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I What happens to intensity during light interference?

  1. Feb 11, 2017 #1
    Dear Every Smart Person,

    I am not a scientist or a physicist by any means, but I have a concept that has been literally driving me crazy all day. No matter how much I read, I can't make sense of it. Here it goes.

    I work in the LED Lighting space. In that space, there are two types of LEDs: a "COB" and an "SMD". A COB essentially has numerous LED light sources placed very close together (i.e. several light emitting diodes), whereas an "SMD" is just one light source (a single light emitting diode). The recent trend we are noticing is that people prefer building light products using SMD technology because apparently you get higher lumens per watt compared to using COB technology.

    After reading online about light interference and Young's double split experiment, I would have thought that when you have numerous light sources (like multiple LEDs on a COB) pointing in the same direction and placed very close together, you will effectively create higher intensity due to superposition.

    After reading the double-slit experiment, I was thinking that maybe if I brought the slits closer together, I can get higher intensity on the screen, but I can't find a single relationship between "slit distance" and "light intensity".

    My Questions:
    1. Should you have greater luminous intensity because there is multiple light interference happening at the same time causing the amplitude to rise much more quicker, especially since the light sources are placed closer together?

    2. Why would the efficiency (lumens per watt) of the light source reduce when you have a COB vs. SMD technology? I would have assumed that since you have overlapping LEDs close together would mean more luminous intensity for the same power output, which would mean a higher lumen per watt product?

    3. I heard that because of heat there is light loss using COB light sources over SMDs...but I can solve for the heat problem. And if I did, shouldn't my efficiency from a COB be higher than that from a COB?

    Any help is appreciated.
     
  2. jcsd
  3. Feb 11, 2017 #2

    tech99

    User Avatar
    Gold Member

    If you have separate lamps of any sort, the phase of light from each is not of exactly the same and wanders around, so we can say they are incoherent. This means that when the light falls on a screen we do not see a stationery interference pattern. But with, say, two coherent sources we can say that the fields add, giving peaks of twice the field strength and four times the power flux density. With incoherent sources we say that they add on a power basis, so we have twice the PFD. The average PFD across the screen for both cases is just power addition.
    For the case of two sources, the peaks are not dependent on the slit spacing. In the case of Young's slits, however, the two sources are reduced in power as the spacing increases due to diffraction loss caused by the geometry.
     
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