Image resolution

Image resolution describes the detail an image holds. The term applies to digital images, film images, and other types of images. Higher resolution means more image detail.

Image resolution can be measured in various ways. Basically, resolution quantifies how close lines can be to each other and still be visibly resolved. Resolution units can be tied to physical sizes (e.g. lines per mm, lines per inch), to the overall size of a picture (lines per picture height, also known simply as lines, or TV lines), or to angular subtenant. Line pairs are often used instead of lines; a line pair comprises a dark line and an adjacent light line. A Line (or TV line, TVL) is either a dark line or a light line. A resolution of 10 lines per millimeter means 5 dark lines alternating with 5 light lines, or 5 line pairs per millimeter (5 LP/mm). Photographic lens and film resolution are most often quoted in line pairs per millimeter.

Resolution of digital images

The resolution of digital images can be described in many different ways.

Pixel resolution

The term resolution is often used for a pixel count in digital imaging, even though American, Japanese, and international standards specify that it should not be so used, at least in the digital camera field. An image of N pixels high by M pixels wide can have any resolution less than N lines per picture height, or N TV lines. But when the pixel counts are referred to as resolution, the convention is to describe the pixel resolution with the set of two positive integer numbers, where the first number is the number of pixel columns (width) and the second is the number of pixel rows (height), for example as 640 by 480. Another popular convention is to cite resolution as the total number of pixels in the image, typically given as number of megapixels, which can be calculated by multiplying pixel columns by pixel rows and dividing by one million. Other conventions include describing pixels per length unit or pixels per area unit, such as pixels per inch or per square inch. None of these pixel resolutions are true resolutions, but they are widely referred to as such; they serve as upper bounds on image resolution.

According to the same standards, the number of effective pixels that an image sensor or digital camera has is the count of elementary pixel sensors that contribute to the final image, as opposed to the number of total pixels, which includes unused or light-shielded pixels around the edges.

Below is an illustration of how the same image might appear at different pixel resolutions, if the pixels were poorly rendered as sharp squares (normally, a smooth image reconstruction from pixels would be preferred, but for illustration of pixels, the sharp squares make the point better).

An image that is 2048 pixels in width and 1536 pixels in height has a total of 2048×1536 = 3,145,728 pixels or 3.1 megapixels. One could refer to it as 2048 by 1536 or a 3.1-megapixel image.

Unfortunately, the count of pixels isn't a real measure of the resolution of digital camera images, because color image sensors are typically set up to alternate color filter types over the light sensitive individual pixel sensors. Digital images ultimately require a red, green, and blue value for each pixel to be displayed or printed, but one individual pixel in the image sensor will only supply one of those three pieces of information. The image has to be interpolated or demosaiced to produce all three colors for each output pixel.

Spatial resolution

File:1951 USAF Resolution Test Target.JPG

The 1951 USAF resolution test target is a classic test target used to determine spatial resolution of imaging sensors and imaging systems.

The measure of how closely lines can be resolved in an image is called spatial resolution, and it depends on properties of the system creating the image, not just the pixel resolution in pixels per inch (ppi). For practical purposes the clarity of the image is decided by its spatial resolution, not the number of pixels in an image. In effect, spatial resolution refers to the number of independent pixel values per unit length.

The spatial resolution of computer monitors is generally 72 to 100 lines per inch, corresponding to pixel resolutions of 72 to 100 ppi. With scanners, optical resolution is sometimes used to distinguish spatial resolution from the number of pixels per inch.

In geographic information systems (GISs), spatial resolution is measured by the ground sample distance (GSD) of an image, the pixel spacing on the Earth's surface.

In astronomy one often measures spatial resolution in data points per arcsecond subtended at the point of observation, since the physical distance between objects in the image depends on their distance away and this varies widely with the object of interest. On the other hand, in electron microscopy, line or fringe resolution refers to the minimum separation detectable between adjacent parallel lines (e.g. between planes of atoms), while point resolution instead refers to the minimum separation between adjacent points that can be both detected and interpreted e.g. as adjacent columns of atoms, for instance. The former often helps one detect periodicity in specimens, while the latter (although more difficult to achieve) is key to visualizing how individual atoms interact.

In Stereoscopic 3D images, spatial resolution could be defined as the spatial information recorded or captured by two viewpoints of a stereo camera (left and right camera). The effects of spatial resolution on overall perceived resolution of an image on a person's mind are yet not fully documented. It could be argued that such "spatial resolution" could add an image that then would not depend solely on pixel count or Dots per inch alone, when classifying and interpreting overall resolution of an given photographic image or video frame.

Spectral resolution

Color images distinguish light of different spectra. Multi-spectral images resolve even finer differences of spectrum or wavelength than is needed to reproduce color. That is, they can have higher spectral resolution. that is the strength of each band that is created ( Lihongeni mulama: 2008)

Temporal resolution

Movie cameras and high-speed cameras can resolve events at different points in time. The time resolution used for movies is usually 15 to 30 frames per second (frames/s), while high-speed cameras may resolve 100 to 1000 frames/s, or even more.

Many cameras and displays offset the color components relative to each other or mix up temporal with spatial resolution:

• 
  digital camera (Bayer color filter array)
• 
  LCD (Triangular pixel geometry)
• 
  CRT (shadow mask)

Radiometric resolution

Main article: Color depth

Radiometric resolution determines how finely a system can represent or distinguish differences of intensity, and is usually expressed as a number of levels or a number of bits, for example 8 bits or 256 levels that is typical of computer image files. The higher the radiometric resolution, the better subtle differences of intensity or reflectivity can be represented, at least in theory. In practice, the effective radiometric resolution is typically limited by the noise level, rather than by the number of bits of representation.

Resolution in various media

This is a list of traditional, analog horizontal resolutions for various media. The list only includes popular formats, not rare formats, and all values are approximate (rounded to the nearest 10), since the actual quality can vary machine-to-machine or tape-to-tape. For ease-of-comparison, all values are for the NTSC system. (For PAL systems, replace 480 with 576.)

• Analog and early digital
  • 350×240 : Video CD
  • 300×480 : Umatic, Betamax, VHS, Video8
  • 350×480 : Super Betamax, Betacam (pro)
  • 420×480 : LaserDisc, Super VHS, Hi8
  • 500×480 : Analog broadcast
  • 670×480 : Enhanced Definition Betamax

• Digital
  • 720×480  : D-VHS, DVD, miniDV, Digital8, Digital Betacam (pro)
  • 720×480  : Widescreen DVD (anamorphic)
  • 1280×720  : D-VHS, HD DVD, Blu-ray, HDV (miniDV)
  • 1440×1080 : HDV (miniDV)
  • 1920×1080 : HDV (miniDV), AVCHD, HD DVD, Blu-ray, HDCAM SR (pro)
  • 2048×1080 : 2K Digital Cinema
  • 4096×2160 : 4K Digital Cinema
  • 7680×4320 : UHDTV
  • Sequences from newer films are scanned at 2,000, 4,000, or even 8,000 columns, called 2K, 4K, and 8K, for quality visual-effects editing on computers.

• Film
  • 35 mm film is scanned for release on DVD at 1080 or 2000 lines as of 2005.
  • 35 mm original camera negative motion picture film can resolve up to 6,000 lines.
  • 35 mm projection positive motion picture film has about 2,000 lines that results from the analog printing from the camera negative of an interpositive, and possibly an internegative, then a projection positive.
  • IMAX, including IMAX HD and OMNIMAX: approximately 10,000×7000 (7000 lines) resolution.

See also

• Dots per inch
• Pixel density
• Display resolution

External links

• Spatial resolution of ionizing particle sensors; Spatial resolution of particle detectors used in High Energy Physics Experiments.
• Pixel & Resolution Tables; practical tables, and theory for beginner's education about resolution issues in digital imaging, photo and printing - Sorry, broken link as of dec 18th, 2010
• Luminous Landscape's Res-Demyst; on why pixel count is not always a good proxy for resolution
• Do Sensors “Outresolve” Lenses?; on lens and sensor resolution interaction.