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Illumination & Displays

Comparing vision performance testing techniques

A new active-matrix display technology shows promise in testing imaging systems such as short-wave infrared sensors.
25 October 2007, SPIE Newsroom. DOI: 10.1117/2.1200710.0831

Standard black-and-white printed targets, used for numerous vision-related experiments, are ideal for contrast and spectral uniformity in the visible and near-infrared (NIR) regions of the electromagnetic (EM) spectrum. However, they cannot be refreshed or updated, nor can they perform as real-time, dynamic stimuli. This reduces their usefulness to various techniques in standard vision performance measurement.

One technology used with dynamic stimuli methods is the back-illuminated active-matrix liquid-crystal display (LCD). Research using active displays to present stimuli during visual acuity measurements through night vision devices has aimed at improving the test's psychophysical rigor. However, emissive displays such as LCDs lack the spectral uniformity of printed targets, making them of debatable value for presenting test targets in the NIR and short-wave infrared (SWIR) regions. In addition, to test night vision goggles (NVGs), researchers place compatibility filters over their LCD monitors in order to eliminate unwanted infrared light. Unfortunately, this limits the test to the visible light region of the EM spectrum, a region in which the NVGs are not optimized. In addition, radiance emitted from a filtered LCD is in no way representative of natural night sky irradiance, which restricts the operational relevance of any conclusions to be drawn from this test.

A new option, only recently viable, is the active-matrix electrophoretic ink display (AMEPID), a dynamic, refreshable, and easily manipulated display that performs much like printed targets with respect to spectral uniformity (see Figure 1).

Figure 1. The microcapsule structure of the active-matrix electrophoretic ink display (AMEPID).

Reflective AMEPID technology makes it possible to view the display with NVGs or SWIR sensors in reflected ambient irradiance from 600–1700nm without the complications of LCDs. Since the device is reflective over a wide spectral region, the AMEPID target can be illuminated with a spectral distribution appropriate for the device under both test and relevant operational conditions. In addition, because the target is active, a researcher can implement rigorous psychophysical techniques, improving the quality of the results.

Our recent experiment employed a stepwise acuity procedure known as a psychophysical staircase. A standard LCD display and an AMEPID reflective display were used to display a tumbling “E” visual acuity target (see Figure 2). The opening in the “E” was randomly presented in one of four orientations (up, down, left, right). The subject's task was to input on a keypad the correct orientation. Acuity assessment was based on the subject's ability to correctly determine this orientation at progressively smaller sizes. Once an average minimum acuity level was attained, the computer recorded that level.

The difference between the mean values of visual acuity for the two target display types, as measured through test NVGs, was statistically significant (see Table). Various possible explanations include complications with the NVIS filter placed on the LCD target and a failure to set target contrast and radiance precisely. In future experiments, we intend to verify that contributions due to minor target variability are, in fact, insignificant.

Figure 2. Test stimuli on the liquid crystal display (LCD) and AMEPID displays viewed through the short-wave IR(SWIR) imager (left) and night-vision goggles (NVG)s (right).
Table 1. Visual acuity thresholds and standard deviations (Stdev) in milliradians averaged across all subjects.

Several conclusions may be drawn from this preliminary work. (For further reading please see the references.1–4) First, it is easy to see that LCD technology is inappropriate for testing SWIR sensor-based imaging systems. The total lack of contrast in the SWIR wavelength region renders them useless for this application. The AMEPID display, however, shows potential for use as an active matrix target for testing—using rigorous psychometric techniques—both traditional image-intensifier-tube-based night vision systems and systems that exploit the SWIR region. But the AMEPID display's limited contrast, small size, and comparatively slow response time indicate that improvements to enhance its utility as a test tool are needed. Furthermore, in testing night vision devices with the AMEPID display, the test must compensate for the non-optimal contrast and display refresh time.

Mathew Swinney
Air Force Research Lab (AFRL)
United States Air Force (USAF)
Wright-Patterson Air Force Base (WPAFB), OH

Mathew W. Swinney, 1st Lt., received his bachelor of science degree in applied physics from Angelo State University in 2005, and is a career research physicist at the AFRL, Human Effectiveness Directorate, Warfighter Interface Division, Battlespace Visualization Branch, WPAFB.