Pixel density

Pixel density measures how many pixels occupy a specified physical length or area. For a flat display with square, uniformly spaced pixels, its linear pixel density is often expressed as pixels per inch. Its reciprocal is the pixel pitch, the physical separation between corresponding points of neighbouring pixels.

If the display has horizontal and vertical counts $N_x,N_y$ and diagonal $D$ measured in inches, the Pythagorean theorem gives the linear pixel densityA hypothetical $1920\times 1080$ display with a $24\mathrm{in}$ diagonal has $2202.91/24\approx 91.8$ pixels per inch. Its pixel pitch is $25.4/91.8\approx 0.277\mathrm{mm}$. At the same pixel counts, halving the diagonal doubles the linear pixel density and quadruples the number of pixels per unit area. These calculations assume the quoted diagonal is the active image area.

A stored image's pixel counts do not determine its physical size without a display or printing scale. Likewise, a CSS px is a layout unit rather than a promise of one physical display pixel. CSS fixes 96px = 1in within its unit system, but the mapping onto physical display pixels depends on the chosen anchor and device. That ratio cannot be used to infer a screen's measured pixel density.^[ref]

Pixel pitch is the physical spacing between corresponding centres of neighbouring pixels on a display or sensor grid. For an axis-aligned rectangular grid it can have separate horizontal and vertical values $p_x,p_y$. The pixel pitch describes the grid spacing, rather than the number of bits used for each pixel value.^[ref]

For square pixels on a uniform grid, linear pixel density $\rho$ and pixel pitch $p$ are reciprocal. If $\rho$ is measured in pixels per inch,For example, a hypothetical matrix with 2400 columns occupying a $600\mathrm{mm}$ active width has $p_x=600/2400=0.25\mathrm{mm}$ and horizontal pixel density $25.4/0.25=101.6$ pixels per inch. This calculation treats the width as the edge-to-edge extent of 2400 equal pixel cells; the distance from the first centre to the last centre is instead $2399p_x$.

Doubling the column count over the same width halves the horizontal pixel pitch. Doubling both row and column counts quadruples the number of pixels per unit area. The dimensions and the direction being measured therefore matter when comparing specifications.

A smaller pixel pitch provides a finer sampling grid, but it cannot recover detail already lost from the input image. Optical blur, aliasing and the viewing geometry also affect visible detail. A CSS pixel is a layout unit, so its size must be mapped through the device pixel ratio before it can be related to this physical grid.