[en] The plasma background light in Thomson scattering measurements is often far higher than expected for a pure hydrogen plasma. The spectral distribution of light from three plasmas (duration: 1 ms to steady state; electron density: below 10¹² to over 10¹⁴ cm^-3; temperature: below 20 to over 1000 eV) and signal-to-noise and intensity data from the Thomson scattering systems used on them are compared with analytic estimates to show that in two of these plasmas molecular light dominates the spectrum, while in the other, molecular light is present, but bremsstrahlung is usually more intense. Knowledge of the mechanism for background light can aid in designing detection systems for Thomson scattering and provide information on the neutral species composition and effective charge of the plasma