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Imaging colorimeters give advanced options

05 Apr 2007

CCD-based instruments can provide accurate measurement of spatial colour and luminance information for displays, instrument panels, light sources and luminaires. Sean Skelley, Doug Kreysar and Kevin Chittim weigh up the options when buying imaging colorimeters.

The market for light-emitting products has expanded tremendously over the past decade to include flat-panel displays (FPDs) and high-brightness LEDs for automotive applications. As consumers of these high-volume devices become more discerning about product quality, there is an increasing need for accurate, high-speed metrology equipment to support their development and production. Instrumentation for spatially resolved measurements of colour and luminance is particularly important. This article reviews the basic operating principles and performance trade-offs of one such class of instrument – imaging colorimeters.

The need for speed
For many types of FPD, human visual inspection is by far the most common method to monitor production quality. Specifically, units are removed from the production line and inspected for colour and brightness uniformity, and defects such as dead pixels, for example. The primary drawback of this manual-inspection approach is speed, since it cannot be integrated directly into the assembly line. Human inspection is subjective and can easily vary from operator to operator. This non-quantitative approach also makes it difficult to enforce standards throughout the component vendor supply chain.

A similar speed issue applies to LED manufacture. The final performance of an LED-based illumination system depends on the exact angular and colour output characteristics of the source. However, LEDs are made in high volume so any such characterization must be accomplished quickly and add little incremental cost.

Metrology instruments for both of these applications have been available for many years, albeit with limited capabilities. An established method is to mount a spot photo-meter or colorimeter onto a two-axis goniometer and sweep the measurement head through various arcs to obtain colour and luminance data at a number of viewing angles. This technique is accurate but slow and therefore used to collect data only from a small number of points on the source over a limited range of angles. This makes it possible to miss significant features or defects in the product's output.

Imaging colorimeter basics
Imaging colorimeters were developed specifically to address the limitations of prior instrumentation. The main components of an imaging colorimeter are: an imaging lens; a set of colour filters; a charge-coupled device (CCD) detector; and data acquisition and image-processing hardware/software (see figure 1). Some instruments may include neutral-density filters and a mechanical shutter.

The system acquires an image of the device under test through each of the various colour filters. This is then processed using previously determined calibrations to deliver accurate data about the source's colour and luminance as a function of spatial location. For example, in a single measurement, an imaging colorimeter can measure the luminance and colour of every pixel in a display simultaneously at a given viewing angle. The instrument can also be mounted on a goniometer to automate these measurements at different viewing angles.

Because an imaging colorimeter simultaneously samples multiple points from a source, it is inherently faster than techniques based on spot measurements. In addition, simultaneously measuring the entire surface of a source or display makes the instrument useful for gauging colour and luminance uniformity, as well as identifying very small defects. An imaging colorimeter can even assess display characteristics, such as distortion and focus quality. The ability to render a processed image of the display also can help to reveal subtle features for qualitative analysis by a human operator.

Current commercially available imaging colorimeters come in a variety of configurations. In terms of hardware, the most important differentiators are the type of CCD and colour filters. Understanding how these components impact on performance is essential when specifying an imaging colorimeter.

CCD choices
Virtually all imaging colorimeters are built using one of three types of CCD: full frame; interline transfer; or interline transfer with integrated colour filters (see figure 2). A full-frame CCD is an unobstructed rectangular array of detectors. Electronic charge accumulates in each pixel when light strikes the CCD. Readout occurs by sequentially shifting each row of pixels down into a read-out register until the entire array is cleared. An external shutter prevents light from reaching the CCD during the read-out cycle to avoid image smearing.

In an interline-transfer CCD, alternate columns are masked with an opaque layer. During read out, the accumulated charge in each exposed column is rapidly transferred laterally to a non-imaging column where the charge is shifted into the read-out register. This allows read out to occur while another exposure is being acquired.

Both of these CCD types respond only to incident luminance and cannot measure colour directly. To measure colour, separate exposures must be made through red, green and blue colour filters. However, a variant of the interline-transfer architecture has each pixel overlaid with either a red, green or blue colour filter specifically for direct colour measurement.

There are several important CCD-dependent performance characteristics to be aware of, starting with fill factor. When an image is formed on a full-frame CCD, virtually none of the information is lost. On the other hand, a significant percentage of the surface of an interline-transfer CCD is opaque, so image features that fall on this blocked area will not be seen. In practice, interline-transfer CCDs often use a microlens array to focus some of the light that would normally be blocked into the active area. This can increase fill-factor efficiency by up to 70%. The fill-factor problem is further exacerbated by the presence of integrated colour filters. Figure 3 shows how missing data can cause erroneous results when the output of a single LED is focused onto a 5 x 5 pixel area of the CCD.

Another major difference between CCD types is their dynamic range. The definition of a CCD's dynamic range is the maximum capacity of each pixel in electrons (called the full-well capacity) divided by the RMS dark noise (the number of electrons read from the device with no input light). Full-well capacity increases with pixel size. Full-frame CCDs typically have larger pixels, with full-well capacities of between 200,000 and 700,000 electrons, making dynamic ranges of 14 (16384:1) to 16 bit (65536:1) possible. In contrast, the full-well capacity of most interline-transfer CCDs is in the 10,000–20,000 electrons range, resulting in a dynamic range of 12 bit (4096:1) or less.

There are also several practical differences between CCD types. A full-frame CCD requires an external shutter and active cooling to minimize noise while an interline CCD does not. These features add to system size, weight, cost and complexity. An interline-transfer CCD can be read out faster than a full-frame CCD, which could be a consideration in high-speed production applications.

Calibrated colour
Correlating the colour response of a CCD camera to the human visual system requires measurements to be made in a calibrated colour space, such as the CIE format. This means matching the overall system response to the corresponding CIE colour curves using colour filters. Very close matching is possible by designing external glass filters. Unfortunately, the RGB colour filters typically integrated onto interline-transfer CCDs do not provide a close match to the CIE curves. The resultant colour accuracy is highly dependent on the nature of the source being measured and how the system is calibrated. Accuracy using mismatched filters is usually substantially worse when measuring narrowband sources, such as LEDs. Figure 4 shows an example of the spectral mismatch of a colorimeter using integrated filters with the CIE colour curves.

Choosing the right hardware
In commercial systems, full-frame CCDs are usually paired with CIE-matched colour filters, while interline-transfer CCDs are mated with either CIE-matched filters or on-chip filters. The full-frame systems are typically the most expensive and the slowest, however they deliver superior dynamic range (up to 16 bit), signal-to-noise ratio and colour accuracy. They are well suited to high-contrast applications such as automotive headlamp evaluation or high-end projector contrast testing.

Interline-transfer cameras with integrated colour filters offer low cost and high speed but limited dynamic range (usually 8–10 bit) and colour accuracy. This makes them a better choice for many on-line or at-line production inspection tasks. However, their limited fill factor means that they may not be optimum for examining small-scale features, such as pixel and sub-pixel defects.

Interline-transfer systems with CIE-matched filters offer a good compromise between cost and performance. With a typical dynamic range of 12 bit and excellent colour accuracy, they deliver enough performance for production testing of FPDs, backlights, projection systems and instrument panels, and are suitable for use with LEDs and other narrowband sources.

While hardware choice is important, the accuracy of any imaging colorimeter is only as good as its calibration. Most manufacturers' systems leave the factory fully calibrated but the ease with which the user can perform periodic recalibration can be a major differentiator in long-term system performance and utility.

Typical calibrations include flat fielding, luminance scaling and colour calibration. For instance, allowing users to create on-site colour calibrations when measuring specific spectra can increase the colour accuracy of the system. Prospective purchasers of imaging colorimeters should definitely assess what hardware and software tools are available for a particular system.

In conclusion, imaging colorimetry instrumentation has developed over the past few years to meet the evolving needs of manufacturers of displays and other light sources. These quantitative metrology tools will play an increasingly critical role in display production as volumes and consumer expectations increase.

• This article originally appeared in the March 2007 issue of Optics & Laser Europe magazine.

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