Core body temperature (CT) is a key indicator of an individual’s risk for heat stroke in the field. Multi-parameter sensors are impractical for field testing as they require long data-collection periods and a wide variety of settings. In the simple near-infrared (NIR) imaging method proposed below, 940-nm light emitting diodes are used for NIR illumination. Continuous images are collected by a video camera and the region of interest is extracted from the video file to calculate the changes in mean intensity over the duration of the video. Increases in vein diameter due to heat stress on the dorsal part of the hand can be quantified with these NIR-illuminated videos. A simple NIR optical imaging system for dorsal vein diameter measurements was tested for its effectiveness in monitoring core temperature changes. The technique for measuring the vein diameter is described in detail. Typically, NIR imaging can be used to measure heart rate and vein patterns, but vein diameter can also be used to infer core temperature. A model was trained using data from five volunteers engaged in a two hour long laboratory exercise, in air temperatures 24–36°C, and with CTs ranging from 36– 40°C. The data was collected from ten participants including various combinations of temperature and clothing. Classifications with the model returned an R2 value of 0.897 and root-mean-square error of 0.7428. Though the model for estimating CT cannot serve as a replacement for direct measurement of CT, the results suggest that it is accurate enough for providing practical monitoring of thermal strain in the work place.