Colorimeter visual

Measurement of Whiteness. The Ciba-Geigy Plastic White Scale is effective in the visual assessment of white effects (79), but the availabihty of this scale is limited. Most evaluations are carried out (ca 1993) by instmmental measurements, utilising the GIF chromaticity coordinates or the Hunter Uniform Color System (see Color). Spectrophotometers and colorimeters designed to measure fluorescent samples must have reversed optics, ie, the sample is illuminated by a polychromatic source and the reflected light passes through the analy2er to the detector. 

Visual methods have been virtually displaced for most determinations by methods depending upon the use of photoelectric cells (filter photometers or absorptiometers, and spectrophotometers), thus leading to reduction of the experimental errors of colorimetric determinations. The so-called photoelectric colorimeter is a comparatively inexpensive instrument, and should be available in every laboratory. The use of spectrophotometers has enabled determinations to be extended into the ultraviolet region of the spectrum, whilst the use of chart recorders means that the analyst is not limited to working at a single fixed wavelength. 

Proportionality between colour and concentration. For visual colorimeters it is important that the colour intensity should increase linearly with the concentration of the substance to be determined. This is not essential for photoelectric instruments, since a calibration curve may be constructed relating the instrumental reading of the colour with the concentration of the solution. Otherwise expressed, it is desirable that the system follows Beer s law even when photoelectric colorimeters are used. 

Visual and photoelectric colorimeters may be used as turbidimeters a blue filter usually results in greater sensitivity. A calibration curve must be constructed using several standard solutions, since the light transmitted by a turbid solution does not generally obey the Beer-Lambert Law precisely. 

Colorimeter Also called color comparator or photoelectric color comparator. An instrument for matching colors with results about the same as those of visual inspection, but more consistent. Basically the sample is illuminated by light from the three primary color filters and scanned by an electronic detecting system. It is sometimes used in conjunction with a spectrophotometer, which is used for close control of color in production. 

In actual practice, the so called visual nephelometer (comparator type) have been more or less superseded by the photoelectric instruments Nevertheless, a Duboscq Colorimeter with a slight modification may be used conveniently for nephelometric analysis, for instance ... 

In fact, either visual or photoelectric colorimeters may be satisfactorily employed as turbidimeters. However, the use of the blue filter normally enhances the sensitivity appreciably. It has been observed that the light transmitted by a turbid solution does not normally obey the Beer-Lambert Law accurately and precisely. Therefore, as an usual practice it is advisable to construct a calibration curve by employing several standard solutions. The concentration of the unknown solution may be read off directly from the above calibration curve as is done in the case of colorimetric assays. 

The simplest types of photometric instrument are designed for measurements in the visible region of the spectrum only and rely on coloured filters and simple photoelectric detectors. The name colorimeter is often used to describe such instruments although this is not necessarily correct and the word should probably be reserved for visual comparators rather than photoelectric instalments. 

Simple colorimeters, used for routine analysis, are visual comparators related to Nessler tubes. The latter are made of flat-bottomed glass tubes with a volumetric mask and filled with reference solutions of varying concentrations (and possibly derivatising agents). The solution to be analysed, placed in an identical tube, is inserted beside the series of standard solutions. 

This visual colorimeter was used until 1960. The observer adjusts the transmitted intensities in both paths by moving the containers of the two solutions to be compared along the two transparent bars. 

Full Shade. Full-shade systems are media that contain only a single pigment. The color of the full-shade system in an optically infinitely thick (opaque) coating is referred to as full shade. The mass tone denotes the color obtained when the pigmented medium is applied as a layer that does not hide the substrate completely (e.g., on a white substrate). Evaluation can be carried out visually or by color measurement. For standards, see Table 1 ( Color in full-shade systems ). Apparatus spectrophotometer or tristimulus colorimeter. 

The depth of color is determined as follows. The carbon black is rubbed with a binder (e.g., linseed-oil) to form a paste which is applied in a thick coat on a glass plate. A colorimeter is used to measure the diffuse reflection of light through the glass. The color depth and hue of the paste can be determined from the intensity and spectral distribution of the diffusely reflected light [4.32]. The color depth can also be determined by visual comparison of the paste with pastes of known blackness under very bright illumination. 

Luminescence color is determined by visual comparison, by calculation of the color coordinates x and y from the emission spectrum, or by means of a tristimulus colorimeter under the excitation definitive for the application (UV radiation, cathode rays, etc.). 

Introduction. Air becomes incorporated into juice during processing or upon reconstitution of concentrates. The incorporated air must be removed if the Brix of the juice is to be determined by hydrometer. It is essential to deaerate juice when determining the color with a colorimeter. The color is slightly less affected by air when visual comparisons are made. 

Huggart (17) using a Hunter Citrus Colorimeter developed an equation containing CR and CY values which established a high correlation between instrument color values and U. S. Department of Agriculture visual color scores. The State of Florida... 

The measurement of color on tristimulus colorimeters will depend on the type of machine and its operating instructions. It should be noted that orange juice color readings are affected by incorporated air and by variations in temperature. The juice should be deaerated and brought to 80°F i 2° before readings are made. Color value readings will have to be correlated with visual evaluations to be meaningful. 

As pink or red—fleshed grapefruit ripen, the color of the juice loses the distinct coloration and gives a juice that is definitely amber with a very slight brownish cast. Mixtures of white-fleshed and pink-fleshed grapefruit have a dull appearance and may look grayish to dull amber. There are no visual color standards for grapefruit juice products. However, color characteristics may be measured on tristimulus colorimeters using the L, a, b, color notation. 

Two tests were conducted to determine the influence of the pesticidal chemicals on the fabric. On the dyed fabric samples, color change due to chemical exposure was recorded both visually and with an instrumental colorimeter. On undyed cotton and wool fabrics, change in strength was determined using a tensile tester. 

Figure 3.1. Schematic of a visual colorimeter (from Wyszecki and Stiles).

In Chapter 2 it was demonstrated that the visual system requires three things to function correctly a light source, an object, and a detector. So one designs an instrument with a standard light source, a port for placing a solid sample, and a detector that has the properties of a standard observer. In the visual colorimeter the pri-... 

Figure 3.2. Schematic of an analog simulattion of a visual colorimeter.

Figure 3.4. Schematic of a digital simulation of a visual colorimeter—a spectrocolorimeter.

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