A Guide to CCD-Based Imaging Colorimeters

CCD-Based Instruments Provide Several Benefits for Measuring Color and Light.

When correctly configured, CCD imaging systems are successful at identifying defects in uniformity and pixels, evaluating uniformity, evaluating multiple spots (LED arrays), quickly gathering several data points, and recording the data for further analysis.

With several choices for CCD, lenses and filters, the Radiant Vision Systems ProMetric® Imaging Colorimeters and Photometers are offered in over 100 configurations and can be easily optimized for any application requiring measurement.

Considerations

CCD. The choice of CCD is the single most critical element influencing the performance of the overall imaging colorimeter system.

This selection will establish dynamic range and pixel resolution, and along with lens selection, will determine attainable viewing angles. Engineering considerations that are crucial in CCD selection are outlined in the next section of this article.

Control Electronics. The design of the control electronics utilized to operate the camera system is closely related to the choice of the CCD. The key requirements are that the control electronics reduce electronic noise and assist optical and electrical calibrations.

Filters. Filter selection is normally a choice between standard configurations for either colorimetric or photometric operation.

According to the measurement application, more selections are available for optimized color measurement performance, view-angle measurement (conoscope), radiometric measurement, and near-infrared (NIR) measurement.

There are several options for filter technology available, comprising of external, internal, and onCCD filter arrays.

Lenses. Lenses are chosen based on field-of-view requirements and working distance for the application.

The optical geometries influencing lens selection will be restricted by the particular CCD employed. If individual lens features are not correctly compensated, they may trigger optical aberrations that impact measurement accuracy.

Considered all together, the different CCD, filter, lens, and electronics choices allow for incredible flexibility when establishing the cost and performance of the imaging colorimeter.

A thorough consideration of trade-offs will allow the optimum configuration for any application to be found.

CCD Technology

CCDs (Charge Coupled Devices) are monolithic semiconductor detector arrays that transform light into electrical current. When incident photons holding the charge are absorbed by the material of a detector, they generate electron-hole pairs.

Electrons gather in each specific element of the detector, called a pixel, where they are contained until the charge is read out during exposure. The complete amount of charge that gathers in each pixel is linearly proportional to the amount of light incident upon it.

The selection of the correct CCD-based imaging colorimeter for your application needs an understanding of several simple trade-offs in imaging colorimeter architecture.

There are a few simple types of CCD architecture, with many differences according to particular applications. Radiant’s ProMetric Imaging Colorimeters utilize scientific-grade CCDs that provide especially high performance in terms of low-noise features, while also offering an extremely fast data transfer speed and a high resolution.

Resolution

Resolution is a critical specification to evaluate when considering the features of an imaging colorimeter, but it can easily be misunderstood.

The resolution is essentially the total amount of pixels that a 2D imaging system will gather, both vertical (N) and horizontal (M) pixels.

For standard imaging colorimeters of a high accuracy, several images are taken through various filters that reflect the x-bar (red), y-bar (green), and z-bar (blue) tristimulus curves.

These images are then changed to overlap and create a single image comprising of X, Y, and Z tristimulus values for every MxN pixel of the image. In some examples, Bayer mosaic (RGB) systems are calibrated to analyze color.

These systems do not have the color precision of a CIE filter-based system and are restricted to applications like color uniformity.

RGB systems may provide a fast processing speed and cost effectiveness, but they offer a considerably lower (basically half) resolution compared to a CIE-filter based system.

High-resolution CCDs offer spatial resolution for fine-scale measurements on illuminated keyboards, high definition displays, and for the detection of tiny surface defects.

For applications where resolution and accuracy are crucial, CIE filter-based systems are the perfect selection. The high resolution of the ProMetric systems allow them to carry out pixel-level detection.

Noise Reduction

Noise reduces the repeatability and accuracy of CCD images. The signal-to-noise ratio (SNR) analyzes the amount of background noise compared to the preferred signal. SNR is important when considering slight variations in light frame measurements.

Radiant Vision Systems mura and defect detection systems, for example, are heavily reliant upon it. The main sources of noise in CCD detectors are read noise, thermal noise, pattern noise, and shot noise.

Thermal noise

Along with free electrons being generated by incident photons (known as photoelectrons), thermal influences can also generate free electrons. Image noise from the charge that has been thermally created becomes especially challenging at high temperatures, or throughout long exposures.

As thermal noise is highly dependent on temperature (every 6 °C decline in temperature decreases it by around a factor of two), the cooling of a CCD decreases the noise floor significantly.

This allows for longer image integration times, which assists the measurement of low light levels for example a display dark state. Cooling is frequently utilized in high-dynamic-range or low intensity applications.

The most frequently used cooling technique for commercial CCD imagers is thermoelectric cooling (TEC, or Peltier coolers).

According to the noise floor desired, this may be attained in multiple stages. For example, the initial TEC cools the CCD itself, while the next is utilized to cool the first TEC’s heat sink, and so on.

Trade-off: RGB systems provide imaging speed at a small cost, but with a much lower resolution than CIE-filter based colorimeters, and they have restricted application use.

Read noise

Read noise is the uncertainty present in the signal throughout the process of reading out the pixels.

This uncertainty happens because of multiple issues, but is dependant on the readout speed and the quality of the electronics: the faster the readout, the bigger the noise. The design trade-off for an imaging colorimeter is then between the cost of electrical systems, and the speed and accuracy of measurement.

Shot noise

The quantum nature of light results in the amount of photons gathered from a ‘constant’ output light source to display statistical differences over time. This uncertainty in signal level, known as shot noise, is the same as the square root of the number of photons contained in every pixel.

The signal to noise ratio is better if the number of photoelectrons collected is higher.

The highest number of photoelectrons that every CCD pixel can gather, its well depth, is directly related to the physical size of the pixel.

Larger pixels can contain more electrons, and generate lower noise images. Various CCDs have alternative pixel areas, with those having larger pixel areas traditionally being more costly, but also providing enhanced performance.

For example, for low light applications, a CCD-based system’s larger pixel size is desired over CMOS systems that have small pixels.

Pattern noise

Pattern noise is the outcome of imperfections in pixels (slight variations in the specific brightness of each pixel) that becomes apparent throughout longer exposure shots, imparting a pattern on the image.

If repeatable, pattern noise can be called ‘Fixed Pattern Noise’ (FPN). The FPN created by non-uniform pixels normally can be calibrated out of CCDs utilizing flat-field or different calibration methods.

One reason that Radiant has not yet added CMOS sensors to its range of imaging colorimeters is that temporal noise and FPN are both higher with CMOS, reducing the present capabilities of performance.

Trade-off: Electrical system cost and measurement speed versus measurement accuracy.

Trade-off: Performance and speed (larger pixel area) versus cost.

Luminance Calibration

To analyze luminance, imaging colorimeters utilize internal filters, produced to ensure that the complete spectral response closely matches that of the human eye.

The ProMetric I-Series colorimeters and any lenses offered with it are factory calibrated across all potential distances and two distinct aperture settings.

This factory calibration means that measurements can be taken at any distance with those apertures, with confidence that the measurement is properly calibrated.

The colorimeter can instantly supply the proper flat-field calibration data at the working distance specified, offering more accuracy and flexibility. To get the most precise luminance measurements for the Device Under Test (DUT), it can be recalibrated under the particular lighting conditions.

Color Accuracy

A CCD is sensitive to electromagnetic radiation in the range of around 300 nm to 1080 nm. The acquisition of color images needs the incoming light to be filtered so that only the desired wavelengths reach the surface of the CCD surface, and the component images must also be aggregated into a whole color image.

There are multiple ways of mechanically accomplishing this, each of which essentially entails capturing single red (x-bar), green (y-bar), and blue (z-bar) filtered images.

For effective color accuracy, the filters must provide a very strong match to CIE response curves. Utilizing only a green filter, photopic measurements are a subset of color measurements.

The CIE XYZ color space represents all colors visible to the human eye.

Figure 1. The CIE XYZ color space represents all colors visible to the human eye.

On-detector filters

Another popular method is to put a rectangular array of red, green, and blue color filters directly onto an interline-transfer CCD’s surface in what is called a Bayer pattern. This is highly similar to the technique employed in consumer digital cameras.

The benefits of this technique are that it is inexpensive, there is no additional hardware to be purchased, and no moving parts are required. These benefits are outweighed by multiple limitations for the majority of imaging colorimetry applications.

As the color filters are located across the CCD, the amount of pixels gathering information for each specific color is only a fraction of all the pixels on the CCD. This decreases the effective resolution of the image, making it harder to record fine details.

The interline-transfer CCDs commonly employed in this technique have small pixels, which restricts the dynamic range of the detector and reduces the signal to noise ratio.

The accuracy of color is also compromised as the filter technology available cannot offer an accurate match to the CIE spectral response curves.

Moving filter wheel

An additional method is the use of a motorized filter wheel. By sequentially spinning the different color filters into place in front of the CCD, the requisite series of component images can be recorded to offer a whole color image.

In this system, a mechanical or electronic shutter is required to stop light from reaching the detector between the exposures.

Trade-off: Using a filter wheel enhances complexity, but provides greater stability and precision, and the choice to use NIR and other specialized filters are viable.

CIE-matched color filter (left) and RGB filter (right) configurations.

Figure 2. CIE-matched color filter (left) and RGB filter (right) configurations.

Use of a filter wheel can deliver very high-dynamic-range, low-noise, high-spatial resolution, and high-fill-factor color images when paired with high-performance CCDs.

This method utilizes different filters for red, green, and blue, making it possible to attain a very strong match to the individual CIE color curves.

As light travels through the filter at different angles of incidence, it is also critical that the filters have minimal variations in performance over the range of incidence angles.

Due to this, absorptive filters allow more precision than thin film filters. Employing a filter wheel has multiple benefits over alternative techniques. First, it is the most stable and precise filtering technique available at an affordable cost.

Second, the method can be easily tailored to use in other specialized wavelength filters, for example NIR filters. Realizing these benefits depend on accurate mechanical calibration and design, introducing some cost and complexity to the imaging colorimeter.

Dynamic Range

Dynamic range is a further critical aspect of colorimeter performance. Dynamic range entails the range of values from lightest to darkest (number of shades of gray) that an imaging system can discern.

For CCD-based measurements, dynamic range is frequently analyzed in decibels (dB) to outline the ratio of maximum possible signal level to read noise level.

High Dynamic Range (HDR) measurement combines several images at various exposure times to offer information for lower light level areas in a measurement, without missing information on high light level areas because of saturation of the CCD.

The dynamic range of the Radiant ProMetric colorimeters is what allows them to meet the visual perception of the human eye for the most precise measurement.

How are all of these Design Choices Integrated into an Optimal Imaging Colorimeter?

The table below outlines the sources of measurement error for an imaging colorimeter. This details all of the challenges raised earlier and adds a few additional, esoteric, but critical, factors as well.

Radiant Vision Systems imaging colorimeters have been specifically created to provide solutions to the errors listed. For example, they have a larger pixel size to assist counter shot noise, factory calibration to reduce pattern noise (non-uniformity), an ND filter wheel, and a CIE-matched color filter wheel.

These design specifications mean that the ProMetric family is the best range of imaging colorimeters for applications that need a high level of precision and accuracy to match human visual perception.

For high-value devices and products where customer requirements are similarly high, there is no substitute for a ProMetric Colorimeter.

  Source of Error Countermeasure
Electronic CCD Noise - Photon (Shot) Noise Use larger pixel size - or “binned” measurements
CCD Noise - Dark (thermal) Noise Cool camera, dark image subtraction
CCD Noise - Read Noise State-of-the-art camera electronics
CCD Pixel - Non-uniformity Flat-field calibration
CCD Nonlinearity Measure and correct in software
Optical Lens Vignetting and Cosine Falloff Flat-field calibration
Spectral Response CIE-matched color filter wheel, color calibration
Light Scattering Software-based stray light correction
Lens Distortion Software-based lens distortion correction
View Angle Software correction / Multiple-angle data
Screen Effects (Illuminance measurements only) Illuminance flat-field calibration
Image Off-Axis Distortion (Keystoning) Geometric software correction

 

Applications

No single technology is the best solution for all measuring requirements. CCD-based instruments are the most successful in applications that involve the following criteria. Anywhere that human visual perception is the standard of quality, CCD imaging colorimeters are essentially the ideal solution.

  • Identifying defects (uniformity and pixel)
  • Measuring uniformity
  • The rapid collection of several data points
  • Measuring multiple spots (for example LED arrays)
  • Determining distortion, focus quality, and dimensions
  • Carrying out advanced analysis

ccd-based instruments

imaging colorimeters

Advantages of the ProMetric Family of Imaging Colorimeters

The ProMetric I-Series and Y-Series families provide several design benefits that assist in taking accurate measurements at high speeds:

1) Electronically controlled and interchangeable lens. Electronically controlled lenses are given with calibration data that produces highly accurate measurements for a varied range of settings.

Unlike a manual lens, which is normally supplied with calibration data for one working distance, Radiant’s imaging colorimeters are all factory calibrated. The first selection of focal distance and aperture is simple and fast.

If the user specifies several conditions of measurement, these are simple to access. Interchangeable lenses provide the imaging colorimeter’s field-of-view to be selectively changed to enhance the working distance to the imaged object.

1. Lens mount
2. Color filters
3. ND filters
4. Cooled CCD
5. Control electronics

2) CIE-matched color filters. This technology has been outlined above. The filter wheel can help up to six filters.

3) ND filter wheel. For light sources and displays that are especially bright, the light intensity can sometimes saturate or ‘wash out’, image areas acquired by the CCD, particularly for long exposure times.

It is crucial to regulate the light reaching the CCD to enable adequate exposure time for sufficient discrimination of color and luminance.

To perform this, Radiant has introduced a Neutral Density (ND) filter wheel to its ProMetric models. The ND filter functions as ‘sunglasses’ for the CCD. By default ND0, ND1, and ND2 filters are normally provided. The wheel has six positions, so further ND filters can be introduced if required.

4) Cooled CCD with built-in electronic shutter. The CCD is thermoelectrically cooled to enhance grayscale resolution and to decrease thermal noise.

The interline CCDs in the ProMetric I and Y achieve superior repeatability with an electronic shutter that provides High Dynamic Range (HDR) image acquisition. HDR mode enables detail from both dark and light regions to be captured.

5) Distributed control electronics. The main electronics board in an imaging colorimeter offers important communications and control, ProMetric’s electronic controls are created to provide years of trustworthy operation.

So, Which Imaging Colorimeter is Right for My Application?

Radiant Vision Systems imaging colorimeters and photometers are founded on scientific grade interline CCDs to give customers selections between factors for example accuracy, measurement speed, dynamic range, and pixel resolution.

ProMetric I

These high-performance imaging colorimeters are engineered to meet the most difficult requirements in a manufacturing environment or an engineering laboratory.

ProMetric I utilizes a cooled interline CCD that offers quick measurements with a high resolution and a 61 dB dynamic range.

Binning 1 x 1 produces a 61 dB dynamic range. Every I-Series camera employs a scientific-grade CCD sensor that is thermoelectrically cooled to offer low-noise measurements that are repeatable and precise. The I-Series is available with four alternate CCD selections:

  • ProMetric I2 utilizes a 2 megapixel (MP) 1600 x 1200 CCD sensor
  • ProMetric I8 offers more resolution with an 8 MP 3320 x 2496 CCD sensor
  • ProMetric I16 provides an even higher resolution with a 16 MP 4920 x 3288 CCD sensor
  • ProMetric I29 brings the ultimate resolution with a 29 MP 6576 x 4384 CCD sensor

imaging colorimeters

These sensors are high resolution and can assist in performing very detailed scale measurements on a varied range of lighting devices, displays, and illuminated components. A multi-exposure High Dynamic Range mode produces precise measurements in low level in high-contrast situations.

ProMetric I comprises of the industry leading Smart Technology™, which assists electronically controlled lenses (24, 35, 50, 100 and 200 mm), along with having an LCD touchscreen interface, which enables the capture and set up of measurements right on the imaging colorimeter.

ProMetric Y

The high-performance ProMetric Y Imaging Photometers utilize cooled interline CCDs to provide quick measurements with a high resolution and a 61 dB dynamic range. Rugged and compact, they are designed for use in production line environments. Three CCD selections are provided:

  • The ProMetric Y2 employs a 2 megapixel 1600 x 1200 CCD sensor to offer repeatable and accurate measurements
  • The ProMetric Y16 utilizes a 16 MP 4920 x 3288 CCD sensor for additional resolution
  • The ProMetric Y29 brings an even greater resolution with a 29 MP 6576 x 4384 CCD sensor
  • The ProMetric Y43 offers the ultimate resolution with a 43 MP 8040 x 5360 CCD sensor

high-resolution sensors

These high-resolution sensors allow for accurate measurements on a varied range of surfaces, backlit symbols, illuminated components, and displays.

ProMetric Y cameras also integrate Smart Technology™, including lenses that are controlled electronically to assist in the automatic image calibration of a range of working distances and apertures.

An electronic shutter provides superb long-term reliability and high measurement speeds. Several lens selections (24, 35, 50, 100 and 200 mm) are available. Both radiometric and photopic models are provided.

This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.

For more information on this source, please visit Radiant Vision Systems.

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