Standards quality irradiance meters have been developed at the National Institute of Standards and Technology to realize a detector based spectral irradiance scale. The design criteria and the optical and radiometric characterization of diffuser-input irradiance meters are discussed. The input geometry optimization is described for a broad-band InGaAs irradiance meter when measuring a 2856 K incandescent light source. The directional responsivity of the fabricated InGaAs and Si irradiance meters matches the cosine function with a maximum deviation of 0.15% within a 12 degree(s) field-of-view. Irradiance from sources of different sizes (incident beams with different f/numbers) can be measured over a wavelength range of 350 nm to 1800 nm.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.