[HH] How much do LEDs dim over time?

[Tom Metro]

How much do LEDs dim over time? A hacker (Bogdan Raducanu, an EE
student) set out to measure it. (Link via @dangerousproto,
https://twitter.com/dangerousproto)

http://www.electrobob.com/led-logger/

  One important aspect that comes to mind is about the lifetime of [LED
  strips], are they better than bulbs, overall? Obviously there's no
  specification you can trust anywhere. So, how do I know if cheap
  strips are good enough or it is worth spending the money on more
  expensive ones? Measure some data for the cheap ones and see if they
  are good enough.

  I setup a very simple device, for first measurement attempt: a metal
  project box houses 15 cm of unused strip that shines light on a
  TLS2550 sensor, everything being isolated from ambient light.

  This sensor is really great: it has I2C interface, high dynamic range
  and approximates the human eye response, giving the result directly in
  lux (after some math). I've used a microcontroller and an EEPROM
  memory with a couple of years of space, storing data every 6 hours.
  ... The used board is Arduino compatible, but it was not used with the
  Arduino environment, just your regular C.

  The light has been on continuously for 1400 hours, two months. ... So
  far, the LED strip has dropped about 18% in brightness compared to the
  beginning. Current has stayed the same, meaning that they are actually
  reducing their efficiency.

  I decided to try to predict the life of the LEDs. Since they don't
  burn but rather fade, I think that the point where they dropped to 70%
  of initial intensity is a good mark of their end of life in
  applications where they are used for lighting and not decoration.

  Using an exponential projection I found that it will take just 2200
  hours until the LEDs drop to 70% of their initial intensity. This is
  rather disappointing, even for cheap LEDs.

See link for the graph.

The author goes on to do some comparisons of the light output and power
efficiency of the cheap LED strips he is using with fluorescent and
incandescent lights, and do a cost comparison, given the projected life
span of the LEDs.


It's long since been well known that fluorescent backlights on LCD
displays significantly diminish in brightness over time, but I wasn't
aware that LEDs had the same problem, to that extent, even if less
pronounced. I thought consistent light output (and power savings) was
one of the big reasons for switching to LED backlights.

So that means our household bulbs that have been upgraded to LEDs with
supposed 100,000 hour/20-year lifespans will likely need to be replaced
far sooner due to diminished output?


The author also has a good posting collecting the common power supply
circuits for driving LEDs with a decent explanation of the theory behind
each:
http://www.electrobob.com/linear-led-power-supplies/

 -Tom