116 TECHNOLOGY / LED Figure 5: Calculated time to achieve maximum exposure at a distance of 30cm.
observed in one child in 500. A study undertaken by Colman et al. suggested that repetitive behaviour can be aggravated by the flickering nature of fluorescent illumination, had interpretative problems and could not be replicated.
Effects of Light – Myalgic Encephalomyelitis (Chronic Fatigue Syndrome)
Chronic fatigue syndrome is one of several names given to a potentially debilitating disorder characterized by profound fatigue which lasts for at least six months. It has a prevalence that varies from 0.2% to above 2%. According to self-reporting, about 52,500 people in the UK (= 21% of myalgic encephalomyelitis) have increased sensitivity to light.
Flickering of Artificial Light Sources The dissemination and understanding of the effects of flickering of artificial light sources is in its infancy. However there are clearly some benefits for the lighting industry to take more notice especially with the opportunity afforded to it through the new technologies of LEDs and OLEDs. There are several aspects to flickering of the light output as follows: • Modulation Frequency
- Occurrences, or cycles of a periodic signal per unit of time
• Modulation amplitude, Modulation depth, depth of modulation - Peak-to-peak magnitude variation of a periodic signal, sometimes referenced to signal mean
• DC Component, DC Value, DC Offset, Offset
- Mean value of a periodic signal
• Flicker, flutter, shimmer - Light source modulation
- Not associated with any periodic signal metric
Figure 6 generated by Northwest National Laboratory defines the key parameters of flicker.
Interestingly the flicker of a light source is defined within a standard EN12464-1:2002 which states that flicker causes distraction and may give rise to physiological effects such as headaches. Stroboscopic effects can lead to dangerous situations by changing the perceived motion of rotating or reciprocating machinery. Lighting systems should be designed to avoid flicker and stroboscopic effects. It is noted within the
standard that this can usually be achieved for example by use of DC electrical supply for incandescent lamps, or by operating incandescent or discharge lamps at high frequencies (around 30 kHz). Unfortunately the majority of LED drivers used today have very poor flicker characteristics and a typical constant current LED driver ripple current may be seen in figure 7. Here a nominal 700mA LED driver actually provides 652mA current out with a peak to peak ripple current of 142mA or 21.8% with a ripple frequency of 100Hz (2x Mains frequency). They key aspect for designing or specifying LED products is to ensure that the LED drivers are not overdriving the required maximum forward current specified by the LED manufacturer. Typical measurements of LED ripple current have been seen up to about 85% of nominal current so this could be a major cause of stress on the LED and wire bonds for some manufacturers so make sure you check! Of course if your LED driver uses Pulse Width Modulation technology then your ripple current is actually 100% as the current through the LED varies between 0 and full current at a specific PWM frequency. If you require high resolution current control then unfortunately a major disadvantage with PWM is that the flicker frequency drops to only a few hundred Hertz at best. A recent flicker study by the Lighting Research Center in New York State tested how 10 humans detected and more importantly accepted flicker using both the flicker frequency and also the degree of percentage flicker with very interesting results as shown in figures 8 and 9. In figure 8, the lower the flicker frequency enabled the subjects to detect stroboscopic effects of the light source especially if the flicker percentage is high. For example, the majority (80-100%) of the subjects would be able to see stroboscopic effects of a
Figure 6: The parameters for defining flicker of a light source.
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