a photometer for measuring electromagnetic radiation absorption of a sample utilizing a source of the electromagnetic radiation . electromagnetic radiation is conducted from the source , to a sample sensor , and is passed through the sample utilizing a sample cell . the electromagnetic radiation is then directed to a detection system which determines absorbance of electromagnetic radiation by the sample at the sample cell . the detection system includes a beam splitter which receives the electromagnetic radiation and outputs first and second beams . a first detector having a first wavelength filter and a second detector having a second wavelength filter receives the two beams from the splitters . one detector may be employed to produce a reference signal . the remaining detector or detectors produce an output signal which is a representation of a characteristic , such as absorbance of the sample . the output signals are operated on by a reference signal , and converted into a measurement absorbance .