INTRODUCTION This test describes a procedure to determine the numerical aperture of step-index@ glass core@ glass clad and plastic clad optical fibers (fiber material class Ic@ IIa@ and IIb as listed in EIA-4920000-A) from the fiber far-field radiation pattern. The applicability and usefulness of this procedure for other classes of step index fiber has not been determined. This procedure describes three methods by which the angular radiant intensity (far field) distribution from an optical fiber can be measured. Methods A and B are angular scans of the far field pattern; Method C is a scan of the spatial transform of the angular intensity pattern.