Light Measurement
The whole discipline of optical measurement techniques can be roughly subdivided into photometry and radiometry.
Whereas photometry focuses on determining optical quantities that are closely related to the sensitivity of the human eye, radiometry deals with the measurement of energy per time (= power, given in watts) emitted by light sources or impinging on a particular surface.
Thus, the units of all radiometric quantities are based on watts (W). According to CIE regulations, symbols for radiometric quantities are denoted with the subscript “e” for “energy”. Similarly, radiometric quantities given as a function of wavelength are labelled with the prefix “spectral” and the subscript “λ” (for example spectral radiant power Φλ).
To choose the proper light measurement instrument user should understand the fundamental differences between photometers and spectral light meters, exploring their unique functionalities, applications, and the contexts in which each is most effectively used.
For more detailed description of analysis method, refer to our blog post
Photometers: Measuring Light Intensity
Photometers are designed primarily to measure the intensity of light. They operate based on the principle of photometry, which involves the quantification of visible light as perceived by the human eye. A typical photometer includes a light sensor, often a photodiode, that converts light into an electrical signal. This signal is then processed to give a reading of light intensity, usually expressed in lux or lumens.
Photometers are widely used in various industries and applications, including:
Applications
- Illumination Design: Ensuring that spaces such as offices, streets, and public areas have appropriate lighting levels.
- Photography and Videography: Adjusting lighting to achieve the desired exposure and visual effects.
- Environmental Monitoring: Assessing natural light levels in ecosystems and urban environments.
- Health and Safety: Monitoring workplace lighting to adhere to safety regulations and standards.
Advantages
- Simplicity: Photometers are generally easy to use and provide quick readings.
- Cost-Effectiveness: They are typically less expensive than spectral light meters.
- Practicality: Ideal for applications where the primary concern is light intensity rather than its spectral composition.
Spectral Light Meters: Analyzing Light Spectrum
Spectral light meters, on the other hand, provide a comprehensive analysis of the light spectrum. These instruments measure the intensity of light across different wavelengths, offering a detailed spectral distribution. They consist of a diffraction grating or a prism to disperse light into its constituent wavelengths, along with an array of sensors to measure the intensity at each wavelength.
The detailed spectral data provided by spectral light meters make them indispensable in fields such as:
Applications
- Color Science and Quality Control: Ensuring the accurate color reproduction in printing, painting, and display technologies.
- Lighting Design and Research: Developing and testing new lighting solutions, including LEDs and other advanced light sources.
- Agricultural Lighting: Optimizing light conditions for plant growth in controlled environments.
- Art Conservation and Restoration: Analyzing and replicating the original lighting conditions of artworks.
Advantages
- Comprehensive Data: Provides detailed spectral information, enabling precise color analysis and light source characterization.
- Versatility: Suitable for a wide range of scientific and industrial applications.
- Accuracy: Offers high precision in measuring and analyzing the spectral properties of light.
Key Differences Summarized
Applications
Photometers are ideal for general lighting assessments
Spectral light meters are used for detailed spectral analysis and color-related applications.
Measurement Focus
Photometers measure overall light intensity
Spectral light meters analyze the distribution of light across different wavelengths
Complexity and Cost
Photometers are simpler and more affordable
Spectral light meters are more complex and typically more expensive.
Photometric Units
Visible light metrics or ‘photometric’ measurements are referred to as illuminance, luminance, luminous intensity, and luminous flux.
An appropriate entrance optic is required for the different metric, e.g. diffuser for lux, sphere for lumens, lens for cd…. But all require the application of the photometric response to the absolute ‘radiometric’ measurement.
| Radiometric Quantity | Radiometric Unit |
| Irradiance | W/m2 |
| Radiance | W/(sr.m2) |
| Radiant intensity | W/sr |
| Radiant flux | W |
| Photometric Quantity | Photometric Unit |
| Illuminance | lux |
| Luminance | cd/m2 |
| Luminous intensity | cd |
| Luminous flux | lumens |
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