Effects of relative humidity conditions in the irradiation chamber and of temperature of the chamber on the response of 3 mm thick light blue poly (methyl methacrylate) sheets have been studied. The response was measured spectrophotometrically at 402, 450, 596 and 612 nm. The response of the dosimeter is independent of the relative humidity (12–97%) during irradiation if the response is measured within 24 h of irradiation. If the response is measured after longer storage time, the absorbance at 402 and 450 nm increases significantly. At these two wavelengths the response of the dosimeter is also independent of the temperature of the irradiation chamber in the range of 0 to 40 °C. However, at higher temperatures (60, 80 °C), the response is not uniform.