This work presents results of experimental studies of the spectral and photometric characteristics of optical radiation generated by a pulse-periodic microwave discharge close to ECR (2.45 GHz, average power of up to 200 W, argon pressure of 10–4–10–1 Torr). Under these conditions, dense (ne = 1010–4 × 1011 cm–3) low-temperature (Te = 3–5 eV) plasma is produced in the working volume at an ionization rate of 10–3–5 × 10–5. It is shown that the increase in the electron density near the upper boundary of the pressure range at a constant level of the input power leads to a drastic change in the type and spectral composition of plasma radiation and a jumplike increase in the light flux. The results of probe and optical measurements made it possible to determine the range of the operating parameters defining the character and parameters of the radiation processes under study. © 2017, Pleiades Publishing, Ltd.