The use of the laser induced fluorescence (LIF) method to study helium-like carbon ions is described, together with the radiation source, which employs doubling of the dye laser frequency by means of a potassium pentaborate crystal. The source is intended for optical pumping of the 23S1 → 23P0,1,2 transitions in the C V ion. A diagnostic system based on the LIF method in the T-10 tokamak is presented. The regimes in which the measurements were made are described briefly and some of the experimental results obtained are given.