Light reflectance, contaminant

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

Visual detection is the simplest and quickest way to detect and monitor contamination. This method may be enhanced by an inspection tank equipped with glass portholes. A light should be mounted outside one porthole so that a person can look through the opposite one and see the light reflected on the water. Oily contamination, unless it is truly soluble, will produce a rainbow eifect on the water surface. 

For SFM, maintaining a constant separation between the tip and the sample means that the deflection of the cantilever must be measured accurately. The first SFM used an STM tip to tunnel to the back of the cantilever to measure its vertical deflection. However, this technique was sensitive to contaminants on the cantilever." Optical methods proved more reliable. The most common method for monitoring the defection is with an optical-lever or beam-bounce detection system. In this scheme, light from a laser diode is reflected from the back of the cantilever into a position-sensitive photodiode. A given cantilever deflection will then correspond to a specific position of the laser beam on the position-sensitive photodiode. Because the position-sensitive photodiode is very sensitive (about 0.1 A), the vertical resolution of SFM is sub-A. 

The kind of information provided by reflectance-based optical data depends on whether the measurement is specular or nonspecu-lar and on the spectral range involved. Nonspecular (scattered light) data carry Information about macroscopic, spatially resolvable extrinsic features such as pits, scratches, or particulate contamination. The characteristic dimensions of these features must be of the order of or larger than the Rayleigh length 1 - 0.6IX, where X is the wavelength of light. 

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