Understanding Spectrophotometric Parameter Measurement and Its Definitions

Working across different industries, spectrophotometers are crucial for measuring and analyzing objective color data. In this guide, we will focus on the basic parameters and specifications of color spectrophotometers to provide insight into the algorithms used for measuring, analyzing, and reporting data.

What Do Spectrophotometers Measure?

Spectrophotometers measure how light interacts with materials by measuring reflected, transmitted, or absorbed light at various wavelengths. Necessary measurements consist of:

• Spectral Reflectance: the percentage of light reflected from a surface within the 380–780 nm spectral region.

• Colorimetric Values: colors or representations in CIE Lab*, CIE XYZ, and RGB spaces.

• Color Difference Calculations: the quantitative differences by ΔEab, ΔE94, and ΔE*00 formulas.

• Opacity and Transparency: transmission versus reflection light measurements. 

Where are Spectrophotometers Used?

Please note some examples of where Spectrophotometers are used in industry: 

• Automotive: paint matching, coating development, and quality assurance.

• Textile and Fashion: fabric color matching, dye consistency, and garment production.

• Food and Beverage: monitoring, consistency, and adherence to standards.

• Plastics and Polymers: color control during manufacturing, quality assurance.

• Printing and Graphics: control and management of color and color reproduction in the printing process.

• Cosmetics: formulation and batch consistency.

How Does a Spectrophotometer Measure Units of Color?

Spectrophotometers transform physical measurements of light into color units based on standards of theoretical and applied optical physics and color theory. The process consists of: 

• Illumination of the sample with a light source at a certain angle

• Transmitted and reflected light are separated into constituent wavelengths using diffraction gratings.

• Determination of light intensity at discrete spectral intervals of 10−20 nm by photodetectors.

• Raw spectral data processed using CIE color matching functions 

Results converted to tristimulus values (X, Y, Z) and color spaces like Lab

What is a Spectral Reflectance Curve?

A spectral reflectance curve is a graph showing the percentage reflectance of light for a surface in relation to each light wave within the visible range. This curve is a fundamental optical property color “fingerprint” description for a material.

Key characteristics:

• Red colored materials exhibit high reflectance values at 700nm with a dip in reflectance from 400 – 500nm. 

• Neutral gray materials exhibit a form of neutrality showing relatively flat reflectance across all wavelengths exerting minimal variation.

• The curves demonstrate and provide insight to color shifts resulting from the application of different illumination set ups.

• This is essential for determining metamerism of different samples and is crucial for metamerism and value shifts between two samples.

What’s the Difference between a Reflectance and a Transmission Spectrophotometer?

The major difference between reflectance and transmission spectrophotometers comes from their optical arrangement and the nature of the samples they are intended to measure. Reflectance Spectrophotometers are made for opaque materials. 

Their functioning is based on light reflected from the sample surface. Both light and the sample under measurement are positioned at a specified angle. This instrument is most suited for opaque materials such as paints, textiles, plastics, and paper. These instruments use either directional (45°/0° or 0°/45°) illumination or diffuse (sphere) illumination geometries.

What is Color Measurement Parameters 

Color measurement parameters relate to specific details like the specifications and conditions which have to be followed to make sure the color measurements are accurate, precise, and repeatable. These parameters are set to explain the functions of instruments and how measurement data should be processed. 

Color Scale

Every color measurement parameters have a data measurement range. In specific, color measurement parameters limit to only twelve data points to form a complete color measurement parameters color chart. 

CIE Illuminant

Color scales that have been CIE coordinates and subdivisions still provide measures of relative distance between colors. Important scales such as CIE XYZ still have value and should be documented as having set foundations to many color spaces, RGB red, green, blue, Hunter Lab, preceeding CIE but still used in legacy applications.

CIE Standard Observer 

Mathematically modeling an average human's color vision utilizes the CIE Standard Observer functions. There are two standard observers: the “2° observer,” which is based on a 2° field of view, and the “10° observer,” which is based on a 10° field of view. 

In colorimetric calculations, the choice of observer can be limiting, particularly for peripheral vision. The 10° observer is usually favored for large sample sets and is becoming the standard for most other color measurement applications. 

Instrument Geometry 

Instrument geometry pertains to illumination and collection angles, that is: how light illuminates the sample and the angle from which the light is collected by the detector. This parameter particularly affects measurement results for a sample with a textured, metallic, or pearlescent finish. 

Common geometries include 45°/0° (45° illumination and 0° viewing), 0°/45° (0° illumination and 45° viewing), and diffuse/8°. Each geometry is appropriate for a given sample type and application. 

Sample Preparation 

Accurate and repeatable measurements rely on the precision of the sample prepared. This includes the surface cleanliness, surface finish, sample volume, sample and layer thickness uniformity (in the case of transmission measurements), and alignment to the measurement aperture.

Sample Presentation

Sample presentation is how the sample is held and positioned during the measurements. Standard sample presentation improves precision and repeatability, enhancing the reliability of the results. 

Relevant factors include the sample backing (for the translucent materials), sample flatness, standard force, and alignment with the measuring device's optics axis. 

Color Difference Formula

Mathematical computations on color difference quantify the variation by computing the difference between two measurements. Different formulas put different weights on different factors of color difference. 

The most widely adopted formula until its limitations were realized was ΔEab (CIE 1976). Further development led to ΔE94 and ΔE00 (CIE 2000) – both which provide improved correlation with visual assessment, and ΔECMC which is better for textiles than its predecessors. 

Spectrophotometers Optical Geometry

The optical design of a spectrophotometer affects how light interacts with the sample, and influences the measurement results. Some geometries are better suited for particular materials and applications than others.

Directional Geometries (45°/0° and 0°/45°) utilize light beams focused at specific angles. Because they omit specular reflection, these geometries are perfect for measuring the color of smooth and matte surfaces. Under normal viewing circumstances, results are consistent with human visual evaluation. 

Spectrophotometers Integrating Sphere

An integrating sphere (often used with spectrophotometers) is a hollow sphere coated on the inside with a highly reflective coating, creating uniform diffuse illumination. The sphere’s geometry guarantees that the sample is illuminated from every angle, which eliminates shadows, and yields uniform illumination, consistent illumination regardless of the sample’s texture or shape. 

The sphere is generally equipped with several ports. These are one for the light source, one for the sample, one for the detector, and optionally reference standards or different measurement geometries. 

Spectrophotometers Light Source 

Spectrophotometers are equipped with light sources that are xenon flash lamps or LED arrays. These sources must provide uniform emission of broad-spectrum illumination across the whole visible range.

Xenon flash lamps emit intense light for short periods of time, ideal for measuring critical color precision. However, these lamps have a limited operational life and may need recalibration from time to time. 

Unlike flash lamps, LED arrays have a longer operational life, need less power, and need less time for measurements. However, they may need careful engineering to eliminate gaps in spectral coverage. LED sets unlike flash lamps, combine multiple LEDs from differing spectral outputs to provide broad-spectrum illumination. 

What is the Spectrophotometric Mode?

Each mode is customized for different sample types and applications. Spectrophotometric modes use distinctive measurement settings of the device along with data processing methods for every mode. 

Reflectance Mode for opaque samples, paints, textiles, plastic,s and papers. The mode calculates the percent of light that is reflected and sets color values from the reflected light spectrum. 

For liquids, films and glass, Transmission Mode is used to determine the percent of light that is transmitted through opaque boundaries. The transmitted light spectrum is used for color calculations. 

What is the Sensor of Spectrophotometers?

As with any modern technology, today's spectrophotometers utilize sensors that convert light intensity into electrical signals to convert them to a digital format. Most modern devices contain one or more of the following sensor technologies: 

• Silicon photodiode arrays: These sensors offer incredible responsiveness across the entire visible light spectrum and have good signal-to-noise ratios alongside stable performance. 

• CCD (Charge-Coupled Device) Arrays offer high sensitivity and high resolution, though they tend to be a bit slower than photodiode arrays, which is why they are found in high precision devices for research. 

• CMOS: These sensors are making their way into portable devices because they have lower power requirements and offer good value in performance. 

What is a spectrophotometer's Wavelength?

Wavelength specification for a spectrophotometer defines their measuring range of electromagnetic radiation. Most applications dealing with measurement of color focus on the visible light range which is from 380nm to 780nm, but some instruments even go to the near ultraviolet (UV) and near infrared (NIR) regions. 

• Extended UV Range: This range is important for any devices concentrating on detection of materials having UV brighteners or fluorescent characteristic features of materials. These are commonly found in paper, textiles, and even detergents. 

• Extended NIR Range: This range is useful for some materials research which may require it, or for some special applications.

The accuracy and stability of an instrument's calibration for its color measurement is critical for ensuring consistent results, as even the slightest calibration errors may result in significant measurement inaccuracies.

What is Spectrophotometer's Wavelength Interva?l

Wavelength interval (spectral resolution) is the distance or the spacing between two consecutive measurement points across the spectrum. Some common intervals are 1nm, 5nm, 10nm, and 20nm.

Finer intervals (1nm, 5nm) Practitioners obtain increased color accuracy and detailed spectral data. However, this requires sophisticated optical systems and longer measurement times. These intervals are favorable in research and critical color matching needs.

Coarser intervals (10nm, 20nm) offer a simpler instrument design and faster measurement. These intervals are sufficient for most routine color measurement tasks.

What is Spectrophotometers Measured Reflectance?

Measured reflectance is defined as the percentage of incident light that a sample reflects at a given wavelength. It is the most basic measurement in colorimetry and the basis for all calculations performed.

Reflectance can be measured accurately by high-grade instruments within the visible spectrum with better than 0.1% accuracy and 0.05% precision. Generally, the measurement range is from nearly 0% (very dark materials) and over 100% (for materials with fluorescent properties).

What is the Spectrophotometer's Measuring Aperture?

The measuring aperture refers to the illuminated and analyzed sample's area. Standard sizes of the measuring aperture vary from 1mm to 30mm, with the former being suitable for smaller samples and the latter being ideal for average measurements from larger samples with textured surfaces.

Smaller apertures (1-4mm) are useful to capture small sample features but are not ideal for capturing the sample’s overall color or the color of textured samples.

What is a Spectrophotometer's Colorimetric Index?

Colorimetric indices are derived from analyzed and computed data that describe details of color or quality. These are known as common indices.

• Whiteness Index (WI) measures the intensity of whiteness of a material which is pivotal in the production of paper, textiles, and plastics.

• Yellowness Index (YI) gauges the intensity of yellowing, which is essential while monitoring color variations induced by aging or weathering.

• The concentration of colorants in materials such as dyes and pigments is measured using Color Strength (K/S values). 

These indices simplify color evaluation and decision-making in quality control by providing single-number metrics. 

What is the Spectrophotometer's Observer Angle?

The observer angle a,s defined by CIE as either 2° or 10°, is the region of space where the color is viewed and measured. This angle also defines how color matching functions calculate the importance of the different portions of the spectra. 

The 2° observer was set in 1931 and defines color vision as viewed within a small central field of view. This is still used in some legacy systems and niche applications. 

The 10° observer, set in 1964, defines color vision over a broader field. This is increasingly becoming the default standard for most color measurement applications. 

What is the repeatability of Spectrophotometers?

Repeatability describes the measurement precision of an instrument by how consistently a measurement is done of the same sample several times under the same conditions. It is most often given as the standard deviation of the measured values.

High-end color measurement spectrophotometers achieve repeatability of better than 0.02 ΔE*ab units, which means that the measurement of the same sample will not differ by more than this figure. This degree of accuracy helps in identifying very minor color changes and supports stringent quality control requirements. 

What is a Spectrophotometer's Inter-instrument Error?

Inter-instrument error (or inter-instrument agreement) defines the extent how closely measurements performed by different instruments of the same model agree for the same given samples. This measurement is very important for remote quality control and color communication. 

Spectrophotometers are now capable of achieving inter-instrument agreement better than 0.2 ΔE*ab units which allows consistent color measurement irrespective of location and instrument used. Achieving this inter-instrument agreement is only possible with stringent manufacturing tolerances, consistent calibration methodology, and documented reference standards. 

What are the Different Types of Spectrophotometers?

Spectrophotometers can be classified by various criteria, including their optical design, measurement geometry, sample handling capabilities, and intended applications.

Benchtop Instruments offer the highest precision and most comprehensive feature sets. They're typically used in laboratories and quality control facilities where measurement accuracy is paramount. These instruments often include multiple measurement geometries, extensive calibration capabilities, and sophisticated software for data analysis and color matching.

Classify Spectrometers by Measurement Angle

0:45/45:0 Spectrophotometers

These instruments use directional illumination and viewing angles, either illuminating at 0° and viewing at 45°, or vice versa. This geometry excludes specular reflection, making measurements correlate well with human visual assessment of matte surfaces.

The 0:45/45:0 geometry is ideal for measuring paints, coatings, textiles, and paper, where surface gloss should not affect color measurement. It provides excellent repeatability and is less sensitive to slight variations in sample positioning.

Sphere Spectrophotometers

Sphere spectrophotometers use integrating spheres to create diffuse illumination from all directions. They can operate with specular reflection included (SPIN) or excluded (SPEX), providing flexibility for different sample types.

SPIN measurements include all reflected light and are useful for measuring the total appearance of glossy materials, metallic finishes, and pearlescent coatings.

SPEX measurements exclude specular reflection and focus on the diffuse color component, similar to directional geometry instruments but with the advantage of accommodating textured and curved surfaces.

Multi-Angle Spectrophotometers

Multi-angle spectrophotometers measure color and appearance at multiple viewing angles simultaneously, typically 15°, 25°, 45°, 75°, and 110° from the specular reflection angle. These instruments are essential for characterizing special effect materials like metallics, pearls, and interference coatings.

Where to Learn More about Spectrophotometers

Mastering spectrophotometric measurement involves integrating theory with hands-on experience. For continuing education, conferences, and publications, the Inter-Society Color Council (ISCC) and the Colour Group (Great Britain) are professional color associations to consider.

What Does a Spectrometer Measure?

Modern spectrophotometers do more than measure basic color. They offer advanced spectroscopic characterization of a material’s appearance. In addition to color coordinates in different color spaces, colorimetric parameters such as color strength, whiteness, yellowness, and opacity are computed.

Understanding Spectrophotometric Measurement Parameters to Ensure Color Consistency

Achieving color measurement consistency demands a close relationship between all measurement parameters and their interactions. Each choice of a defined parameter interacts with specific results and must suit the application and the sample type.

Meaningful color communication and quality control in an organization requires standardization of measurement and procedural frameworks. These include proprietary instrument parameters, sample preparation, environmental conditions, and operator calibration and training.  

Regular measurement of instrument parameters against certified benchmarks ensures accuracy and early detection of drift or problems which compromise measurement quality and verification of performance.