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Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next PageABE FMaGSlighting | DIMMINGDimming: critical term definitionsMeasured vs. perceived light:Measured light output is the quantifi ablevalue of light measured by a light meteror similar device. This is the dimmingpercentage indicated on LED productspecifi cation sheets. Perceived light is theamount of light that your eye interprets dueto dilation. The eye’s pupil dilates at lowerlight levels, causing the amount of perceivedlight to be higher than measured (e.g. 20%measured = 45% perceived). The equationfor determining perceived light is to take thesquare root of the measured light percentage(e.g. = 0.447) — see fi gure.Pop-on:If the voltage at which a light source beginsoperation is higher than the voltage of thedimmer’s lowest setting, then the lights willinitially “pop-on”. This means there will be asection of the dimmer that will not turn onthe light until suddenly the light “pops-on.”Drop-out:If the voltage at which a light source stopsoperating is higher than the lowest settingon the dimmer, the light source will turn offbefore the dimmer has turned off. The lightswill effectively “drop-out” before the slideron the dimmer reaches the bottom.Flicker:Flicker is the unexpected modulation oflight level that is visible to the humaneye. Flicker can be caused by multiplefactors, including: line noise, control noise,circuit noise, component tolerance, andcircuit design. Flicker can be continuous(happening all of the time), or intermittent(only happening some of the time).Smooth and continuous:Smooth and continuous describes theexpectation of performance based onthe public’s knowledge of incandescent10080601% Architectural dimming5% Hi performance dimming10% Lighting managementdimming401 5 10 200 102232405060708090100MeasuredPerceived light (%)light (%)Determining perceived light. Source:IESNA Lighting Handbook, 9th Edition(New York, 2000), 27-4.dimming. A proportional change in control(dimmer) position should be refl ected by anequal change in light level. There should beno abrupt change in light level as the lightsource is being dimmed.Dead travel:Dead travel is the amount of change in thecontrol before there is any change in thelight source. In a vertical slide dimmer, ifyou can slide the dimmer halfway downbefore the light starts to dim, there is 50%“dead travel” on the dimmer. ◀ ____________ 22 APRIL 2009 LEDsmagazine.comPrevious Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next PageABE FMaGS