Background <p>Light-activated and dual-cure resin systems require sufficient transmitted irradiance for adequate polymerization. During cementation, saliva, blood, or oxygen-inhibiting agents may be interposed between the curing light and the restorative material. This in vitro study evaluated the influence of intervening media type and thickness on transmitted irradiance from a dental light-curing unit.</p> Methods <p>Four intervening media were evaluated: artificial saliva, deionized water, an oxygen-inhibiting agent (Oxyguard II), and porcine whole blood, at thicknesses of 1, 3, and 6&#xa0;mm. Cylindrical acrylic containers (4&#xa0;mm internal diameter, 6&#xa0;mm length) sealed with an acrylic base were filled to the target thickness. Each specimen was positioned over a radiometer sensor, and transmitted irradiance from an LED light-curing unit (Demi™ Plus) was measured during a 10-second exposure (<i>n</i> = 10 per condition; total = 120). Data were analyzed by two-way analysis of variance with Bonferroni post hoc tests (α = 0.05).</p> Results <p>Media type and thickness significantly influenced transmitted irradiance (<i>p</i> &lt; 0.001), with a significant interaction (<i>p</i> &lt; 0.001). Values ranged from 258.8 to 715.4 mW/cm². Porcine whole blood and the oxygen-inhibiting agent showed progressive reductions with thickness (379.0–258.8 and 658.4–475.4 mW/cm², respectively), whereas artificial saliva and deionized water maintained higher values (696.0–715.4 and 657.3–712.5 mW/cm²) without progressive decreases.</p> Conclusions <p>Media type and thickness significantly affected transmitted irradiance. Porcine whole blood and the oxygen-inhibiting agent produced the greatest attenuation, potentially reducing light available for resin polymerization, though polymerization outcomes were not directly measured. Effective field isolation may help preserve transmitted irradiance during clinical light-curing.</p>