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Sphere

This a fully enclosed sphere containing a web cam, white led and photo resistor in exactly the geometric arrangement shown. The led glows dimly (via fixed dropper resistor). It runs at about 9 mA. It's light reflects off the inside of the sphere and the web cam takes pictures of the inside of the sphere. The photo resistor is there to measure relative light levels. The system is theoretically in a steady state (including temperature which hovers around 29 deg.C). and there is no extraneous light. This is what the camera sees:-

web cam image

and this is that image's histogram:-

web cam histogram

The thing is, it's not steady state. These are the photo resistances and histogram means in two experiments each taking three weeks The initial number is at the start, and the second number is at the end of three weeks. They gradually change from one to the other as I measure them.

  1. 69.4k -> 136.5k, 33 RGB -> 1 RGB
  2. 28.7k -> 55.7k, 49 RGB -> 21 RGB

Both times the photo resistance and histogram mean drops very significantly, in experiment 1 almost to zero. The inference is that the inside of the sphere is getting darker as time goes by (measured in weeks). This question regarding photo resistor stability was inconclusive but cannot explain the shifting histogram.

It's difficult to judge by eye, but I think that the LED is getting slowly dimmer. Occam would suggest so, but I have used two brand new LEDs so they would have both behaved like this. The generic LEDs were both from a cheapo bulk eBay purchase.

Q1. Can this be explained from an electronics perspective?

Q2. Can anyone suggest any further investigation(s) I can undertake?

EDIT.

I came across something called current crowding which seems to fit my problem. I can't follow the link though due to a flight failure...

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    \$\begingroup\$ LEDs do degrade over time, though your time period seems short. However, degredation is increased if the LED is over-driven or over-heated. Are you over-driving the LED? Have you taken measures within your closed sphere to ensure the LED is not cooking? \$\endgroup\$
    – Trevor_G
    Commented Mar 22, 2017 at 1:27
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    \$\begingroup\$ Repeat with a primary colour LED - or 3 - R,G and B - from a reputable manufacturer - and see if the drift stops. White LEDs use phosphors to generate the yellow component, pumped by a blue LED. And phosphors do decay - especially if you bought the white LED from eBay... \$\endgroup\$
    – user16324
    Commented Mar 22, 2017 at 11:07
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    \$\begingroup\$ @BrianDrummond Ah ha! It was extra cheap from eBay, bought in bulk. Is it possible that phosphor decay could be so apparent over the course of 3 weeks? Can you say more? Do you have any anecdotal evidence? Both LEDs were from the same bulk batch... \$\endgroup\$
    – Paul Uszak
    Commented Mar 22, 2017 at 13:04
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    \$\begingroup\$ The LED and camera are constantly on, 24 /7. \$\endgroup\$
    – Paul Uszak
    Commented Mar 22, 2017 at 13:20
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    \$\begingroup\$ @AndrewMorton It's probably been 6 months since I painted it. Nothing special, just ordinary brilliant white matt emulsion in several coats. I pondered mixing my own barium sulphate paint but opted for emulsion as the visible light reflectivity characteristics are very similar. \$\endgroup\$
    – Paul Uszak
    Commented Mar 22, 2017 at 13:51

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