It is dark and wet, no streetlights. The windshield wipers are going. The road junction you are approaching has manifold merging traffic streams, and you need to change lanes for your exit. You check for vehicles right, left, ahead, behind. Some are already braking, and their rear light intensity is changing. Your memory map of the vehicles in front is in place, and with a final sweeping glance, you make your move. But suddenly, the bus has jumped position, and the truck seems to straddle two lanes. Another rapid look over the target area only confuses the picture further. Are you hallucinating? Are vehicles shape-shifting? This experience is not psychedelic but may lead to a close encounter with unidentified metallic objects. You have been LED astray.

Remember those moments on a disco dance floor when the strobes repositioned the dancers like puppets? Your body might have been dancing in one reality, but your mind might have been in quite another. If your head moves or your eyes sweep the scene, or if you blink at a certain point in the cycle, a light that is in one trajectory this moment is in another the next. Being entertained by feeling like we're somewhere else on a dance floor is one thing, but traveling on dark wet roads in speeding chunks of steel is another. It is an undesirable experience.

I am not talking about the LED lights of cyclists, flashing away two or three times a second. Flashing bike lights do not appear continuous, and the brain treats them accordingly, whereas the pulse-modulated LED-based rear and brake lights of vehicles seem to be continuous but break up with movement.

The first LEDs we saw on the roads in England were experimental lane boundary markers. Solar-charged during the day, they conserved battery power by way of a strange mark-space ratio with a tiny on period compared to its off period. The frequency was relatively high, so I have no doubt that, on the slab in the lab, the indicators appeared to be steady. Indeed, there are viewing conditions on the highway that render LEDs steady state, but when you move your head or eyes, the picture disintegrates as the pulse width modulation (PWM) is stretched into a series of dashes.

It is a shame that Morse code has just been officially discontinued — a missed opportunity to encode heady messages in lane markings. In case you suspect messages are already exuding from an LED array near you, there is a really cool Morse decoding tree at You could be deciphering the DIT, DAHs in a moment!

My first experience with light modulation was as a child, shaking my head from side to side while looking at the star Sirius. The twinkling of the star was transformed into a color-speckled band. Sirius is not a pulsar; its light does not have an off period, so the colored dashes form a continuous ribbon. To see this color modulation, try it at home, but do not allow the eyes to lock onto the target or the trick of the eyes will not work. The gaze must move with the head.

Before LEDs were available, I constructed devices to experiment with neon indicators whirling at high speeds using multi-way telephone jacks and sockets as the slip-ring assemblies. The jacks were not exactly rated for 240V. Even the 50Hz sine wave of the mains from the local power station was delicately displayed in dashed, coral pink hoops.

In our early color-changing tubes, the LEDs were analog driven, only resorting to pulses — albeit between pixels — when we wanted to create a special illusion, i.e., to display the red disk on white ground of the Japanese flag when the tube was waved or the head shaken (as in the Sirius experiment). For the record, this was years before the PWM wars, before the big LED companies in our industry were even glitter in the eye of the beholden (sic). The boy racers may claim that their pulse-drive frequencies are way beyond persistence of vision speeds. Therefore, the light should appear continuous. But if there is movement on the part of the source or viewer, there will be break-up, so there are conditions for which the steady-state drive of LEDs can be recommended. In LED rear lights of vehicles, the square wave of PWM renders absolute off periods, and depending on the frequency, the illusion of vehicles doing a deadly dance in space can distract the eyes and brain.

We are not home free even if the viewed and viewer are static. There are situations where the lightscape subtended by the macular — the sweet-spot of our vision — may appear constant to us while that witnessed by our peripheral vision is jumping. For example, mains frequency fluorescent tubes can be seen to flicker in the corner of our eye, but if we focus on them, they appear steady. Our peripheral vision does not seem to be subject to the same timescale as the macular view of our surroundings, or perhaps the effect is down to a different refresh/decay/pole rate of the different areas of the retina.

Questions regarding pulsed lights and their effect upon the eye and brain are relevant to the public debate surrounding radio cell phone technology and RFI. Few people recommend sleeping near a cell radio antenna. Many cell phone users even hold their phones as far as possible from their heads or switch from ear to ear during a call to reduce the build-up of a microwave hotspots in the brain. Since the new Tecra digital radio format has been introduced to the UK police force, manifold complaints have arisen among users who feel nauseous just carrying the handheld radios. As with radio technology, pulsed light technology may be producing a range of unseen side effects, as well as the visible ones like the disorientation on the highway.

With the sharp rise in output of LED sources, work is being done to establish safe naked-eye exposure levels to certain sources. Perhaps there should also be discussion of possible ill effects of exposure to modulated sources. Beat-frequency oscillations may arise from multiple sources pulsing at different clock speeds, certain relationships may overlap to create a sort of subliminal moiré, and harmonics may effectively transpose the action of high frequency down to the human response spectrum.

Would it be prudent to devise tests to establish the disorienting effects of pulsed sources, particularly on the highway, before PWM LED vehicle lighting becomes so ubiquitous that there is no going back?

Peter Wynne Willson is co-founder with Tony Gottelier of WWG, which seeks to innovate theatrical, entertainment, and architectural lighting. (