The amount of small particles of solid matter suspended in water as measured by the amount of scattering and absorption of light rays caused by the particles. Turbidity blocks light rays and makes the water opaque. Turbidity is measured in nephelometric turbidity units (NTU). Potable water should not exceed 0.5 NTU. Turbidity cannot be directly equated to suspended solids because white particles reflect more light than dark-colored particles and many small particles will reflect more light than an equivalent large particle.