The centroid wavelength of a spectrum can be determined directly from the ratio of quantum and radiant flux values. This method is a fundamental spectral measurement since it satisfies the mathematical definition of centroid wavelength. Using broad-band methods for measuring quantum and radiant flux, the centroid wavelength can be determined from a ratio of photocurrents. This simple technique yields spectral information, which is normally available only by spectroradiometry. The technique is extendable beyond the visible spectrum, and has useful applications beyond LED measurements. LEDs are used to demonstrate this method for determining centroid wavelength. The LEDs are characterized by both broad band methods and spectroradiometry. The results are discussed and evaluated. The measured LEDs range from red to blue and also include white. The broad-band measurements are made without prior knowledge of the spectral distribution. For LEDs, the accuracy can be improved by using correction techniques. The broad-band centroid wavelength is within +/- 5 nm of the value determined by spectroradiometry.