Author Topic: Why are older LCD backlight so power hungry...  (Read 1659 times)

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Offline alank2

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Why are older LCD backlight so power hungry...
« on: January 08, 2015, 09:26:12 pm »
Hi Everyone,

I've got a 4x20 LCD display that is new off of eBay from China and it is hugely bright for 60mA or so.  Then I've got some older displays - they run about 150mA and aren't nearly as bright.  Is it just the efficiency of the LED's used for the backlight?  Have they really gotten this much more efficient?

Also, how can you date code something like this.




Offline danadak

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Re: Why are older LCD backlight so power hungry...
« Reply #1 on: November 26, 2016, 02:58:31 pm »
1994 was a turning point in LED development with InGAS process. It still
took a few years before the high brightness led became mainstream. The
jump in brightness went from tens of candelas to thousands. As time
evolved the older processes disappeared as the new process volume
surpassed the older.

Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer

Offline Buriedcode

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Re: Why are older LCD backlight so power hungry...
« Reply #2 on: November 26, 2016, 08:06:29 pm »
Older backlights were arrays of LED's in epoxy, with a diffuser sheet over the top.  This means they are a series/parallel grid, and as danadak pointed out, often low efficiency LED's (or rather the most efficient green at the time, which were yellow/green).

Newer backlights are often just an edge lit light pipe.  It doesn't need to be as bright for transmissive displays, as there is no partial reflector foil (that lets some light through, but reflects some, allowing for transflective displays).  In the early 2000's newer software allowed for better lightpipe design, meaning that more of the light injected into the lightpipe could be ejects out the front, rather than just going out the back or sides where a reflector was place - these reflector sheets absorbed some light to.

Polarisers have also got better, so when pixels are transparent, they absorb only slightly more than 50%, rather than the 60-70%.

So I think its a combination of things, LED efficiency, better lightpipe design, better optical films (3M was the big player here) that all added up to the point where a 80mA backlight really can be as good as a 240mA.  Also, a 20x4 display is obviously much larger than the standard 16x2 and so will draw more power.  But 20mA seems to be standard for 16x2 these days.
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