Color Measurement Lecture 6 PDF

Color measurement Lecture 6 Color measurement instruments  The instruments used for color measurement The measurement of color according to the CIE colorimetric system is known as color measurement. The values obtained (tristimulus values (X, Y, Z), chromaticity coordinates (x, y),...

Color measurement Lecture 6 Color measurement instruments  The instruments used for color measurement The measurement of color according to the CIE colorimetric system is known as color measurement. The values obtained (tristimulus values (X, Y, Z), chromaticity coordinates (x, y), etc.) are known as colorimetric values. The methods used can be classified into either direct measurement of tristimulus values or spectral measurement followed by calculation. 1. Direct measurement of tristimulus values This method directly measures the tristimulus values (𝑋,𝑌,𝑍) using a set of filtered sensors that mimic the color-matching functions (𝑥‾(𝜆), 𝑦‾(𝜆), 𝑧‾(𝜆)) of the human eye. 2. Spectral measurement (Spectral colorimetry) Spectral measurement captures the full spectral power distribution (S(λ)) of the light source across different wavelengths.  Colour – measuring instruments: Variety of instruments are available for color measurement. They can be broadly classified as: 1. Colorimeters (Direct measurement ) 2. A spectroradiometer (Spectral measurement ) 3. Spectrophotometers (Spectral measurement ) 4. Goniophotometers 5. Integrating Spheres 6. Digital Imaging Devices 7. Densitometers  The Beer-Lambert Law Beer’s Law: According to Beer’s law when monochromatic light passes through the colored solution, the amount of light transmitted decreases exponentially with increase in concentration of the colored substance. 1 𝐴 = 𝑙𝑜𝑔10 ( ) or 𝐴∝𝑐 𝑇 Lambert’s Law: According to Lambert’s law the amounts of light transmitted decreases exponentially with increase in thickness of the colored solution. 𝐴∝𝑙 The Beer-Lambert Law is a fundamental principle in analytical chemistry that describes the relationship between the absorption of light and the properties of the material through which the light is passing.  This law is commonly used in spectrophotometry, which is a technique that measures how much a substance absorbs light at different wavelengths. The Beer-Lambert Law is expressed by the equation: A=ε⋅c⋅l where: A is the absorbance of the sample (the amount of light absorbed by the sample). ε (epsilon) is the molar absorptivity or molar extinction coefficient, a constant specific to the absorbing substance. c is the concentration of the absorbing substance in the solution, usually expressed in mol/L (molarity). l is the path length of the cuvette or container through which the light passes, typically measured in centimeters.  The relationship between absorbance (A), concentration (c), and path length (l) is linear when the concentration is low, and the substance follows Beer's law. Beer's law is given by: 1 𝐴 = 𝑙𝑜𝑔10 ( )= ε⋅c⋅l 𝑇 where T is the transmittance of light through the sample. This law is particularly useful in quantitative analysis, where the concentration of a solute in a solution can be determined by measuring the absorbance of light at a specific wavelength. 1. A colorimeter A colorimeter is a device that measures the absorbance of particular wavelengths of light by a specific solution. This device is most commonly used to determine the concentration of a known solute in a given solution by the application of the Beer-Lambert law.  Tristimulus colorimeters Principle: A colorimeter quantifies color based on the tristimulus theory, which states that any color can be described using three primary colors: red, green, and blue (RGB). It measures the intensity of light transmitted or reflected from an object through RGB filters, simulating the response of the human eye. They give a direct measure of colorimetric quantities (tristimulus values). They are used for color measurement of both surface colors and self-luminous colors. Surface colors refer to the appearance of an object as a result of the way it interacts with light. The color of an object is determined by the wavelengths of light that it reflects, absorbs, and transmits. Self-luminous colors are colors produced by objects that emit their own light, rather than colors that are visible due to illumination by an external light source. Technical Details: Operates using photoelectric sensors that detect filtered light. Calculates color coordinates in systems like CIE XYZ. Their advantages are their speed, ease of operation, and good measurement repeatability. Applications: Quality control for textiles, paints, plastics, and food. Evaluating the uniformity of surface coatings. Limitations: Accuracy depends on calibration and lighting conditions. Cannot measure spectral data, limiting its ability to detect metamerism (two colors appearing the same under one light but different under another). 2. A spectroradiometer Principle: A spectroradiometer is an instrument designed to measure radiometric quantities (irradiance, radiance) in a narrow spectral bandpass as a function of wavelength. Irradiance is the power of electromagnetic radiation (light) incident on a surface per unit area. Radiance measures the power of electromagnetic radiation traveling in a specific direction, per unit area of the source, per unit solid angle. A spectral bandpass refers to the range of wavelengths of electromagnetic radiation that can pass through a filter or optical system. Technical Details: The term "bandpass" implies that only a specific band or range of wavelengths is allowed to pass through, while others are either absorbed or blocked. Applications: These spectroradiometric data are used for color measurement of self-luminous colors and for determination of correlated color temperature. Limitations: The disadvantage of these instruments is that their calibration procedures are tedious.  A tele-spectroradiometer A tele-spectroradiometer is similar to a spectroradiometer, but it incorporates a telescopic input module. This instrument is used when it is desired to measure the color of a distant object at its usual observing position and under its usual viewing conditions. This makes them well suited for color vision experiments. 3. A spectrophotometer Principle: A spectrophotometer is designed to compare, at each wavelength, the radiant power leaving an object to that incident on it. Radiant power refers to the total energy radiated, emitted, reflected, or transmitted by a source in the form of electromagnetic waves over a certain period of time. It is measured in watts (W), which are equivalent to joules per second. The formula for radiant power is: P =ΔE/Δt where: P is the radiant power, ΔE is the change in energy, Δt is the change in time. These radiant power ratios are dimensionless quantities used for color measurement of surface colors. Note: the main advantages of a spectrophotometer or spectroradiometer over a colorimeter are their greater accuracy, and their ability to detect metamerism.  A spectrophotometer measures either the amount of light reflected from a sample object or the amount of light that is absorbed by the sample object. The instrument operates by passing a beam of light through a sample and measuring the intensity of light reaching a detector. A molecule or substance that absorbs light is called a chromophore. Chromophores exhibit unique absorption spectra and can be defined by a wavelength of maximum absorption, or λmax. A large number of biological molecules absorb light in the visible and ultraviolet (UV) range. The net affect of absorption is that the intensity of the transmitted light decreases as it passes through a solution containing a chromophore. The amount of light absorbed depends on the nature of the chromophore, the concentration of the chromophore, the thickness of the sample, and the conditions (eg., pH, solvent,etc.) under which absorption is measured.  The source is usually an incandescent lamp or a continuous or pulsed xenon arc lamp. The output spectrum of an incandescent lamp is smooth and predictable, whereas that of a xenon lamp is irregular, making wavelength calibration more critical with a xenon lamp. The wavelength separation may be accomplished by various means, such as interference filters or wedges, prisms or gratings. The choice of wavelength separation method will depend upon the desired spectral range, resolution, stray light level and cost. For measurements over an extended wavelength range from the ultraviolet to the near-infrared, or requiring higher resolution, a grating is used. The detectors used with color-measuring instruments are generally photomultipliers, ex; silicon photodiodes. Photomultipliers (PMTs) are traditionally used in spectrophotometers and spectroradiometers because of their superior sensitivity and low-noise performance in low light level conditions. Applications: Color matching in automotive, printing, and textile industries. Analyzing pigments, coatings, and dyes. Measuring the metameric index (how a sample matches a reference under different illuminants). Advantages: High precision and repeatability. Ability to detect subtle color differences. 4. Goniophotometers Principle: Goniophotometers are used to analyze the angular dependence of light intensity and color. They measure how light interacts with a surface as the angle of incidence or observation changes. Technical Details: Utilizes a rotating arm to measure reflected or transmitted light at varying angles. Captures bidirectional reflectance distribution function (BRDF) or transmittance. Applications: Analyzing surfaces with anisotropic properties, such as metallic or glossy finishes. Evaluating display screens, LEDs, and reflective materials.  5. Integrating Spheres Principle: An integrating sphere diffuses light uniformly inside a hollow sphere to measure total reflectance and transmittance, regardless of sample geometry or surface texture. Technical Details: The sphere is coated with a material with nearly 100% diffuse reflectance (e.g., barium sulfate or Spectralon). A light source illuminates the sample, and a detector measures scattered light inside the sphere. Applications: Measuring color opacity and hiding power in paints and coatings. Evaluating materials with complex geometries, such as powders or fabrics. Benefits: Removes errors due to directional reflectance or texture. 6. Digital Imaging Devices Principle: Digital imaging devices capture color using a combination of optics and image sensors, typically charge-coupled devices (CCDs) or complementary metal-oxide semiconductors (CMOS). They analyze RGB values at every pixel, creating a digital representation of the color. Technical Details: Paired with software for analyzing color uniformity, distribution, and trends. Can perform spatial color analysis over large areas. Applications: Dermatology (skin tone analysis). Quality assurance in cosmetics. Limitation: Highly sensitive to lighting conditions and camera settings. 7. Densitometers Principle: Densitometers measure the optical density of a sample, which correlates to its light-absorbing properties. While not primarily designed for color measurement, they are widely used in printing to monitor ink density and ensure consistency. Technical Details: Measures the ratio of incident light to transmitted or reflected light. Applications: Ensuring consistency in printed materials. Quality control in photographic and graphic arts industries.  Some Notes: All color measurement instruments basically consist of a source of radiation, an object (for surface colors), an optical system for wavelength separation, and a photodetector. For highest accuracy, the CIE recommends that the spectral range should be 360-830 nm. However, for most colorimetric applications of surface colors, a spectral range of 380-760 nm will suffice. For colorimetry of samples containing fluorescent whitening agents, the color measurements should begin at 300 nm. These considerations are important in selecting options of instrument light source, optical system and detector, which are appropriate to a given application.