Colorimetry involves the transmission, absorption, or reflectance of visible light and can be extended into the ultraviolet (UV) and near-infrared (NIR) spectrum. Gratings used in current spectrophotometers can be specified over a range of 200 to 1200 nm, with the visible range being from 400 to 700 nm. Visible color can be measured as an indication of a concentration or as an indication of pH in titration using dyes. Colorimeters and spectrophotometers associated with autotitrators and wet chemistry analyzers are discussed in a separate section of this chapter. Visual colorimetry involves the matching of the sample color against standards in cuvettes, Nessler tubes, or thin films. Blood glucose meters use glucose oxidase on test strips and measure multiple-wavelength absorbances proportional to glucose concentration. Simple color wheel kits are used in water quality analysis to match depth of color in the liquid sample with a reference color calibrated in the concentration of the specific chemical end group for such measurements as pH or arsenic concentration. These are being replaced with low-cost hand-held colorimeters and spectrophotometers, which can be programmed for field use. This includes methods for turbidity measurement by nephelometry, which uses the ratio of scattered to transmitted light in suspensions, as shown in Figure 8.15a. If colored filters such as RGB (red, green, and blue) are used, the device is a colorimeter. If a grating or prism is used, the device is a spectrophotometer. Due to the current technology of matching of CCD and diode arrays to entire spectral images, an entire spectral curve may be measured continuously.