Smoke and smoke opacity meters are instruments measuring the optical properties of diesel exhaust. These instruments have been designed to quantify the visible black smoke emission utilizing such physical phenomena as the extinction of a light beam by scattering and absorption. In general, smoke and opacity meters are much simpler (some of them very simple) and less costly in comparison to most other instruments used for PM measurement. They are often used to evaluate smoke emissions in locations outside the laboratory, such as in maintenance shops or in the field.