Abstract: A spectrophotometric sensor assembly that may find particular use in a capnometer, a medical device for measuring the concentration of carbon dioxide in the exhaled breath of a patient, includes at least one microlamp. A microlamp is a very small source of electromagnetic radiation including a heated filament disposed over a substrate. The microlamp may be constructed using semiconductor fabrication techniques. The microlamp typically emits broad-band infrared radiation. Radiation from the microlamp usually passes through a filter, which preferentially transmits radiation of a preselected wavelength. The radiation then passes through a sample chamber and onto a detector. The concentration of a substance of interest within the chamber may be computed by determining the degree to which the radiation is absorbed in the chamber. In a preferred embodiment, an array of microlamps is sequentially triggered in a rapid manner.