Radiation trapping of the He I UV resonance transition (λ = 53.7 nm) has been experimentally confirmed in weakly ionized (ionization degree of ∼1–10%)helium plasmas in the PISCES-A linear divertor plasma simulator by using visible line emission with the same upper state as the UV transition. To study the effect of UV opacity on visible line ratios, the radiation trapping of UV resonance transitions was included in a He I collisional–radiative code with the optical escape factor method. The modified code is used to derive electron density ne and temperature Te with the standard He I line intensity ratios, 667.8 nm/728.1 nm and 728.1 nm/706.5 nm. The predicted values of ne and Te are compared with Langmuir probe measurements. Best agreement (within ±30%) is obtained when including radiation trapping and the correct (measured) neutral temperature and using the vacuum chamber radius (as opposed to the smaller plasma radius) for the characteristic length of the system. Neglecting radiation trapping entirely is found to give errors as large as 10 × (3 ×) in the predicted values of ne (Te).