Light Measurement Devices: From Spectral Data to Imaging Colorimeters

Anne Corning

Spectroradiometers are powerful tools for measuring the brightness (luminance) and color (chromaticity) of a light source or illuminated display at a single point. Imaging colorimeters are likewise powerful tools for measuring light and color using images for spatial context. Each of these LMDs (light measurement devices) measures color in different ways and captures a different subset of data about the visual characteristics of a device under test (DUT). For complete performance and quality assessment of lighting and display products from the lab through final assembly, developers and manufacturers often need a full set of data that encompasses both spectral and colorimetric characteristics. 

Composite of DUTs

Examples of products and devices that require luminance and chromaticity measurement in development and manufacturing.

Many inspection applications demand the capabilities of both a spectroradiometer and an imaging colorimeter for device qualification. This often requires manufacturers to source and purchase both systems. But only one LMD can be used at a time for measurement, requiring multiple stations or processes—an inefficient and costly approach. Additionally, any manual setup and data transfer between systems can cause significant downtime or introduce errors at any stage of the inspection process.

Integrating the two measurement devices into a single system, however, offers developers and manufacturers a more robust, multi-functional, accurate, and efficient metrology solution. To understand these two types of LMDs, how they each characterize light and color, their complimentary capabilities, and the importance of both spectral and colorimetric data for assessing device performance, we start with a discussion of light. 

The Light & Color Spectrum

The electromagnetic spectrum characterizes all of the light energy radiating from the sun—indeed, all of the light energy in the universe, both visible and invisible to the human eye. One way to characterize light is by its wavelength. Light waves are made up of sub-atomic particles called photons, which oscillate at different frequencies. The greater the frequency, the shorter the wavelength.

The entire spectrum ranges from very energetic gamma rays on one end (short wavelength), all the way to low-energy microwaves and radio waves on the other end (long wavelength). The very small band in the middle of the spectrum is the range of visible light that the human eye can perceive. Visible light has wavelengths ranging from about 380 nanometers (nm) to 700 nm, bounded by ultraviolet (UV) at the low end of the range and infrared (IR) at the high end.

Electromagnetic spectrum

The electromagnetic spectrum from high-energy, high-frequency, short-wavelength gamma rays to low-energy, low-frequency, long-wavelength radio waves. The visible light spectrum—the range of wavelengths that can be perceived by the human eye—is just a small sliver of the electromagnetic spectrum, extending from roughly 380 nm – 700 nm.

Radiometry, Spectrometry, and Spectroradiometry

Radiometry is the science of measuring all wavelengths of light, including those beyond the human visual range. Radiometers are instruments that characterize the amount of energy of a given wavelength range, for example UV or IR. They are typically inexpensive, portable, and can take quick measurements of a light source’s total power, or radiant flux (measured in Watts). 

Spectrometers are also used to measure wavelength ranges, but are specifically applied to measure the composition of light using internal optical diffraction gratings, prisms, or filters to separate light into its constituent wavelengthsAlthough incredibly precise, spectrometers are not complete systems. Rather, they need to be paired with additional optics to output relevant measurement values. 

Combing a spectrometer and radiometer yields a spectroradiometer, which harnesses the high precision of the spectrometer to measure various characteristics of light sources including radiance, luminance, and chromaticity values.  A spectroradiometer can be used to capture spectral values alone or applied as a reference instrument to provide extremely accurate data to calibrate other measurement instruments such as imagers. 

One drawback of all three of these LMDs is that they can measure only a small area or “spot” –they are sometimes referred to generically as “spot meters.” This means that they can’t measure a full display panel or the complete distribution of a light source all at once. Instead, multiple individual measurements must be taken to capture all relevant light output.

Imaging Photometers and Colorimeters 

Photometry is the science of measuring wavelengths of light in relation to how they are perceived by the human eye; colorimetry applies the same principles to measuring wavelengths based on how we perceive color. In comparison to the spot meters described above, imaging photometers and colorimeters are instruments that capture and measure an entire area at once in a single image, which can then be analyzed for multiple parameters simultaneously. 

Imaging photometers measure both luminance values and spatial resolution; colorimeters measure these properties plus chromaticity. These LMD systems enable more sophisticated image processing techniques such as display mura analysis and defect detection, which rely on being able to compare measurement values across a DUT to assess uniformity.

Spectral and Colorimetric Measurement

There is a key difference between spectral and photometric/colorimetric measurements. Spectral data, as measured by a spectroradiometer, captures absolute values: the raw energy of each wavelength of a light source. Photometric/colorimetric measurements, by contrast, characterize a light source with reference to the perceptive capabilities of the human eye. This data captures the amount of energy per wavelength perceived by a standard human observer. 

Spectral data can be used to create a spectral power distribution (SPD): a graphical representation of the energy and wavelength properties of the source, as shown in the images below. The y-axis is typically normalized on a scale from 0 -1 indicating relative power at each wavelength, not absolute unit measurements.

Three example SPDs

Spectral power distributions of three example light sources: the sun at full daylight, a typical incandescent bulb, and a typical fluorescent bulb. Wavelength is shown on the x-axis, and relative intensity (power) is shown on the y-axis. (Image source: Luminus Devices, Inc.)

Colorimetric data can be used to represent the human visual system’s response to a light source’s SPD, which we perceive as a single “color.” In reality, the color we see is based on how the structures in our eye (cones) respond to three primaries—roughly, red, green, and blue, as shown in the image below. For a detailed explanation of SPDs and the human response, watch the webinar from Photonics Spectra, “Colorimetry: A Primer on the Science of Light and Color Measurement.”

Rods and cones_human visual perception

The human eye has three types of cones (S, M, and L) that are each sensitive to a range of wavelengths of light roughly corresponding to red, green, and blue. The combined response of our 3 cones enables us to perceive the “color” of an SPD at a given power per wavelength.

There are multiple uses for SPDs and spectral data in scientific, engineering, industrial, and commercial endeavors. For example, LED manufacturers and lighting designers refer to SPDs to help create lighting schemes that are optimal for human activity and comfort (human-centric lighting). Colorimetric data is particularly useful is to assess manufactured products—such as printed materials, painted surfaces, light sources, and illuminated displays that are intended to be seen and used by humans. In these cases, absolute color specifications are less important than how the color will appear to customers.

Calibration Ensures Accuracy

Light and color data is only as useful as it is accurate. Any measurement device or piece of metrology equipment must be calibrated to a known standard to ensure that its results are and remain accurate. For example, Radiant’s measurement systems are calibrated at our factory, again at installation, and we further recommend annual recalibration to ensure continued performance. 

But some applications require more frequent calibration, for example, if different types or colors of DUTs are being tested. When using an imaging colorimeter, absolute values from a spectrometer or spectroradiometer are needed to reset the colorimeter’s response for every new scenario. Pulling out a spectroradiometer device to perform frequent manual calibrations in these circumstances could be a very laborious and time-consuming process. 

An imaging colorimeter with an integrated spectrometer that provides rapid, automatic calibration eliminates these challenges. Device makers can enjoy efficient, accurate measurement of the comprehensive data set they require. 

An Integrated Imaging Colorimeter + Spectroradiometer Solution

Radiant’s new ProMetric® I-SC is the integrated solution that satisfies this need, pairing a high-resolution ProMetric I Imaging Colorimeter for rapid visual inspection with a high-end spectroradiometer for real-time color calibration and spectral data measurement. This innovative design is based on patented technology developed by Radiant Vision Systems (US Patent No. 8482652). 

Using a measurement image acquired by the connected imaging colorimeter, the ProMetric I-SC simultaneously measures spectral data at the center point of a DUT while quantifying and comparing spatial luminance (cd/m2) and chromaticity (CIE x,y and u′v′) values across the image to evaluate a device. Both systems are controlled using a single software platform, our TrueTest™ Software, which also provides a centralized interface for data visualization and output.

When used with TrueTest™ the ProMetric I-SC system captures both spectral data and accurate, repeatable color measurement across devices, testing environments, and over time. Device makers can:

  • Capture spectral and spatial measurement data simultaneously using a single solution
  • Maintain continuous operation including fully automated data capture and system calibration
  • Capture and store repeatable, CIE-matched measurements
  • Choose from multiple sensor resolution options and lens choices with Smart Calibration™ for a wide range of distance and aperture settings
ProMetric I-SC Solution - Imaging Colorimeter and Integrated Spectrometer

Radiant’s ProMetric® I-SC integrates a ProMetric Imaging Colorimeter with a high-end spectroradiometer.

With the ProMetric I-SC, Radiant offers the advantage of both systems’ capabilities in one solution, eliminating the burden of purchasing multiple systems or learning separate platforms. Direct integration also enables system functionality without manual intervention. Users can measure spectral and spatial data at the exact same point on a source, monitor output, and directly apply color calibrations, without moving equipment.

Combining system capabilities in the ProMetric I-SC also maximizes capability of the solution as a whole. Through the context of measurement images, users can define and view the precise location of the spectrometer’s spectral measurement point as a spot in the image (not possible when using a spectrometer separate from an imaging system). Similarly, because the spectroradiometer uses the imaging colorimeter’s optics, it can acquire spectral data from the exact position and angle relative to the DUT that the imager captures.

From absolute accuracy in product design to optimal efficiency for in-line quality control, the ProMetric I-SC Solution is engineered specifically to address end-to-end metrology applications.

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