The new spectrophotometers used a prism to separate light into its absorption spectrum and a phototube to electrically measure the light energy across the spectrum. They allowed the user to plot the light absorption spectrum of a substance, giving a standardized "fingerprint", characteristic of a compound. With Beckman's model D, later known as the DU spectrophotometer, National Technical Laboratories successfully provided the first easy-to-use single instrument containing both the optical and electronic components needed for ultraviolet-absorption spectrophotometry. The user could insert a sample, dial up the desired wavelength of light, and read the amount of absorption of that frequency from a simple meter. It produced accurate absorption spectra in both the ultraviolet and the visible regions of the spectrum with relative ease and repeatable accuracy. The National Bureau of Standards ran tests to certify that the DU's results were accurate and repeatable and recommended its use.