I took apart an Amazon Fire tablet (SR043KL) retired by cracked touch digitizer glass, seeking to salvage its display backlight and I was successful. I am fascinated by the optical behavior of modern LED backlights, even those used in products with a low price target like this tablet. After fussing with light diffusers for my Glow Flow project, I have a great deal of appreciation and respect for how evenly these backlights distributed their LED light.
I had a lot of time invested in the earlier LG laptop backlight project and was timid about fully exploring all its backlight layers, fearing that I would break something. Now that I have a smaller backlight with lower stakes, I’m going to take the layers apart and see how they act and interact with each other.
[UPDATE: This Hackaday post A Hacker’s Introduction to DIY Light Guide Plates has more details about these backlight layers, as well as making custom plates out of acrylic sheets with a laser cutter.]
At first glance the layers for this backlight are arranged slightly differently from the LG laptop backlight. I’m too new into this field to guess what tradeoffs are involved. What I do know is that the bottom-most layer on the Fire backlight appears to be non-removable. When acting alone, I could see a dotted pattern almost like dithering.
Above this layer is a sheet of smooth matte translucent white that I would have expected to be the top layer, but here it is.
When in place, it blended the dotted pattern together into something smoother. I think this looks great as-is, but we have two more layers to make it even better.
The third layer looks wild, with the optical characteristics I associate with Fresnel lenses and lenticular lenses, but this pattern looks different and I wished I knew the right name for it so I could read more about it.
When installed, it imparted a bit of pattern along with a rainbow-like sheen.
The fourth and final layer also has that optical property, but dialed back a bit. It also has a matte top finish similar to the second layer.
When in place, we have our backlight, providing an impressively even illumination across the entire area with all light provided by a row of LEDs on just one edge.
Speaking of those LEDs, I count eighteen of them. Given that they start illuminating at around fifteen Volts, my guess is that we’re looking at three parallel strings of six LEDs each. I don’t have anything to accurately clamp current at 3*20=60mA (my bench power supply current limit is only guaranteed to be +/- 10mA) but I estimate that would be somewhere near eighteen volts which makes this barely over one watt at maximum brightness. Pretty neat!
I’m setting this aside for later use. Emily Velasco has said she has a project idea that might make use of a small backlight, so it might go to her instead. If it does, I’m sure we’ll get something really weird and cool out of it because that’s what Emily builds. But in case this backlight isn’t what she needs, I can salvage others so we have alternatives.
UPDATE: Emily Velasco used this backlight in a mini X-ray film viewer.