Relevance of Quality and Quality Related Customer Complaints - Dr M S Pa...

Relevance of Quality and Quality Related Customer Complaints - Dr M S Parmar

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https://www.youtube.com/watch?v=V0s54tauQeY

Spectrophotometer

Every chemical compound absorbs, transmits, or reflects light (electromagnetic radiation) over a certain range of wavelength. Spectrophotometry is a measurement of how much a chemical substance absorbs or transmits. Spectrophotometry is widely used for quantitative analysis in various areas (e.g., chemistry, physics, biology, biochemistry, material and chemical engineering, clinical applications, industrial applications, etc). Any application that deals with chemical substances or materials can use this technique. In biochemistry, for example, it is used to determine enzyme-catalyzed reactions. In clinical applications, it is used to examine blood or tissues for clinical diagnosis. There are also several variations of the spectrophotometry such as atomic absorption spectrophotometry and atomic emission spectrophotometry.

A spectrophotometer is an instrument that measures the amount of photons (the intensity of light) absorbed after it passes through sample solution. With the spectrophotometer, the amount of a known chemical substance (concentrations) can also be determined by measuring the intensity of light detected. Depending on the range of wavelength of light source, it can be classified into two different types:

• UV-visible spectrophotometer: uses light over the ultraviolet range (185 - 400 nm) and visible range (400 - 700 nm) of electromagnetic radiation spectrum.
• IR spectrophotometer: uses light over the infrared range (700 - 15000 nm) of electromagnetic radiation spectrum.

In visible spectrophotometry, the absorption or the transmission of a certain substance can be determined by the observed color. For instance, a solution sample that absorbs light over all visible ranges (i.e., transmits none of visible wavelengths) appears black in theory. On the other hand, if all visible wavelengths are transmitted (i.e., absorbs nothing), the solution sample appears white. If a solution sample absorbs red light (~700 nm), it appears green because green is the complementary color of red. Visible spectrophotometers, in practice, use a prism to narrow down a certain range of wavelength (to filter out other wavelengths) so that the particular beam of light is passed through a solution sample. 

Devices and mechanism

Figure 1 illustrates the basic structure of spectrophotometers. It consists of a light source, a collimator, a monochromator, a wavelength selector, a cuvette for sample solution, a photoelectric detector, and a digital display or a meter. Detailed mechanism is described below.  

A spectrophotometer, in general, consists of two devices; a spectrometer and a photometer. A spectrometer is a device that produces, typically disperses and measures light. A photometer indicates the photoelectric detector that measures the intensity of light.

• Spectrometer: It produces a desired range of wavelength of light. First a collimator (lens) transmits a straight beam of light (photons) that passes through a monochromator (prism) to split it into several component wavelengths (spectrum). Then a wavelength selector (slit) transmits only the desired wavelengths, as shown in Figure 1.
• Photometer: After the desired range of wavelength of light passes through the solution of a sample in cuvette, the photometer detects the amount of photons that is absorbed and then sends a signal to a galvanometer or a digital display, as illustrated in Figure 1.

Adams Chromatic Valence Color Space

Adams chromatic valence color spaces are a class of color spaces suggested by Elliot Quincy Adams. 

Two important Adams chromatic valence spaces are CIELUV and Hunter Lab.

Chromatic value/valence spaces are notable for incorporating the opponent process model, and the empirically-determined 2½ factor in the red/green vs. blue/yellow chromaticity components (such as in CIELAB).

In 1942, Adams suggested chromatic value color spaces. Chromatic value, or chromance, refers to the intensity of the opponent process responses, and is derived from Adams' theory of color vision.

A chromatic value space consists of three components:

·         {\displaystyle V_{Y},}V_Y the Munsell-Sloan-Godlove value function: {\displaystyle V_{Y}^{2}=1.4742Y-0.004743Y^{2}}(V_Y)² = 1.4742Y – 0.004743Y²

·         {\displaystyle V_{X}-V_{Y}}V_x – V_y,  the red-green chromaticity dimension, where {\displaystyle V_{X}}V_x is the value function applied to {\displaystyle (y_{n}/x_{n})X}(y_n/x_n)X  instead of Y

·         {\displaystyle V_{Z}-V_{Y}}V_Z - V_Y, the blue-yellow chromaticity dimension, where {\displaystyle V_{Z}}V_Z is the value function applied to {\displaystyle (y_{n}/z_{n})Z}(y_n/z_n)Z  instead of Y

A chromatic value diagram is a plot of {\displaystyle V_{X}-V_{Y}}V_X - V_Y (horizontal axis) against {\displaystyle 0.4(V_{Z}-V_{Y})}0.4(V_Z – V_Y) (vertical axis). The 2½ scale factor is intended to make radial distance from the white point correlate with the Munsell chroma along any one hue radius (i.e., to make the diagram perceptually uniform). For achromatic surfaces, {\displaystyle (y_{n}/x_{n})X=Y=(y_{n}/z_{n})Z}(y_n/x_n)X = Y = (y_n/z_n)Z  and hence {\displaystyle V_{X}-V_{Y}=0}, {\displaystyle V_{Z}-V_{Y}=0}V_X – V_Y = 0, V_Z – V_Y = 0. In other words, the white point is at the origin.

Constant differences along the chroma dimension did not appear different by a corresponding amount, so Adams proposed a new class of spaces, which he termed chromaticvalence. These spaces have "nearly equal radial distances for equal changes in Munsell chroma".

Kubelka-Munk equations:

When light is fall on some sample, some part of the light reflected, some are absorbed and some are scattered. If the sample is having opacity more than 70% then as per Kubelka Munk (established in 1931) following is the relation between reflectance, scattering and absorption of light:

K/S= (1 – 0.01R)²/2(0.01R)

The mathematical basis for all color matching software is the Kubelka-Munk series of equations. These equations state that for opaque samples such as textile materials, the ratio of total light absorbed and scattered by a mixture of dyes is equal to the sum of the ratios of light absorbed and scattered by the dyes measured separately. Where absorption is defined as "K" and scattering is defined as "S", Kubelka-Munk states that _:

(K/S) mixture = (K/S) dye 1 + (K/S) dye 2 + (K/S) dye 3 + ...

K/S is not a readily measurable quantity, but it can be calculated from the reflectance of a sample -- "R" -- by the Kubelka-Munk equation that states

 K/S= (1-R)²/2R

K/S  value is proportional to dye  concentration in the substrate

K/S=kC

Where k is constant and C is concentration of dyes or colorant 

Delta E Differences and Tolerances.

The difference between two colour samples is often expressed as Delta E, also called  DE, or ΔE. 'Δ' is the Greek letter for 'D'. This can be used in quality control to show whether a dyed or printed sample, such as a colour swatch or proof, is in tolerance with a reference sample or industry standard. The difference between the L*, a* and b* values of the reference and sample will be shown as Delta E (ΔE). The resulting Delta E number will show how far apart visually the two samples are in the colour 'sphere'

 CIE L*C*h°

This is possibly a little easier to comprehend than the Lab colour space, with which it shares several features. It is more correctly known as  L*c*h*.  Essentially it is in the form of a sphere. There are three axes; L* , c* and h°.  

The L* axis represents Lightness. This is vertical; from 0, which has no lightness (i.e. absolute black), at the bottom; through 50 in the middle, to 100 which is maximum lightness (i.e. absolute white) at the top.

The c* axis represents Chroma or 'saturation'. This ranges from 0 at the centre of the circle, which is completely unsaturated (i.e. a neutral grey, black or white) to 100 or more at the edge of the circle for very high Chroma (saturation) or 'colour purity'.

The h* axis represents Hue. If we take a horizontal slice through the centre, cutting the 'sphere' ('apple') in half, we see a coloured circle. Around the edge of the circle we see every possible saturated colour, or Hue. This circular axis is known as h° for Hue. The units are in the form of degrees (or angles), ranging from 0° (red) through 90° (yellow), 180° (green), 270° (blue) and back to  0°. 

The Lch colour model is very useful for retouching images in a colour managed workflow, using high-end editing applications. Lch is device-independent.

CIE XYZ:

The basic CIE colour space, or colour model, is based on a 'Standard Observer and 'Standard Illuminants' (D50, D65, etc.). This is a numerical model of colour sensitivity based on research commenced in the 1920s on a sample of people with normal colour vision. It is a 'universal colour space' representing the colour spectrum visible to the 'average human'. The light-sensitive retina at the back of the eye has three types of receptors near the centre, known as cones. They are sensitive to the three primaries, 'red, green and blue'. The CIE XYZ tristimulus values are assigned to the red, green and blue curves respectively. These approximate to the cones in the eye. The relative response of each is plotted on a diagram against the wavelength in nanometers. The eye also has rods, outside of the retina's centre, which are sensitive to low-wavelength light and which only operate at low levels of illumination. There are two axes: vertical and horizontal.

The vertical axis represents Relative Response 0 - 2.0 (shown here) or Reflective Intensity 0 - 120% (not shown).

The horizontal axis represents Wavelength in nanometers, usually from about 380 to about 720nm.

It should be emphasized that this is a 'device-independent' colour space in which each primary colour (X,Y,Z) is always constant, unlike  RGB which varies with every individual device (monitor, scanner, camera, etc.). XYZ is typically used to report the spectral response of a sample measured by a colorimeter or a spectrophotometer. A colorimeter may contain as few as three sensors, one each for red, green and blue, (or X, Y and Z), and will typically be used for display calibration and profiling. A spectrophotometer will report the entire spectral response at frequent intervals along the spectrum, say every 10 nanometres, and will typically be used to measure printed sheets to control a press or create an ICC profile.

While CIE XYZ is used to report colour from measuring instruments, it is not so useful for humans to describe colour. Another use is as the Profile Connection Space (PCS) within an ICC profile, where it may be used instead of CIE Lab.

You may notice that the Y ('green curve') covers the widest wavelength. This corresponds to the overall human visual response to all colours, or lightness. It is therefore also used to indicate luminance ('lightness').

CIE L*a*b*: In such case the vertical L* axis represents Lightness, ranging from 0-100.  The other (horizontal) axes are now represented by a* and b*. These are at right angles to each other and cross each other in the centre, which is neutral (grey, black or white). They are based on the principal that a colour cannot be both red and green, or blue and yellow. 

      The a* axis is green at one extremity (represented by -a), and red at the other (+a). The b* axis has blue at one end (-b), and yellow (+b) at the other. 
The centre of each axis is 0. A value of 0, or very low numbers of both a* and b* will describe a neutral or near neutral. In the case of paper, the white point in terms of a* and b* is usually carried through to the black, being gradually reduced towards '0'.

In theory there are no maximum values of a* and b*, but in practice they are usually numbered from -128 to +127 (256 levels).

The CIE Lab colour model encompasses the entire spectrum, including colours outside of human vision. CIE Lab is extensively used in many industries apart from printing and photography. Its uses include providing exact colour specifications for paint (including automotive, household, etc.), dyes (including textiles, plastics, etc.), printing ink and paper.