The Threshold of Clarity: PPI
PPI (Pixels Per Inch) measures the pixel density of a display. While resolution (like 1920x1080) tells you the total number of pixels, PPI tells you how 'tightly packed' those pixels are. A high PPI is what makes a screen look 'Retina'—where the individual pixels are so small that they are invisible to the human eye at a normal viewing distance.
The PPI of Different Devices
- Smartphones: Usually 400 to 500+ PPI. Because we hold them very close to our faces, we need extremely high density.
- Laptops: Usually 150 to 220 PPI.
- Desktop Monitors: Usually 90 to 110 PPI (for 1080p 24") or 140 to 160 PPI (for 4K 27").
- TVs: Often as low as 40 to 80 PPI. Because we sit 8-10 feet away, low density is not noticeable.
The Formula
To calculate PPI, we find the diagonal resolution in pixels using the Pythagorean theorem and then divide it by the physical diagonal size of the screen.
PPI = √ (w² + h²) / d
The 'Screen Door' Effect
In Virtual Reality (VR), PPI is critical. Because the lenses in a VR headset magnify the screen, low pixel density results in a 'screen door effect' where you can see the black gaps between the pixels, which breaks immersion and can cause motion sickness.