It is important to have a monochromatic source of radiation for the light incident on the sample cell. Monochromaticity is measured as the width of the "triangle" formed by the intensity spike, at one half of the peak intensity. A given spectrometer has a spectral bandwidth that characterizes how monochromatic the incident light is. If this bandwidth is comparable to (or more than) the width of the absorption line, then the measured extinction coefficient will be mistaken. In reference measurements, the instrument bandwidth (bandwidth of the incident light) is kept below the width of the spectral lines. When a test material is being measured, the bandwidth of the incident light should also be sufficiently narrow. Reducing the spectral bandwidth reduces the energy passed to the detector and will, therefore, require a longer measurement time to achieve the same signal to noise ratio.