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Illumination & Displays

High flux LEDs enable novel light sources for night vision

The first flashlight ever designed for seeing at night preserves the dark adaptation and peripheral vision of the eyes.
24 June 2007, SPIE Newsroom. DOI: 10.1117/2.1200706.0760

Under well-lit conditions, photopic vision, mediated by cone cells, allows color perception by the human eye. In dim light however, the eye switches to scotopic vision, which, mediated by rod cells, does not allow color perception. Both forms of vision combine as mesopic vision under intermediate light conditions. Human night vision is a very complex phenomenon and the design of efficient light sources represents a very challenging field.

Here is an example of a common problem: while cross country skiing at night, there isn't quite enough light to maintain balance on uneven and somewhat featureless terrain. A solution is to use a head lamp to illuminate the near field. However, the spot of light on the snow is bright enough to destroy dark-adapted vision, thereby limiting the field of vision to the headlight-illuminated snow. Without a visible horizon, skiing is now unbalanced and cramped, like skiing in a tunnel.

A blue-green flood and yellow-red spotlight

To avoid this tunnel vision, white or blue-green hot spots must be eliminated. A single collimated high flux LED reflected off a rough surface 50 feet away can create a retinal illuminance sufficient to destroy dark adaptation. The viewer will see a hot spot afterimage—an image that stays with the eye even after it has stopped looking at an object—for a minute or more. Changing the hot spot to yellow or red can give a comparable photopic response from retinal cone cells, but with the benefit that the scotopic response of the rod cells is reduced by two orders of magnitude. This means that no yellow-red induced afterimages are seen, even for illuminated objects as close as three feet.

Combining the yellow-red spot with a blue-green floodlight avoids the four problems of the pure-red-light solution to the night-vision problem: poor peripheral vision, color blindness, color induction (in the form of green shadows), and poor acuity.

This technique is referred to as ‘retinal mapping’ and it involves placing the long wavelength illumination components on-axis in the central field of view where the cones are, and the short wavelength components in the periphery where the retinal rod cells are. This translates into a more efficient light—measured as the ratio of total retinal photo-current to flashlight current— that can preserve the users’ dark adaptation for virtually all illuminated object distances, angles, and reflectivities.

Structured beamforming optics and thermal LED design

Retinal mapping is just one of three areas of high-flux-LED driven innovation in the field of night vision. Efficient secondary optics can also shape the LED beam profile as a function of rod and cone number densities and of the dynamics of seeing. For instance, catadioptric collimating optics, which deflect and reflect light at the same time, have a beam divergence five times wider than the retinal cone cell field of view. At first sight, this looks like a gross mismatch between source divergence and target acceptance angle. But this would only be the case if the eye sat still, which it does not. The eight degree beam divergence covers the range of saccadic eye motion: the extremely fast, small, and jerky eye movements that redirect the line of sight and allow the eyes to fix on a still object as the head turns or moves. The blue-green floodlight is achieved with a catadioptric diffuser having a non-Lambertian emission with fall-off tailored so that an intensity 90° off-axis is high enough to see the next footfall but low enough to preserve the eye's dark adaptation, even on snow.

The multi-watt heat dissipation of high-flux LEDs and their need to run cool is stimulating innovation in light-source thermal design. An incandescent bulb runs efficiently when its hot filament is thermally isolated from the bulb socket and flashlight body. In contrast, LEDs require a low thermal resistance coupling to the metal body of the light. A good LED-to-flashlight thermal contact results in a perceptible warming of the flashlight head and barrel, which incidentally alleviates another problem of the night-time search: cold hands.

The mesopic flashlight

The human eye is sensitive to intensity variations over fourteen orders of magnitude. Night vision covers the middle five orders of magnitude between full moon reflectance on snow to starlight on dirt. In this mesopic regime, the whole retina is active, using both scotopic and photopic systems. The night vision mesopic flashlight keeps the eye firing on all cells without retreating to the photopic system by bleaching the rods (as a white light does) or feeding only the cones (as a red light does). Since the rods represent 95% of the light sensitive cells in the retina, it makes sense to use them.

Using mesopic vision is both a lesson in the physiology of vision, and a slowly dawning realization that the lack of blue or white hot spots allows a more ‘eyes wide open’ and less squinty type of vision as well as opening up the distant horizon. Holding such a flashlight horizontal allows walking as in daylight, i.e. using the peripheral vision to guide the next foot step. Illuminated objects from one to one hundred yards ahead are also visible without compromising the fully-dark-adapted visibility of the horizon, stars, and shadows.

While color visibility is good, color rendering however, is poor. This is characteristic of the dark-adapted eye. Where color rendering is critical, a white light should be used in the photopic or light-adapted regime. As the eyes move from room light into darkness, what was a dim blue ring around a distinct yellow spot now seems to blend (the colors becoming less apparent) while increasing in apparent brightness (Purkinje effect) and extending to the periphery. The dark-adapted eye does not strongly register a blue-to-yellow color change, but instead perceives a continuous brightness increase from the periphery towards the optical axis. The mesopic light seems to open up the night, making it more transparent, rather than closing it down into a tunnel vision. Strangely enough, this appears to be the first flashlight designed for seeing at night.

The Light Diagnostics mesopic light (patent pending) is currently undergoing psychophysical testing at the US Army night-vision research lab. Demo units are available to North American defense, search and rescue, and law enforcement agencies upon request.

Dan McGraw
Light Diagnostics, Inc.
Salt Lake City, UT

Dan McGraw, President of Light Diagnostics, Inc., is the inventor of the fluorescence comparator ring light.