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How does a spectrophotometer measure color?

A spectrophotometer is an instrument used to measure the color and light intensity of a sample. It works by passing light through a prism or diffraction grating to split it into different wavelengths, and then measuring the intensity of each wavelength after it passes through the sample. This allows the spectrophotometer to determine how much light is absorbed by the sample at each wavelength, which reveals the sample’s color and light absorbing properties.

How a Spectrophotometer Works

Here are the main components and functioning of a spectrophotometer:

  • Light source: This is typically a tungsten-halogen lamp or LED that emits a broad spectrum of light wavelengths.
  • Monochromator: This device splits the light into different wavelengths. It consists of a prism or diffraction grating to disperse the light, and an adjustable slit to select a narrow band of wavelengths.
  • Sample holder: This holds the sample material so light can pass through it. Sample types can be solid, liquid, or gas.
  • Detector: This measures the intensity of light transmitted through the sample. Photodiodes, photomultiplier tubes, and CCD arrays are common detectors.
  • Readout: This displays the amount of light absorbed by the sample at each wavelength. It may show the spectra visually or display numeric absorption values.

In operation, a spectrophotometer shines light from the source through the monochromator, which scans across the wavelength range. At each incremental wavelength, the intensity of the transmitted light through the sample is measured by the detector and sent to the readout. The absorption spectrum showing absorbance vs. wavelength is then plotted.

Measuring Sample Color

Spectrophotometers measure color based on how much light the sample absorbs at different wavelengths in the visible spectrum. The visible spectrum ranges from about 380-750 nm wavelength and contains all the colors of light detectable by the human eye.

When white light passes through a colored sample, it preferentially absorbs certain wavelengths depending on the sample’s chemical composition. This selective absorption of wavelengths makes the light appear as the color observed when it reaches the eye. For example:

  • A red sample absorbs blue and green light, transmitting more red.
  • A yellow sample absorbs blue light, transmitting more red and green.
  • A purple sample transmits red and blue, absorbing more green.

By measuring transmittance across many narrow wavelength bands, a spectrophotometer produces a spectrum showing which wavelengths are absorbed and to what degree. This quantitative absorption spectrum acts like a “fingerprint” to identify the sample’s characteristic color.

Color Space Systems

To mathematically specify color, the absorption measurements at key wavelengths are used to calculate color coordinate values. There are several color space systems that spectrophotometers use:


This system is based on how the human eye perceives color, using imaginary primary colors X, Y, and Z. It forms the basis for many other color spaces. Spectrophotometers measure the reflectance or transmittance curve, then calculate the X, Y, Z tristimulus values by integrating the product of the sample’s spectral curve and the color matching functions for X, Y, and Z.


This color space describes color using Lightness (L) and two color channels a and b. L ranges from 0 (black) to 100 (white). The a axis represents green to red, while b represents blue to yellow. CIE Lab allows for uniform color spacing, where a given distance represents approximately the same perceived color difference.


A similar system to CIE Lab that describes color using L for lightness, u for the red-green axis, and v for the blue-yellow axis. A key difference from CIE Lab is it aims to be perceptually uniform even when colors get very bright or dark, providing better spacing at the extremes of lightness.


This represents amounts of Cyan, Magenta, Yellow, and Black ink required to match a color, and is commonly used in printing. Spectrophotometer color software can convert the absorption spectrum to CMYK values. Each color is represented on a 0-100% scale, with higher values meaning more of that ink is needed.


This system describes colors in terms of Red, Green, and Blue light on a 0-255 scale. It is additive color used for electronic displays. A spectrophotometer system can calculate the RGB values corresponding to the measured absorption spectrum.

Color Space Application
CIE XYZ Basis for many other color systems
CIE Lab Uniform color difference representation
CIE Luv Perceptually uniform color space
CMYK Printing ink formulas
RGB Electronic display colors

This allows the numerically defined color to be communicated and reproduced accurately across different mediums. The spectrophotometer color data can also be used to check for consistent color quality in manufacturing.

Measurement Modes

Spectrophotometers can measure reflected light, transmitted light, or both depending on the model. The main measurement modes are:


In reflectance mode, the spectrophotometer shines light on the sample and measures the percentage of each wavelength reflected. This is used for opaque solid samples like painted surfaces, fabric, paper, and plastics. An integrating sphere helps collect the reflected light accurately.


In transmittance mode, light passes through the sample in a cuvette or film holder and the spectrophotometer measures the fraction transmitted. This is useful for transparent liquid samples like inks, dyes, and biological assays.


This measures light reflecting off a sample backed by a non-reflective black surface, combining aspects of transmittance and reflectance modes. It allows colored transparent film or plastic samples to be measured.

Double Beam

This uses a split beam, one passing through the sample and one through a reference. The reference beam intensity is compared to the sample beam to automatically account for lamp fluctuations. It improves the accuracy of quantitative absorbance measurements.

Mode Sample Types
Reflectance Opaque solids
Transmittance Transparent liquids
Transflectance Transparent films
Double Beam Precise quantitative absorbance


Spectrophotometers are widely used in many industries and fields to analyze the color and light absorption properties of materials:

Manufacturing Quality Control

Spectrophotometers allow manufacturers to objectively measure color consistency in their products to maintain quality control standards. This includes applications like:

  • Checking batch-to-batch consistency of paint, plastic, textiles
  • Matching colors in production to established specifications
  • Evaluating color fastness and durability

Scientific Research

Researchers use spectrophotometers for assays and quantitative analysis in fields like:

  • Chemistry – Measuring reagents and reaction products
  • Biochemistry – Nucleic acid and protein assays
  • Microbiology – Cell density measurements
  • Forensics – Identifying samples by spectroscopy

Color Matching and Measurement

Industries that need accurate color reproduction use spectrophotometers, including:

  • Printing – Matching proofs to press runs; measuring ink and paper color
  • Textiles – Matching fabrics to color standards
  • Cosmetics – Formulating makeup shades
  • Retail paint – Matching paint colors for customers

Remote Sensing

Satellite-mounted spectrophotometers measure light reflected off the earth’s surface, allowing mapping of ground characteristics like vegetation and mineral content.

Medical Diagnostics

In pulse oximetry, differences in light absorption of oxygenated and deoxygenated hemoglobin allows measurement of blood oxygen levels.

Key Specifications

There are several technical specifications to consider when selecting a spectrophotometer:

  • Wavelength Range – Typically around 340-800 nm for color measurement.
  • Wavelength Accuracy – How closely it measures the intended wavelength; 1-2 nm is good.
  • Wavelength Resolution – The smallest increment it can select, such as 1 nm.
  • Photometric Range – The range of light intensities it can measure before saturating.
  • Photometric Accuracy – How precisely it measures light intensity, such as within 0.5%.
  • Bandwidth – The width of each wavelength band measured, smaller is better resolution.
  • Noise – Ratio of stray light signal to source intensity for sensitivity.
  • Baseline Flatness – Evenness of spectrum for quantitative work.

Benchtop spectrophotometers provide better performance than portable handheld units. Higher cost research-grade systems offer very fine wavelength resolution down to 0.1 nm and high photometric accuracy.

Calibration and Standardization

To ensure accurate measurements, spectrophotometers need periodic calibration. This accounts for effects from aging lamps, drift, and baseline inconsistencies. Calibration involves measuring reference light sources or standard color tiles with known properties.

For transmittance, blank samples like air or water provide 100% transmission references. In reflectance mode, certified reflectance standards with values traceable to NIST are used. Some spectrophotometers can also self-calibrate internally. Proper standardization confirms the instrument is performing to specifications and allows adjustment if needed.


A spectrophotometer is a versatile instrument that uses spectral analysis of light passing through a sample to quantify its color and absorbance characteristics. By measuring the degree of light absorption at different wavelengths, it produces a unique spectral fingerprint of the sample color. This data can be used to ensure color quality control, match colors precisely, identify materials, perform chemical assays, and more. With the proper calibration and standardization, spectrophotometers provide highly quantitative color measurement across industries like manufacturing, textiles, printing, cosmetics, food science, chemistry, biology, and medicine.