Measured effect of radiation trapping on He I line intensity ratio technique used for estimating electron density and temperature

He I line intensity ratios are commonly used to estimate electron density, $n_{e}$, and temperature, $T_{e}$, in helium plasmas. Previously, UV radiation trapping has been invoked to obtain better agreement on $n_{e}$ and $T_{e}$ with other diagnostics. In the linear divertor simulator PISCES-A we have experimentally confirmed radiation trapping of the He I UV resonance transition (1$^{1}$S - 3$^{1}$P: 53.7 nm) from the radial profile of He I visible line emission (2$^{1}$S - 3$^{1}$P: 501.6 nm), which has the same upper state as 53.7 nm; and also from the radial profile of plasma radiated power measured with an AXUV photodiode array. The radiation trapping effect has been implemented into a collisional-radiative code by using the optical escape factor method. $n_{e}$ and $T_{e}$ derived from He I line intensity ratios (667.8nm/728.1nm and 728.1nm/706.5nm) calculated with the modified code agree well with those from probe measurement at neutral pressure of above a few mTorr. The discrepancy at low pressure may be due to the presence of nonthermal electrons.