Nonthermal atmospheric pressure plasmas are widely used in biomedicine and agriculture. Currently, radical density is used as an important parameter for the development of plasma biomedicine and agriculture. Especially, nitrogen dioxide (NO2) and nitric oxide (NO) radicals are significant species in these applications. These could be measured by using the cavity‐enhanced absorption spectroscopy (CEAS) technique. For the NO2 and NO radical density measurement, we used a visible LED (light‐emitting diode) of 660 nm and a mid‐infrared LD (laser diode) of 5238.6 nm, respectively. Radical densities can be calculated with the transmission ratio using Beer–Lambert law, which is obtained by the absorbed amount of laser intensity passed through the gas from the plasma jet in an optical cavity of CEAS.
