The National Bureau of Standards, the FAA, and the Illuminating Engineering Society use candela effective in specifying intensities of flashing light source because this rating is the most meaningful when it becomes necessary to predict the visible range of flashing warning lights versus steady burning light sources. If a flashing light has a candela effective rating of 100 then it will be visible at the same distance as a 100 candela steady burning source. A light with a higher candela second rating will appear brighter than a light with a lower candela second rating even if the lower rated light has a much higher peak candela rating.ģ) CANDELA EFFECTIVE or EFFECTIVE CANDLEPOWERĬandela effective is based on candela seconds and attempts to equate the brightness of a flashing light source to the brightness of a steady burning source. Candela seconds is merely a relative measure of how bright a flash of light will appear to a human eye. Candela seconds is used by the Society of Automotive Engineers and the California Highway Patrol to specify the minimum requirements for light output from a flashing light because flash energy has been shown to be a relatively accurate and fair way of comparing radically different types of lights such as incandescent rotators and xenon strobe lights. This quantity is the actual light energy contained in a pulse of light. strongly discourages the use of peak candela ratings when comparing warning lights.Ģ) CANDELA SECONDS or CANDLEPOWER SECONDS In addition there is no set multiplication factor for converting peak candela, a unit of luminous intensity, to either candela seconds or effective candela, both units of luminous energy. Peak candela alone cannot be used to directly compare two warning lights. It indicates NOTHING ABOUT HOW BRIGHT THE LIGHT APPEARS TO THE HUMAN EYE. This quantity is the maximum light intensity generated by a flashing light during its light pulse. Let’s briefly discuss three different commonly specified “intensity” ratings: When comparing two different warning lights, the first question usually asked is how bright are these lights and how do they compare to one another? This can be a complicated question when one is comparing very different light sources such as rotating incandescent lights and xenon strobe lights. PEAK CANDLEPOWER, CANDELA SECONDS AND CANDELA EFFECTIVE d = Distance in feet that a light intensity can be seen.Lb = Foot-Lamberts background illuminance.Q = Light output of flashtube in lumen seconds (empirically derived for helix flashtubes).φ = Efficiency of flashtube in lumen seconds/watt seconds.M = Lens or reflector amplification factor.Ieff = Effective Intensity (Also known as candela effective).F = Flashes per second (flash frequency).Download Light Intensity Specs XENON STROBE FORMULAS The term “Effective Intensity” or “Candela Effective” is used by signal engineers to describe a flashing signal light which has the same signaling effectiveness as an equivalent steady burning light. The decrease goes as r squared because the area over which the light is spread is proportional to the distance squared.The purpose of this reference section is to promote understanding of light intensity specifications for flashing signal lights. Notice that as the distance increases, the light must spread out over a larger surface and the surface brightness decreases in accordance with a "one over r squared" relationship. This relationship can be illustrated by the diagram below, which shows the apparent brightness of a source with luminosity L 0 at distances r, 2r, 3r, etc. Suppose that some time later the brightness of the light is either greater or lesser if the intensity diminished you would know that the source was moving away from you and if it became brighter you would know that the source was moving towards you (assuming the light source itself remained the same). Suppose you were to use a light meter to measure an initial intensity I i, or brightness, a distance r from a light source. The intensity or brightness of light as a function of the distance from the light source follows an inverse square relationship. More on Brightness as a Function of Distance
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