, a single beam, dc driven, transmittance output detector can be constructed within the body of a LED. At the high performance end, fully referenced, computer interfaced detectors are described that are pulsed at high speeds to attain measurement standard deviations in the range of 2-3 x 10(-6) absorbance.