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Abstract: We demonstrate that high power light-emitting diodes (LED’s) exhibit low-frequency noise characteristics that are clearly superior to those of quartz tungsten halogen lamps, the non-coherent light source most commonly employed when freedom from intensity variation is critical. Their extreme stability over tens of seconds (combined with readily selectable wavelength) makes high power LED’s ideal light sources for DC recording of optical changes, from living cells and tissues, that last more than a few hundred milliseconds. These optical signals (ΔI/I0) may be intrinsic (light scattering, absorbance or fluorescence) or extrinsic (absorbance or fluorescence from probe molecules) and we show that changes as small as ~8 × 10-5 can be recorded without signal averaging when LED’s are used as monochromatic light sources. Here, rapid and slow changes in the intrinsic optical properties of mammalian peptidergic nerve terminals are used to illustrate the advantages of high power LED’s compared to filament bulbs. [Copyright &y& Elsevier]