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John E Clark

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Everything posted by John E Clark

  1. All lights have fall off from the center... just some lights are worse than others... take your measurements from the beam center, and then set up the light on an even surface, and see if you can characterize the fall off from beam center.
  2. The way the 'inverse square law' is stated, is in reference to a point source, and distance from said source. Think of a light bulb, perhaps 2 inches in diameter and you are standing a 5 feet. At that distance, the light bulb diameter is 'small' relative to the distance to the observer, and so is a 'point source' and so the inverse square law applies. With something like a soft box, where the diameter of the surfice is 'large', relative to the location of the subject, then the inverse square law does not apply... I'll leave it as an exercise to figure out at what distance the a 4 foot diameter soft box become essentialy a 'point source'... In any case, one can perform a gedankenexperiment in that case and not that if one is 1 foot away from a 4 foot softbox, that by going to 2 feet... one does not '1/4' the light that one had at 1 foot... I've seen some highschool science fair type experiment were someone has performed the metering at various distances until the readings begin to follow the inverse square law... I think it's about 2-3 x the diameter of the illuminating surface... To understand a 'focused beam', via a 'Frensel lens', and note that the beam can be viewed as a point source, but the point source is not located where the lamp head is placed, but at some distance to the 'back' of the housing, say 200 feet... because the beam is diverging only a small amount due to the focusing lens. With the idea that the 'point source' location for the frensel lens lamp is 200 feet away, and the lamp directed to a subject at 10ft, and then moved to 20 feet, because the 'virtual point source' is 200 feet distant, I've only moved the subject 'virtually' from 210 feet to 220 feet from the virtual point source location... and so I've not 'doubled' the disance from the point source virtual location. This also 'works' for a laser, where the 'point source' is infinitely far away... well... not really, but definitely must further than most things...
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