Apparatuses for reducing radiation thermometry bias errors in enclosures such as CVD reactors. In one embodiment, a radiation thermometer utilizes an off-focus telecentric lens arrangement. The off-focus telecentric arrangement is focused at infinity, but is utilized to capture radiation from a relatively proximate target (e.g., within a couple meters) that is out of focus. The capture of collimated radiation from the target diminishes the contribution of stray radiation. In another embodiment, scattered radiation originating from a designated segment of a peripheral heating element can be reduced locally by one of several mechanisms, including reducing the emission (e.g., operating temperature) of the designated segment, or capturing or deflecting a portion of the radiation originating from the designated segment. Radiation thermometers fixed proximate an axis that extends from the center of the wafer carrier and across the designated segment are subject to less stray radiation, thus providing a more reliable temperature reading.