A near-room-temperature capacitively-coupled plasma (CCP) glow discharge cell is used for fundamental studies of low and intermediate pressure Ar plasmas. The cell consists of two cylindrical plate electrodes powered by a 13.56 MHz radio-frequency source, forming a diffuse low-pressure plasma between the electrodes inside a cylindrical glass envelope 10 cm in diameter. The cell is operated at pressures from 0.1 to 10 Torr and electrode voltages up to \(\sim \) 2 kV depending on pressure, producing plasmas in which the free electron temperature ranges up to several electron-volts while the gas remains close to ambient temperature ( \(\sim \) 300K). The emission lines from Ar in the visible and near infra-red (NIR) correspond primarily to transitions from 4p states to the metastable and resonant 4s Ar states. Calibrated Emission Spectroscopy using Absolute Radiance (CESAR) measurements on these transitions are used to determine population densities of Ar 4p states. Tunable laser absorption spectroscopy is used to measure the metastable and resonant Ar 4s state populations. These densities are presented in combination with electron density and energy distribution function measurements performed with a Langmuir probe. These measurements enable us to assess the degree of non-equilibrium in the plasma as a function of pressure and electrode voltage. We use Monte Carlo techniques to quantify the uncertainty of all the measurements. The experimental results are used to enhance our understanding of non-equilibrium plasmas and to validate kinetic models of excitation and ionization processes in plasmas.