Cary 14 Spectrophotometer
The Cary Model 14 UV-VIS Spectrophotometer was a double beam recording spectrophotometer designed to operate over the wide spectral range of ultraviolet, visible and near infrared wavelengths (UV/Vis/NIR). This included wavelengths ranging from 185 nanometers to 870 nanometers. (The Cary Model 14B, almost identical in exterior appearance, measured wavelengths from .5 to 6.0 microns.) The Cary 14 spectrophotometer was first produced in 1954 by the Applied Physics Corporation, which later was named the Cary Instruments Corporation after co-founder Howard Cary. The instrument was a successor to the Cary 11, which was the first  commercially available recording UV/Vis spectrophotometer. It was produced until 1980, and refurbished models can still be obtained. Design and use. The double beam design of the Cary 14 provided rapid, simplified analysis by simultaneously measuring the transmittance of both the sample and the reference over the entire spectral range. The optics of the Cary 14 were a key feature. The double monochromator in particular was described and patented. The Cary 14 was one of the first instruments to incorporate high-quality gratings into its monochromators. To take readings in the ultraviolet or visible range, either a deuterium or tungsten  lamp was used, with the light focussed into the entrance slit. The light passed through the first monochromator, which used a 30° Littrow prism, through the intermediate slit, and then into the second monochromator, which used an echellete grating with 6000 grooves/cm to the exit slit. The beam from the monochromator then reflected from a rotating semicircular mirror and beam chopper, sending the light alternately into compartments for the sample and the reference, separated by dark periods. The beams from the sample and reference alternately registered on the photomultiplier (pmt) detector, with the pmt output in the dark  intervals subtracted from both measurements. The measured absorption or transmittance was calculated from the difference in the dark-corrected sample and reference measurements. To take readings in the near infrared, a tungsten ribbon-filament lamp was used instead of the photomultiplier tube. An additional mirror was used to direct the light beam onto a lead sulfide photoconductive cell and reverse the light's path through the monochromators. The instrument had a built-in chart recorder for displaying the analog signal on paper. By using a double-pen mechanism, an effective chart width of 20 inches could be obtained. The original  hydrogen discharge lamp was water cooled. Samples and reference solvent were held in the sample chambers by a variety of means, most typically using 1 centimeter pathlength cuvettes made of glass or quartz. By combining the Littrow prism and the echellete grating, the Cary design minimized noise and interference (stray light) while obtaining high resolution measurements over a very wide dynamic range. The instrument was widely used for studies of chemical bonding, quantitative analysis, and rates of chemical reaction. The use of the instrument generally necessitated that substances being studied are in the solution state. Production. The Cary 14 was  produced until 1980. Its selling price in 1960 was approximately US $20,000. Cary Instruments replaced production of the Cary 14 with the Cary 17 beginning in 1970. Cary recording spectrophotometers, including the Cary 14, were contemporary to the single beam, non-scanning Spectronic 20 spectrophotometer. These instruments were complementary and were used in academic and analytical settings through the 1950s, 1960s, and 1970s. Although the Cary 14 is out of production, refurbished versions of it that retain the original optics but with modernized, digital electronics are commercially available as of 2017. Versions of modernized Cary 14 spectrophotometers extend the  wavelength range to 2500 nanometers in the near infrared spectrum.