The chemical process of CN formation in a CO-N2 mixture is studied through temporally-resolved intensity measurement of CN Violet radiation occurring in the reflected-shock region of a shock tube. A 78% CO-22% $N_2$ mixture is driven by cold hydrogen to a shock speed of up to 3.45 km/s, to produce a reflected-shock temperature corresponding to Martian entry flight of up to 6.4 km/s. Absolute calibration of spectrometer is performed using a standard lamp of radiance. A reaction model is constructed by combining four existing models and multiplying the $C_2$ dissociation rate by a factor of five. Spectral radiation from the shock layer ahead of a flat circular-disk model is measured along the stagnation line with the sodium-salicylate phosphor coated on the test window to measure the vacuum-UV radiation as well. The intensity of the chemiluminescence due to the radiative recombination of CO and O is stronger than that of the CN-violet band by an order of magnitude. The phosphorescent output reveals that the intensity of the CO4+ band is weaker than the existing numerical prediction by as much as a factor of 4.3.