Optical Measurement Technology Explained Simply

What is a photometer?

A photometer is an optical measuring instrument that measures the intensity of light. It determines how much light a sample transmits, absorbs, or reflects.

Transmission photometers measure the light passing through the sample. Reflection photometers measure the light reflected from a surface.

Lambert-Beer's Law Transmission and Reflection

Optical Path Length Measurement active

I₀ Incident light

I Filtered Light

_IR Reflected Light

Studyflix Educational Video

Photometry Explained in a Video

The Studyflix educational video explains the photometric measurement principle, the structure of a photometer, the Lambert-Beer law, and the determination of substance concentrations.

Open the video directly on Studyflix

The video will open directly on Studyflix in a new browser window.

Definition

What does a photometer measure?

A photometer compares the intensity of the emitted light with the light intensity that reaches the detector after interacting with a sample.

Light Becomes Measurable Information

When light strikes a sample, it can pass through it, be absorbed, scattered, or reflected. Based on the change in light intensity, the photometer calculates a measured value.

In this way, it is possible, for example, to determine concentrations, water content, color intensity, surface properties, film thicknesses, and the components of a mixture of substances .

Depending on the measurement task, measurements can be made in the ppm, vpm, or percent range. The achievable measurement range depends, among other things, on the wavelength, the film thickness, and the optical properties of the sample.

Light source

Emits light in the ultraviolet, visible, near-infrared, or infrared spectrum.

Filter or monochromator

Select a measurement wavelength and, often, a reference wavelength as well.

Sample or measurement medium

For example, liquid, gas, powder, granules, film, paper, or a solid surface.

Detector and Analysis

The detector converts the light intensity into an electrical signal, which is then digitally evaluated.

Physical Basis

Lambert-Beer's Law

Quantitative transmission photometry is often based on Lambert-Beer's law. It describes the relationship between light absorption, concentration, and optical layer thickness.

The more particles there are, the less light there is

If there are more absorbing particles in the sample, or if the path length of light through the sample increases, the measured light intensity decreases.

A = ε · c · d

A =_−log₁₀(T)

T = I / I₀

A Absorbance or Extinction

ε Substance-dependent molar extinction coefficient

c Concentration of the absorbent material

d Optical layer thickness or path length of light

I₀ Intensity of the incident light

I Intensity of the transmitted light

Important: The classic Lambert-Beer law applies in particular to homogeneous, sufficiently dilute solutions and, as far as possible, monochromatic light. Significant scattering, chemical interactions, and very high concentrations can cause deviations.

In reflection measurements, the relationship between the reflection signal and the material content is typically determined through test measurements and an application-specific calibration curve.

Interactive Simulation

Vary the concentration and layer thickness. The simulation shows schematically how the absorbance and transmittance change.

Relative concentration c 35%

Layer thickness d 10 mm

0.63 Absorbance A

23.4% Transmission T

76.6% Light attenuation

The simulation is for illustrative purposes only and does not replace actual calibration.

Measuring principle

How a Photometric Measurement Works

The basic measurement principle is similar for laboratory and process photometers.

Generate light

A suitable light source produces the required measurement light.

Select Wavelength

Filters select the measurement and reference wavelengths.

Examine the sample

The light passes through the sample or strikes its surface.

Capturing Light

The detector measures the transmitted or reflected light.

Calculate the measured value

The analyzer calculates, for example, concentration, moisture content, color, or film thickness.

Animated filter wheel with measurement and comparison filters

Dual-wavelength method

Measurement and Reference Wavelength

Industrial photometers often use at least two interference filters. One filter determines the measurement wavelength, and the second filter determines the reference wavelength.

The filter wheel rotates and alternately filters out light of the two wavelengths from the light beam. Then the two detector signals are compared with each other.

At the measurement wavelength, the component of interest should absorb as much as possible or significantly alter its reflection behavior.
At the reference wavelength, the component should absorb or reflect as little as possible.
The ratio of the two signals is a measure of the substance to be determined.
Uniform effects on both wavelengths can be partially compensated for.
Lamp aging, detector drift, normal cuvette contamination, or extraneous light often have less of an impact on the measurement.

Compare Measurement Methods

Transmission or Reflection?

The most important difference lies in where the detector is positioned and whether the light can pass through the sample.

Through-light measurement

Transmission Photometer

In transmission measurement, light is passed through a sample. The detector is located behind the sample and measures the remaining light intensity.

Suitable for transparent or partially transparent samples
Typical for liquids and gases
Measurement of concentration, water content, color, and organic components
Quantitative analysis is often performed according to the Lambert-Beer law
Single measurements and continuous process measurements are possible

T = I / I₀ × 100%
T = transmission, I₀ = incident intensity, I = transmitted intensity

Reflectance Measurement

Reflection Photometer

In reflectance measurement, light strikes the surface of a sample. The detector detects the reflected light.

Suitable for solid, opaque, or highly scattering samples
Ideal for powders, granules, fibers, paper, films, and coatings
Measurement of moisture content, color intensity, whiteness, and component proportions
Continuous measurement over a conveyor belt is possible
Analysis Using Application-Specific Calibration Curves

R =_IR / I₀ × 100%
R = reflectance, I₀ = incident intensity, _IR = reflected intensity

Wavelength ranges

UV, VIS, NIR, and IR

Different materials absorb or reflect light at different wavelengths. For this reason, the spectral range is adapted to the measurement task.

Transmission Photometer

Typical measurement range: 200 to 4,700 nm

UV 200–400 nm

VIS 400–700 nm

NIR 700–2,500 nm

IR 2500–4700 nm

Reflection Photometer

Typical measurement range: 400 to 4,700 nm

VIS 400–700 nm

NIR 700–2,500 nm

IR 2500–4700 nm

For accurate measurements

Why is calibration necessary?

Every application is different

Different measurement media have different spectral curves. Therefore, a photometer cannot be calibrated absolutely for every application based solely on general tables.

To obtain precise quantitative results, test measurements using samples with known concentrations or reference materials are required.

Determine the appropriate measurement wavelength
Set the reference wavelength
Measuring reference samples
Create a Calibration Curve
Check the calibration regularly

What factors affect the measurement range?

The possible measurement range and the achievable accuracy depend on several factors.

Cuvette layer thickness during transmittance measurement
Reflective Properties of the Surface
Selected Measurement and Reference Wavelengths
Homogeneity and Composition of the Measurement Medium
Temperature and Pressure, Especially in Gas Measurements
Sufficient light reaching the detector

If the transmission or reflection is too low, insufficient usable light reaches the detector. In this case, measurement accuracy can decrease significantly.

Light Signal / Reflection

Wavelength in nm

1450 nm

1,940 nm

Example of Use

Photometric Water Analysis

Water exhibits characteristic absorption bands in the near-infrared region. This makes it possible to photometrically determine the water or moisture content in numerous materials.

Strong water band at 1940 nm

This wavelength is particularly sensitive to water and is suitable for measuring relatively low moisture content.

Water band at 1450 nm

The weaker band can be used for moderate water content.

Reference wavelength outside the band

The reference wavelength is selected so that water has as little effect as possible on the measurement signal.

Typical Applications

What can be measured photometrically?

Transmission Measurements

Especially for liquids, solutions, and gases.

Proportion of individual components in liquids or gases
Water Content Analysis in Various Materials
Intensity of Color in Solutions
Detecting Color Changes in Chemical Reactions
Detecting Red, Blue, or Yellow Casts
Measurement of organic matter residues in the ultraviolet range
Laboratory, spot, and continuous process measurements

Reflection Measurements

Especially for solid, loose, or opaque materials.

Moisture Measurement in Powders, Granules, and Fibers
Thickness Measurement of Sheets and Films
Determination of Components in Mixtures of Substances
Whiteness and Color Measurement
Measuring the Thickness of Coatings
Measurements of food, animal feed, textiles, and paper
Continuous Measurement Across Conveyor Belts

Process Measurement Technology

Design of Industrial Photometer Systems

Industrial systems often consist of a measuring head, a data acquisition unit, and the necessary connections to the process control system.

Measuring head

Includes a light source, optical components, and a detector. The measuring head is mounted directly on the process or above the sample .

Evaluation device

Processes the detector signals, calculates the measured value, and displays measurement curves, threshold values, and diagnostic data.

Compensation

Additional temperature and pressure sensors can compensate for changes in measured values, particularly in gas measurements.

Process Integration

Measurement values can be transmitted via analog outputs, digital signals, USB, Ethernet, or industrial communication systems .

Fast Measurement and Direct Process Control

Photometers often provide measurement results for single measurements within a few seconds. For continuous measurements, measurement values can be updated in fractions of a second. This not only enables rapid display of results, but also allows for direct control of automated production processes.

Depending on the system, measured values can be recorded in real time, displayed as a graph, archived, and used for quality control or process control.

Knowledge Explained in a Nutshell

Frequently Asked Questions About Photometers

The most important questions about transmission photometers, reflection photometers, wavelengths, calibration, and industrial applications.

What is a photometer?

A photometer is an optical measuring instrument that measures the intensity of light before and after it interacts with a sample. Based on the change in the light signal, it is possible to determine, for example, concentrations, moisture content, color values, film thicknesses, or the components of a mixture of substances.

What is the difference between a transmission photometer and a reflection photometer?

In a transmission photometer, light passes through the sample. The detector is located behind the sample and measures the transmitted light. In a reflection photometer, light strikes a surface. The detector measures the light reflected by the sample.

What law does photometry rely on?

Lambert-Beer's law is a fundamental principle of quantitative absorption and transmission photometry. It describes the relationship between absorbance, substance concentration, and optical layer thickness: A = ε · c · d.

Does Lambert-Beer's law also apply to reflection photometers?

Not exactly in the same way. With solid, granular, or loose materials, light propagation is more complex. The relationship between the reflected signal and the measured value is therefore usually determined through test measurements and an application-specific calibration curve.

What physical quantities can be measured with a photometer?

Depending on the measurement method and calibration, substance concentrations, water and moisture content, color intensities, color changes, whiteness, coating thicknesses, film thicknesses, and the proportions of individual components in material mixtures can be determined.

In what concentration ranges can measurements be taken?

Depending on the application, measurement medium, wavelength, and coating thickness, measurements in the ppm, vpm, and percent ranges are possible. The exact measurement range must be configured and calibrated for the specific application.

What wavelengths does a transmission photometer use?

Depending on the model, transmission photometers can cover a range from approximately 200 to 4,700 nm: ultraviolet from 200 to 400 nm, visible light from 400 to 700 nm, near-infrared from 700 to 2,500 nm, and infrared from 2,500 to 4,700 nm.

What wavelengths does a reflection photometer use?

Reflectance photometers typically operate in a range of approximately 400 to 4,700 nm. This includes the visible spectrum, the near-infrared, and the infrared spectrum.

Why are different wavelengths used?

Different materials absorb or reflect light at different wavelengths. For a specific measurement task, a wavelength is therefore selected at which the component of interest causes the most distinct optical change possible.

What is a measurement wavelength?

The measurement wavelength is the wavelength at which the component to be determined is absorbed as strongly as possible or its reflectance behavior is altered particularly significantly.

What is a reference wavelength?

The reference wavelength is selected so that the component of interest absorbs or reflects as little as possible at that wavelength. The ratio of the measurement signal to the reference signal improves the stability of the measurement.

What is the function of the filter wheel in a photometer?

The filter wheel is typically equipped with at least two interference filters. As it rotates, it alternately selects the measurement wavelength and the reference wavelength from the light beam.

What are the advantages of the dual-wavelength method?

Factors that have a similar effect on the measurement and reference wavelengths can be partially compensated for. These include, for example, aging of the light source and the detector, normal cuvette contamination, and certain external light sources.

Why does a photometer need to be calibrated?

Different measurement media have different spectral curves and optical properties. Only by using reference samples with known measurement values can the relationship between the optical signal and the desired substance content be determined.

How is a photometer calibrated?

First, suitable measurement and reference wavelengths are selected. Next, samples with known concentrations, moisture content, or material properties are measured. Based on this, a calibration curve or an evaluation algorithm is created.

What factors determine the possible measurement range?

In a transmission measurement, the cuvette layer thickness, light absorption, and selected wavelengths are particularly important. In a reflection measurement, the reflection properties, surface structure, color, homogeneity, and composition of the material play a role.

What happens if too little light reaches the detector?

If the transmittance or reflectance falls below a certain minimum value, the usable light output is too low. This degrades the signal-to-noise ratio and can make it difficult to obtain a reliable measurement.

For which applications are transmission photometers suitable?

Typical applications include the determination of individual components in liquids and gases, the measurement of water content, the monitoring of coloration and color changes, and the detection of organic residues in the ultraviolet range.

For which applications are reflection photometers suitable?

Reflectance photometers are frequently used to measure moisture content in powders, granules, and fibers. Other applications include measuring film thicknesses, coatings, component proportions, color intensities, and whiteness levels.

What materials can be analyzed using reflectance photometers ?

These include minerals, construction materials, ceramic compounds, chemicals, powders, granules, foils, films, food, animal feed, filter cakes, fibers, textiles, and paper.

How can water or moisture be measured photometrically ?

Water has characteristic absorption bands in the near-infrared region. If the water content changes, the transmission or reflection signal also changes. Through calibration, the water or moisture content can be calculated from this.

What is the significance of the water band at 1940 nm?

There is a strong water band at approximately 1940 nm. The optical signal is particularly sensitive to changes in water content at this wavelength. This wavelength may therefore be suitable for measuring relatively low moisture content levels.

What is the significance of the water band at 1450 nm?

The water band at approximately 1450 nm is weaker than the band at 1940 nm. It may therefore be suitable for applications with moderate water content.

Can gases also be measured using a transmission photometer ?

Yes. Since gases have a lower density than liquids, longer optical path lengths are often required. The measured value may also depend on pressure and temperature.

Why are pressure and temperature compensations important?

Particularly in gas measurements, pressure and temperature affect the density and, consequently, the optical measurement signal. If these parameters are not kept constant, corresponding sensor values can be used for compensation.

How fast does an industrial photometer operate?

Individual measurements can be performed in just a few seconds. For continuous process measurements, updates are possible in fractions of a second.

Can photometers be used in the laboratory and in the production process ?

Yes. Photometers are suitable for laboratory analyses, spot checks, and batch measurements, as well as for the continuous monitoring of automated production processes.

What components make up an industrial photometer system?

A typical system consists of a measuring head, an evaluation unit, and connecting cables. Depending on the application, cuvettes, optical fibers, temperature sensors, pressure sensors, cooling systems, calibration standards, or purge air systems may also be included.

What is the function of the data logger?

The data acquisition unit processes the signals from the measuring head, calculates the measured value, and can take additional signals such as pressure or temperature into account. Measured values can be displayed, stored, or transmitted to a control system .

Can photometers be connected to a process control system?

Yes. Depending on the device model, analog measurement outputs, digital inputs and outputs, relay outputs, USB, Ethernet, or industrial communication interfaces can be used.

What are the advantages of an open transmission probe?

In an open design, the cuvette is positioned freely between the light source and the detector. In the event of a leak, liquids or gases cannot easily penetrate the instrument housing. Furthermore, the cuvette remains easily accessible.

Are photometers available for potentially explosive areas?

Explosion-proof versions are available for certain applications. Depending on the device model, variants are available for Ex Zones 1 and 2, as well as various ATEX protection concepts.

Why is it a good idea to conduct test measurements before selecting equipment?

Test measurements show whether the desired component, at a suitable wavelength, causes a sufficiently large change in the optical signal. They help to determine the measurement range, wavelengths, film thickness, probe design, and calibration.