As you probably know by now, the pixels in the new iPad's Retina display are so small that they almost can't be seen by the human eye. However, the story is very different indeed when you look at them at 80x magnification under a microscope.
What you see below are the displays of the iPad 3 and iPad 2 as photographed by Lukas Mathis, a Swiss software engineer and UI designer. When he received his iPad 3, the first thing he did was investigate the 2,048 × 1,536 pixel Retina display - first by eye and then under the microscope. Then he checked out the screen on his iPad 2, noting the much larger (and quite differently shaped) pixels. He continued the investigation, taking in his iPod Touch, iPhone 4S, Kindle Fire, Google Nexus One, PlayStation Vita, and more...
Despite the very obvious differences in resolution - they're all photographed at roughly 80x magnification, so the pixel sizes are relative - the layout and shapes of the pixels are very interesting. Take the iPad 2, for example. Not only are its subpixels (red, green, blue) non-rectangular, but the green subpixel is one shape while the red and blue are another. In the case of the PlayStation Vita, the blue subpixels are only half the size of the red and green. On the Google Nexus One you can see the much-maligned PenTile matrix - and in the HP Veer and PlayStation Portable displays (pictured on Mathis' site), each pixel is scarred with dark lines.
What do all of these differences mean? Well, now we're moving into the arcane realm of LCD and OLED technology. The dark gaps between the pixels (and within the pixels in some cases) are mostly caused by control circuitry (each and every subpixel has a few microscopic copper wires running to it) and the need to make sure that each pixel group is 'square' (draw an imaginary square around each group of three subpixels, and you'll see what I mean). The weird shapes are most likely to do with controlling the exact amount of light that each subpixel emits. In the case of the PS Vita, with its skinny blue subpixel, it's safe to assume that the blue light being emitted is twice as strong as red or green; if it was full-width, the resulting image would be very blue.
It's more complicated than that, though. It might be that the backlight has a hint of blue, and so the blue subpixel is made smaller to compensate. The anti-glare coating on the display might absorb more red and green, and so those subpixels need to be larger. There is also the fact that different display manufacturers use different techniques; Samsung might prefer rectangles, while Sharp might prefer more organic shapes.
Finally, jumping back to the topic of resolution, you can clearly see the higher PPI of the iPhone 4S display versus the iPad 3 (326 PPI vs. 264). It's a shame that Mathis doesn't have access to the Galaxy Nexus, as its 1,280 × 720 PenTile display with 316 PPI is probably quite a sight to behold.
Image credit: Lukas Mathis