High-intensity light sources as optical countermeasures against human operators

Simulated test protocols are used to investigate the effectiveness of a novel and non-lethal approach for the degradation of human performance.
23 September 2014
Alexander Toet

Despite the advent of remotely operated and autonomous weapon systems, the requirement of human (direct) vision is still critical for the success many military tasks. There are, for instance, several weapon systems in which a human operator acquires, tracks, and designates a target. Successful piloting of aerial and surface vehicles also still depends on human vision. High-intensity light sources have the potential to generate a variety of disturbing visual effects and may therefore offer a simple and cost-effective means of defense. It is currently unknown, however, if their resultant disturbing effects are robust and powerful enough to seriously degrade human performance in practical settings.

Purchase SPIE Field Guide to IR Systems, Detectors and FPAsThe aim of using high-intensity light sources to suppress human activity is to temporarily obscure a large portion of a person's visual field of view with extremely bright light. Various laser dazzlers are currently available for use as non-lethal deterrence devices. These are commonly used to prevent or block insurgent attacks on convoys or ships, and at checkpoints. Experiences with these devices in military operations (when used within eye-safe limits), however, indicate that they have only limited success in the temporary incapacitation of opponents.

We have therefore developed a test protocol for the qualification of optical non-lethal weapons, and for the study of their immediate effects on intended (human) targets.1 Our test protocols were designed to involve a range of relevant military tasks (e.g., driving a vehicle, piloting an aircraft, as well as target search and detection) so that we could systematically investigate the capability of high-intensity light sources to disturb human vision and degrade task performance in realistic simulated scenarios.2–4 We exposed study participants to high-intensity light distributions with different spatiotemporal (i.e., flicker and scanning patterns) and chromatic (i.e., color flicker) structures, at irregular intervals in our simulations. We quantified the effectiveness of the optical countermeasures by assessing task performance, relative to a no-countermeasure baseline, and with subjective reports given by the participants on the levels of discomfort experienced.

For one of our tests, we conducted a simulated flight task that involved a pursuit flight combined with cognitively demanding secondary tasks. The participants of this simulation felt notably disrupted by the unpredictable high-intensity flash insults, and they experienced vivid afterimages for up to 15 minutes. Surprisingly, we found that the light insults had no effect on the accuracy of the primary and secondary task performances.2 In another test, we simulated a driving task. The participants were exposed continuously to high-intensity luminous flicker with various color combinations, temporal frequencies, and duty cycles.4 The results of this simulation showed that frequencies of about 15Hz were the most discomforting. We also found that chromatic flicker (alternating colors) was more discomforting to the participants than achromatic flicker (turning the same color on and off), with the exception of monochrome red flicker. The most discomforting temporal color combinations were red-green, red-blue, and red-black. In addition, our results show that arrhythmic chromatic flicker is more disruptive than the rhythmic equivalent, but that the opposite is true for achromatic and monochromatic flicker. We again found that the high-intensity lights had no effect on the driving performances of the participants. Some of the light insults, however, were rated as “unbearable.”

The results of a search task—performed with continuous exposure to high-intensity luminous flicker—showed that the mean search time increased significantly in the presence of high-intensity flicker.4 Exposure to achromatic flicker resulted in the longest search times and the highest levels of discomfort, closely followed by red-green chromatic flicker. We also conducted a tracking task under the same conditions. In this test, the largest disruptive effect—20% degradation in tracking performance—was caused by red-green chromatic flicker. Achromatic flicker and red-cyan chromatic flicker had no observed effect.

We have also derived a model that describes how the illumination of a windshield by a high-intensity light source affects driving behavior.3, 5 Our model is based on the assumption that drivers reduce their speed when their view is blocked by veiling glare from a bright light in their visual field (see Figure 1 and Figure 2). We developed an associated protocol to test this model and to assess how high-intensity light sources affect car drivers by inhibiting their view. From measurements of the luminous intensity of the light source, our model can be used to predict the maximum safe speed for normal driving. The model estimates can be directly related to operational requirements and therefore can be used to assess the operational effectiveness of high-intensity light sources.

Figure 1. Example simulation views from a driver's perspective. A high-intensity light source (a narrow-beam laser dazzler aimed at the car windshield) that causes minimal blinding in daylight (b) can seriously degrade vision in darkness (d). The undisturbed daylight (a) and dark (c) views are shown on the left.

Figure 2. Scenes from a night simulation of a driver's view with artificial illumination by a high-intensity light source (laser dazzler). (a) The beam was directed through the car windshield at the driver from a distance of about 15m and at an angle of about 40°. The driver's perspective is shown when the laser dazzler is in a (b) narrow-beam and (c) wide-beam mode.

We have designed and conducted simulation protocols to test the effectiveness of high-intensity light sources as a means of reducing human visibility and decreasing task performance. Our simulations show that the high-intensity light sources have few effects on certain critical tasks. Together with the limited window of deployment, the difficulty of pointing a beam directly at an adversary, and many simple solutions (e.g., closing the eyes, filters, looking away), our results suggest that high-intensity light sources are not robust countermeasures against human operators. Our future research will focus on stimulus characteristics and methods of delivery that can be used to optimize the effectiveness of these countermeasures.

Alexander Toet
Netherlands Organization for Applied Scientific Research (TNO)
Soesterberg, The Netherlands

Alexander Toet works in the Perceptual and Cognitive Systems Department as a senior human factors and electro-optics scientist. His work focuses on multimodal image fusion, image quality, computational methods of human visual search and detection, laser eye safety, and the quantification of visual target distinctness. He is also a Fellow of SPIE.

1. A. Toet, Optical countermeasures against human operators, Proc. SPIE 9251, p. 9251-17, 2014. doi:10.1117/12.2066125
2. A. Toet, J. Walraven, J. W. A. M. Alferdinck, A. S. Wennemers, Effects of temporal blinding on pilot flight performance, Tech. Rep. TM-01-A058, TNO, 2001.
3. A. Toet, J. W. A. M. Alferdinck, Effects of high intensity light sources on vision through windscreens, Tech. Rep. TM-01-A058, TNO, 2011.
4. J. W. A. M. Alferdinck, J. J. Kriekaard, A. Toet, Assessment of chromatic flicker effects on human task performance, Tech. Rep. TNO-DV 2010 A104, TNO, 2010.
5. A. Toet, J. W. A. M. Alferdinck, Effects of high power illuminators on vision through windscreens and driving behavior, Proc. SPIE 8898, p. 88980I, 2013. doi:10.1117/12.2028224
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