Abstract: Minority carrier diffusion lengths, especially long diffusion lengths that exceed the thickness of the wafer, are determined accurately and conveniently using techniques that limit errors due to lateral carrier diffusion, surface reflectivity, temperature variations, and inherent limitations in standard computation techniques that assume a diffusion length shorter than the wafer thickness. In particular embodiments, a probe is provided that senses the photovoltage in an area spaced from the edge of the illuminated region to provide a measurement substantially free of error from lateral carrier diffusion. The probe may also measure surface reflectivity simultaneously with measurement of photovoltage. Reflectivity correction is particularly beneficial in the analysis of wafers with dielectric coatings.