Graticules

For other quantification, specialized graticules are available, including point counting, grids, concentric circles, and special scales. The latest methods of quantification involve automatic image analysis. 

Construct a small rectangle around the point with fine pencil lines connecting the nearest 2 1/2 or 5 graticules. Graticules are intersections of latitude and longitude lines that are marked on the map edge, and appear as black crosses at four points in the interior of the map. 

A graticule is normally provided on the screen to assist in the measurement. Parallax errors can cause problems unless this graticule is actually engraved on the screen itself, and the final accuracy will depend on the degree of beam focusing which can be achieved. Accuracy of better than 1.5 per cent should be possible in most cases. 

The image produced may be viewed either directly by the eye or projected on to a screen. The latter method is usually the more convenient and involves less eyestrain it is less satisfactory than direct observation for fine sizes near the limit of optical resolution. Sizing is commonly achieved by comparison of the particle images with a scale or graticule. Since the process is tedious, sizing is sometimes performed automatically. The smallest resolvable particle size is a function of the wavelength of the light used and varies between 0.5 and 1 p... 

To size particles, graticule circles of known area on the graticule are superimposed until one is found that best fits the particle the surface area is then used to calculate particle size. 

Fig. 7 Common microscope eyepiece graticules used in particle sizing (a) Patterson-Cawood (b) Porton.

Microscopic examination permits measurement of the projected area of the particle and also enables an assessment to be made of its two-dimensional shape. In general, the third dimension cannot be determined except when using special stereomicroscopes. The apparent size of particle is compared with that of circles engraved on a graticule in the eyepiece as shown in Figure 1.3. Automatic methods of scanning have been developed. By using the electron microscope 7, the lower limit of size can be reduced to about 0.001 pan. 

Take images of a stage graticule (1 mm) using both lOx and 4x objectives. 

Import graticule images into the image analysis software to perform image calibration (see Note 18). This step is required for the first time only. For subsequent image analysis, simply select the required calibration settings. 

Image calibration is required in order to obtain accurate measurements. The objective used (lOx or 4x) the units (pm) and the graticule measurements need to be entered into the image analysis software to obtain the correct calibration for the system used. 

An illustration of the illumination used in this type of apparatus, to observe the motion of colloid particles, is given in Figure 6.16. Basically, the illumination system and microscope allow you to observe the motion of the silica particles, which are seen as bright star-like objects on a green background. When an electric field is applied the average time taken for the particles to travel a distance of one square on the eyepiece graticule can be easily measured. One particle is measured each time the field is applied for a short time (i.e. 10-20 s) and the polarity is then reversed and the speed in the opposite direction measured. The... 

The speed of the particles can be easily calculated using the fact that each square of the graticule corresponds to a known distance, typically... 

FIG. 1.10 Characterization of the size of irregular particles (a) a schematic illustration of Martin diameters, (b) the use of a graticule to estimate the characteristic dimension. 

A graticule was used to size sand particles and glass spheres.% The percentage by weight of particles less than the stated size was found to be as follows ... 

The rate of particle migration is determined by measuring with a stopwatch the time required for a particle to travel between the marks of a calibrated graticule in the microscope eyepiece. If the objective of the microscope is immersed during the electrophoresis measurement, the calibration of the graticule should be made with the same immersion liquid. 

Four points were identified per field, and a total of 1000 counts were determined for Rasa coal. This system employs a point count stage and an ocular graticule. The volume percent of macerals was calculated according to ASTM D2799-86 (14). 

Fairs investigation of microscopic measurement consisted of two parts (1) Type of illumination for improving the resolution of the. microscope system, and (2) the development of a series of graticules for the direct comparison of the projected particle... 

This procedure should be followed in cases where a change of magnification is required to cover the size range. Table 9 illustrates the overlapping of the 5-graticule with different objectives. 

Table 9—Effect of Magnification on Size of Circles and Rectangular Areas of Fairs 5-Graticule, Figure 15...

The easiest diameter to measure is the Feret diameter but this is significantly larger than the other two diameters for most powders. It is probably best to reserve this diameter for comparison purposes and for rounded particles. Of the other two diameters, the projected area diameter is preferred since two dimensions are included in one measurement and the projected area is easier to estimate using globe and circle graticules than the length of the chord that bisects the image. 

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