Blade Flicker, Shadow Flicker, Glint: Potential Hazards of Wind Turbines
Dr David McBride & Mr Bruce Rapley
Introduction
Any moving object that passes between a light source and an observer has the potential to cause flicker—a repeating cycle of changing light intensity for the observer. In this context, flicker relates to the perception of fluctuating brightness at frequencies lower than those covered by persistence of vision. Persistence of vision is the retention of an image on the retina of the eye after the optical excitation is ended. This is very important in cinematography that presents a series of very brief images in quick succession on the screen that the eye interprets as smooth, flowing motion, rather than a series of jerky still images. Old movies that were shot at a very slow speed display this flicker. Just think about seeing an old Charlie Chaplin movie to recall observing this effect. Modern movies are shot at 25 frames per second that is faster than the eye can detect the flow of images as a series of individual frames.
People will notice flicker at frequencies usually less than 50 Hz, although this varies with intensity. Above 50 Hz, the brain’s response to the flicker lasts longer than the flicker itself and the persistence of vision takes over and converts the flicker into a continuous image.
It is known that flicker frequencies between 10 to 25 Hz can cause problems such as eyestrain, headaches, nausea and seizures. The latter effect will be covered in more detail in the section on photosensitive epilepsy.
There are many sources of flicker in everyday life, Table 1.
For wind turbines, the rotating blades passing in front of the sun from the observer’s perspective will cause a flashing series of light/dark images to pass across the eye (retina). This is referred to as Blade Flicker. If the observer is looking at the ground or another object, such as a building, the shadows of the rotating blades will cause reflected images to pass across the retina. This is referred to as Shadow Flicker.
The blades of wind turbines are generally white and as such can reflect the sun, like a mirror, and this effect is called Glint. As the blades are often rotating, an observer could potentially observe flicker, shadow flicker and glint almost simultaneously, depending on the exact angle of their vision. The question is: are these phenomena annoying or dangerous? To answer this it is necessary to examine three potential areas: photosensitive epilepsy, visual distraction and annoyance...
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