XPost: sci.electronics.design   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   Rhydian wrote:   
   > On Thu, 10 Feb 2022 11:35:31 -0500, Phil Hobbs wrote:   
   >   
   >> Rhydian wrote:   
   >>> Hi,   
   >>>   
   >>> I'm building an optical instrument that points a 850nm LED at a   
   >>> boundary between two materials at an oblique angle, and measures the   
   >>> (specular) reflection with a photodiode at the same (opposite) angle.   
   >>>   
   >>> The first few prototypes are working well but I want to compare the   
   >>> performance I'm getting with the theoretical limits.  My starting point   
   >>> is the Fresnel equations, but the part I'm having trouble with is that   
   >>> they give separate results for the s and p polarizations.  How do I   
   >>> combine the two into a total reflected power?   
   >>>   
   >>> As the incident angle approaches the critical angle for total   
   >>> reflection,   
   >>> both the s and p numbers approach unity, so clearly I can't just sum   
   >>> them, or take the vector sum, or I would get an answer greater than 1.   
   >>> Average?  Use the highest of the two?   
   >>>   
   >>> I'm assuming here that the photodiode detector (Osram SFH2700) has a   
   >>> response that's insensitive to polarization, but happy to be corrected   
   >>> on this point.   
   >>>   
   >>> I have a copy of "Building Electro-Optical Systems" but there's clearly   
   >>> something I'm missing.  Google is not much help either, it finds   
   >>> pretty- much exactly the same question (but for microwaves rather than   
   >>> IR) from two years ago, and no replies.   
   >>>   
   >>> TIA   
   >>>   
   >>> Rhydian (who should probably have paid more attention in   
   >>> electromagnetics classes 30 years ago)   
   >>>   
   >>>   
   >> You just treat the two polarizations independently and add up the   
   >> photocurrents when you're done.   
   >>   
   >> LEDs are pretty well unpolarized when you look at them from a distance.   
   >>   
   >> There are polarization effects with angle, due to the Fresnel   
   >> reflections from the top surface.  If the LED has a flat top facet,   
   >> p-polarized light escapes better, so there's a tendency for the light to   
   >> be somewhat radially-polarized.  Textured surfaces and lensed packages   
   >> smear that out pretty well, though, so to leading order your LED should   
   >> be unpolarized.   
   >>   
   >> Thus, it's a good guess to assume the LED light has equal amounts of s-   
   >> and p-polarized light.  These don't interfere, so the total photocurrent   
   >> is just the sum of the s and p photocurrents.   
      
   > OK, thanks, makes sense now.   
   >   
   > The LED is an Osram SFH4050, the top surface is slightly frosted so as   
   > you say, hopefully I can just treat it as 50:50 split between s and p   
   > polarization.   
   >   
   > One piece of odd behaviour I did see with this LED - I assumed the output   
   > power would be roughly linear with current, and lose efficiency and tail   
   > off as the die heated up.  But going up in 50 uA steps to about 5 mA (max   
   > is 100) there's a noticeable upward curve.  At first I thought I'd   
   > somehow screwed up the photodiode amp, but I tested it on an Ophir Nova   
   > II and got the same results.  I don't remember seeing this before with   
   > other LEDs.   
   >   
   > So long as the output power is long-term stable to within a few dB it   
   > won't matter (there isn't space for a monitor photodiode in the design).   
   > I will put a few of them on continuously for a few months, just to check.   
   >   
      
   Depends on the device.  There's normally a bit of a toe at low   
   currents--in the low tens of microamps for normal display LEDs--but then   
   it's pretty linear.   
      
   If your LED is something unusual it might behave differently.   
      
   Cheers   
      
   Phil Hobbs   
      
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC / Hobbs ElectroOptics   
   Optics, Electro-optics, Photonics, Analog Electronics   
   Briarcliff Manor NY 10510   
      
   http://electrooptical.net   
   http://hobbs-eo.com   
      
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