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🗓️ 19 giugno 2015

TV Monitors HD, 4K: The Resolution

What is display resolution?

Calculate Viewing Distance Part 2: The Viewing Distance

Display Resolution (or Screen Resolution) is the number of horizontal and vertical pixels present on a display. Full HD resolution, for example, consists of 1920×1080 pixels, meaning 1920 horizontal pixels and 1080 vertical pixels. Multiplying these two values gives you the total number of pixels on the screen, which in this case is more than two million. A higher resolution corresponds to a more detailed image.

The word Pixel is a contraction of Picture Element. These small colored points that make up any type of digital image are each composed of sub-pixels containing RGB (Red-Green-Blue) colors, which, when placed together and illuminated at certain intensities, create all the specific combinations available in the color palette.

The parameter that measures the relationship between resolution (pixel quantity) and display size (diagonal in inches) is called density and is indicated using the acronyms DPI (Dots Per Inch) or PPI (Pixels Per Inch). For example, a 24-inch screen displaying a Full HD image (1920×1080) has a density of 91.79 PPI. The same screen at 4K resolution (Ultra HD, or 3840×2160) has a density of 183.58 PPI.

√(1920² + 1080²) = 2203 / 24 inches = 91.79 PPI
√(3840² + 2160²) = 4406 / 24 inches = 183.58 PPI

Human Eye Resolution

Human sensory organs have insurmountable physical limits. A healthy human ear perceives frequencies from 20Hz to 20kHz. Mixing sounds with frequencies below or above this range would not produce perceptible results. Something similar happens with our eyes¹.

A healthy eye can perceive details down to 1 arc minute (1/60 of a degree). Beyond this distance, objects blend together and appear blurred. As with all optical systems, human eyes also have a minimum resolution angle² (MAR, Minimum Angle of Resolution) below which they can no longer distinguish details. This is the shortest distance at which two adjacent lines are perceived as separate. This distance depends on retinal health and the minimum precision of the ocular optical system (which varies from person to person).

Under normal contrast conditions and at the minimum focusing distance, one can distinguish between 250 and 290 DPI. However, spatial resolution decreases dramatically as distance increases - at 1 meter, it already drops below 75 DPI - and plummets in low contrast situations.

To make this concept less technical, we can perform a simple experiment. Let’s take two images of a red “X” on a black background; both images have the same physical size but are rendered at different resolutions, 8×8 and 16×16 respectively.

Looking at the animation, we can notice how the pixels in the right image blend together more quickly than those in the left image. With higher pixel density, the distance between pixels (measured in pixel pitch) decreases, allowing our eyes to merge them into a single object and giving us the illusion of looking at a uniform image instead of a series of separate but adjacent squares.

Conversely, if we stand in front of two screens of equal size but different resolutions and gradually move closer, the screen with lower resolution will lose quality much faster than the one with higher resolution. We can conclude that:

The higher the resolution, the shorter the viewing distance can be before the image breaks down into visible pixels. The lower the resolution, the greater the distance must be for pixels to form uniform objects.

But there’s more, because the spatial resolution of eyes is not uniform. Near the central region of the retina, visual acuity is very high and degrades quickly as you move away from it. Although the eye receives data from a field of about 200 degrees, acuity over this range is poor. To form high-resolution images, light must fall on the fovea³. This means that our peripheral vision is much, much less detailed (blurred) compared to central vision. If we get too close to the image, the Field of View (FoV) is too wide and doesn’t allow us to see peripheral details well. On the other hand, if we move too far away, spatial resolution decreases dramatically, affecting the image in its entirety.

This information is very important for understanding how to get the most out of high or ultra-high definition TVs and monitors, taking into consideration every possible factor: panel size, pixel resolution, ideal proportions, and correct viewing distance that keeps in mind the organic limitations of human vision.

Calculate Viewing Distance Part 2: Viewing Distance