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Tech Papers

Guide to CCD-Based Imaging Colorimeters

Radiant Vision Systems

How to Choose the Best Imaging Colorimeter
With multiple options for CCD, filters and lenses, the Radiant Vision Systems ProMetric® Imaging Colorimeters and Photometers are available in more than 100 configurations. With this flexibility, an imaging colorimeter or imaging photometer can be readily optimized for any measurement application.

The selection of the CCD is the single most important factor affecting the overall imaging colorimeter system performance. This choice will determine pixel resolution and bit depth, as well as – in conjunction with lens selection – achievable viewing angles. The selection of a CCD, and the engineering considerations critical to its use in imaging colorimeters, is described in the next section of this paper.

Closely related to the selection of the CCD is the design of the control electronics. The main requirements are that the control electronics minimize electronic noise and support electrical and optical calibrations.

Filter selection is usually a choice between standard configurations for either photometric or colorimetric operation. Depending on the measurement application, further options are available for enhanced color measurement performance, radiometric measurement, and Near-IR measurement. There are a number of filter technology options available, including internal, external, and on-CCD filter arrays.

Finally, lenses are selected based on working distance and field-of-view requirements for the application. The optical geometries affecting lens selection will be constrained by the specific CCD used. In addition, if lenses are not correctly compensated for, they may introduce optical aberrations that affect the accuracy of the measurement.

Taken together, the various CCD, electronics, filter, and lens options provide great flexibility in determining the performance and cost of the imaging colorimeter.

Lens CloseupCCD Technology
CCDs (Charge Coupled Devices) are monolithic semiconductor detector arrays. Incident photons are absorbed by the detector material and create electron-hole pairs. During exposure, electrons accumulate in each individual detector element, called a pixel, where they are held until the charge is read out. The total amount of charge that accumulates in each pixel is linearly proportional to the amount of light incident upon it.

There are a few basic forms of CCD architecture, with many variations for specific applications. Colorimeters are generally available with full frame or interline transfer CCDs. These two CCD technologies have varying strengths and weaknesses that determine the measurement applications for which they are most appropriate. Radiant Vision Systems ProMetric Imaging Colorimeters are manufactured with interlinetransfer CCDs. Radiant Vision Systems uses scientific grade CCDs that offer particularly high performance in terms of low noise characteristics and restrictions on the maximum allowable number of dead pixels.

Full Frame CCD Interline Transfer Compare

Full-Frame CCDs
In a full-frame CCD, the entire detector surface is first exposed to light and then the light must be externally blocked off so that readout can occur without further charge accumulation. The name “full frame” comes from the fact that the entire surface of the CCD is exposed to the light source. In addition, in a CCD sensor, in contrast to a CMOS sensor, the active surface area of the CCD is virtually 100% - the boundaries between pixels have negligible area.

Capturing the image stored in a full-frame CCD is a straightforward process of sequentially reading out the individual pixel data. This is accomplished by first shifting the charge in each detector column one row towards the serial register. The last row of charge goes into the serial register itself; this charge is then shifted one pixel at a time out of the serial register to produce an analog signal. This process is repeated until all the accumulated charge has been read from the detector. This readout process is generally regulated to minimize readout noise.

In addition, full-frame CCDs offer slightly better performance for dynamic range, which is the number of gray levels that can be resolved by the CCD. Full-frame CCDs have 72 dB dynamic range, which is equivalent to 16,384 gray levels. With these performance advantages come some requirements on the physical architecture of the Imaging Colorimeter. Because full-frame CCDs require an external means of controlling exposure, typically a mechanical, rather than an electronic, shutter must be used. This increases the mechanical complexity of the system as well as its size and weight. The shutter also introduces a speed penalty.

Interline-Transfer CCDs
On an interline-transfer CCD, alternate columns of the detector array are masked off with an opaque material. To read out the image, the charge in each column of “light” (or illuminated) pixels is shifted, simultaneously, to the adjacent “dark” column that is blocked off from light. The stored image data can then be read out in the same way as described previously for full-frame CCDs. However, new image information can be accumulated in the open “light” pixels while this is occurring. This has the effect of allowing relatively faster exposure times and allows exposure time to be controlled electronically.

Interline-transfer CCDs are preferred for high-speed measurement applications because the image capture and readout processes can occur simultaneously. A trade-off of the interline-transfer CCD architecture is that since part of the sensor surface is opaque; some portion of the incident light is not captured. This can lower the quantum efficiency of the detector and reduce the spatial resolution. Therefore, the interline-transfer CCD may fail to resolve small image features. However, this effect can be offset by using modern high resolution CCD sensors of up to 29 mp combined with 2 x 2 binning of the pixel measurements.

Noise Reduction
Noise limits the radiometric accuracy of CCD images. The primary noise sources in CCD detectors are thermal noise, read noise, and shot noise.

Thermal Noise
In addition to free electrons created by incident photons (called photoelectrons), thermal effects can also create free electrons. Image noise from this thermally created charge becomes particularly problematic at high temperatures, or during long exposures. Since thermal noise is extremely temperature dependent (every 6°C drop in temperature reduces it by a factor of two), cooling a CCD lowers this noise floor dramatically, and enables longer image integration times. Cooling is often used in low intensity or high dynamic range applications.

The most common cooling method for commercial CCD imagers is thermoelectric cooling (TEC). Depending upon the desired noise floor, this may be accomplished in several stages. For example, the first TEC cools the CCD itself, while the next is used to cool the first TEC’s heat sink. This process can be extended.

Read Noise
Read noise is the uncertainty introduced into the signal during the process of reading out the pixels. This uncertainty occurs due to several factors, but the primary factor that influences this is the readout speed: the faster the readout, the greater the noise. The resulting design trade-off for an imaging colorimeter will then be between measurement speed and measurement accuracy.

Shot Noise
The quantum nature of light causes the number of photons collected from a “constant” output light source to exhibit a statistical variation over time. This uncertainty in signal level, called shot noise, is equal to the square root of the number of photons collected in each pixel. Therefore, the higher the number of photoelectrons collected, the better the signal to noise ratio.

The maximum number of photoelectrons that each CCD pixel can accumulate – its well depth – is directly related to the pixel’s physical size. Larger pixels can hold more electrons, and thus produce lower noise images.

Different CCDs have different pixel areas, with those having larger pixel areas generally being more expensive. This results in a cost / performance trade-off when selecting an imaging colorimeter.

Color Accuracy
A CCD itself is not wavelength selective. Therefore, capturing color images requires filtering the incoming light so that only the desired wavelengths reach the CCD surface and then aggregating the component images into a complete color image.

There are several ways to mechanically accomplish this, each of which basically involves capturing individual red, green, and blue filtered images. For good color accuracy the filters must deliver a very good match to CIE response curves.

Photopic measurements are a subset of color measurements, using only a green filter.

Moving Filter Wheel
One approach is to use a motorized filter wheel. By sequentially spinning the various color filters into place in front of the CCD, the requisite series of component images can be captured to provide a complete color image. In this scheme an electronic or mechanical shutter is necessary to block light from the detector between the exposures. This approach can deliver very high dynamic range, low noise, high spatial resolution, and high fill factor color images when coupled with high-performance CCDs.

Filter Wheel

Because this approach uses individual filters for red, green, and blue, it is possible to achieve a very good match to the individual CIE color curves. Because of varying angles of incidence of light passing through the filter, it is also important that the filters have minimal change in performance over the range of incidence angles. For this reason, absorptive filters are used in place of thin film filters.

Using a filter wheel has several advantages over other methods. First, it is the most precise and stable filtering method available at reasonable cost. Second, the approach can be easily extended to use other specialized wavelength filters, such as NIR filters. Realizing these advantages requires precise mechanical design and calibration, adding some complexity and cost to the imaging colorimeter.

Detector FiltersOn-Detector Filters
Another common approach is to place a rectangular array of red, green and blue color filters directly onto the surface of an interline-transfer CCD in what is known as a Bayer pattern. This is very similar to method used in consumer digital cameras. The advantages of this method are low cost, the lack of additional hardware, and the lack of moving parts.

These advantages are outweighed by several disadvantages for most imaging colorimetry applications. First, because the color filters are spread across the CCD, the number of pixels capturing information for each individual color is only a fraction of the total pixels on the CCD, so reducing the effective resolution of the image, making it more difficult to image small details. Next, the interline-transfer CCDs typically used in this approach have small pixels, which limits detector dynamic range and decreases the signal to noise ratio. Finally, color accuracy is compromised because the filter technology available here is not able to provide as accurate a match to the CIE spectral response curves.

How are all of these design choices integrated into an optimal imaging colorimeter?

The table below summarizes the sources of measurement error for an imaging colorimeter. This includes all of the issues raised earlier and includes a few additional, esoteric but important, factors as well.

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 Better camera electronics / Slower readout
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 filter wheel, correction matrices
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) Flat-field calibration
Image Off-Axis Distortion (Keystoning) Geometric software correction

Radiant Vision Systems Imaging Colorimeters have been designed specifically to address the points raised above.

1) Electronically-controlled and interchangeable lenses. Electronically controlled lenses are provided with calibration data that provides very accurate measurements for a wide range of settings. This is in contrast to a Manual Lens, which is typically provided wtih calibration data for just 1 or 2 working distances and 1 or 2 aperture settings. With Radiant’s Imaging Colorimeters, initial selection of focal distance and aperture is quick and easy. If the user defines multiple measurement conditions, these are easily accessible. Interchangeable lenses allow the Imaging Colorimeter’s field-of-view to be selectively modified to optimize the working distance to the object being imaged.

2) CIE matched color filters. This technology has been described above. The filter wheel supports up to six filters. Additional filters can
be used to further improve the accuracy of the color measruement, or to allow radiometric measurements.

3) ND filter wheel. For displays and bright light sources, it is important to regulate the light reaching the CCD to allow sufficient exposure time for adequate discrimination of luminance and color. This is done by adding an ND filter wheel. By default ND0, ND1, and ND2 filters are usually provided. Again the wheel has six positions, so additional ND filters can be added if needed.

4) Cooled CCD with built-in electronic shutter. To improve gray scale resolution and to reduce thermal noise, the CCD is cooled. The interline CCDs in the ProMetric I and Y attain superior repeatability with an electronic shtter. This enable High Dynamic Range (HDR) image acquisition. HDR mode provides actionable detail from both light and dark regions.

5) Distributed control electronics. The main electronics board in the imaging colorimeter provides the essential control and communications. These are designed for years of reliable operation.

G cutaway

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

So, which imaging colorimeter is right for my application?

Radiant Vision Systems Imaging Colorimeters and Photometers are based on interline CCDs to provide choices between measurement speed, accuracy, pixel resolution and dynamic range.

ProMetric I
These high-performance imaging colorimeters are designed for the most challenging requirements in an engineering lab or manufacturing environment. ProMetric I uses a cooled interline CCD that provides fast measurements with high resolution and 61 dB dynamic range. Binning 2 x 2 yields 73 dB dynamic range. It is available with four different CCD choices. The ProMetric I2 uses a scientific grade, 2 megapixel 1600 x 1200 CCD sensor that is thermoelectrically cooled to provide low-noise measurements that are accurate and repeatable. The ProMetric I8 provides more resolution with an 8 megapixel 3320 x 2496 CCD sensor. ProMetric I16 provides even more resolution with a 16 megapixel 4920 x 3288 CCD sensor. ProMetric I29 provides the ultimate resolution of a 29 megapixel 6576 x 4384 CCD sensor. These high-resolution sensors enable very fine scale measurements on a wide range of flat panel, illuminated keyboard, and lighting devices. A multi-exposure High Dynamic Range mode addresses accurate measurements in low level on high contrast situations.

ProMetric I contains industry-first Smart Technology™, which supports Electronically-controlled lenses (24, 35, 50, 100 and 200mm) as well as an LCD touchscreen interface which allows measurement set up, capture and analysis, right on the imaging colorimeter.

ProMetric Y
The high-performance ProMetric Y Imaging Photometers use cooled interline CCDs to deliver fast measurements with high resolution and dynamic range. Compact and rugged, they are optimized for use in production line settings. Three CCD choices are offered. The ProMetric Y2 uses a 2 megapixel 1600 x 1200 CCD sensor to provide accurate and repeatable measurements. The ProMetric Y16 uses a 16 megapixel 4920 x 3288 CCD sensor to provide more resolution. The ProMetric Y29 provides even more resolution with a 29 megapixel 6576 x 4384 CCD sensor. These high-resolution sensors enable precise measurements on a wide range of displays, keyboards and surfaces. The Y-Series incorporates Smart Technology, including electronically-controlled lenses that support automatic image calibration at two specific apertures and any valid working distance. An electronic shutter delivers high measurement speed and excellent long-term reliability. Multiple lens options (24, 35, 50, 100 and 200 mm) are available. Both Photopic and Radiometric models are offered.

Radiant Vision Systems ProMetric® Imaging Photometers and Colorimeters utilize CCD sensors, and most models are available with a choice of different CCDs. In order to choose the right product configuration for a specific application, it is useful to have an understanding of what factors influence CCD-based imaging colorimeter performance.

 

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