XPost: sci.electronics.design   
   From: news@rblack01.plus.com   
      
   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.   
   >   
   > Cheers   
   >   
   > Phil Hobbs   
      
   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.   
      
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