radiation trapping

Process by which the resonance radiation emitted in a lamp is absorbed and re-emitted many times in the plasma prior to striking the phosphor. This process is called radiation trapping or imprisonment.
Radiation trapping lengthens the effective lifetime of emission as viewed from outside the lamp. The control of this trapping is, therefore, an important design consideration for low-pressure lamps.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 410