Imaging

The generation of an optical image often means that light received from points of an object is sent to points on some image plane. More generally, imaging may mean that points in a certain plane (containing any objects or not) are mapped to points in some other plane. In some cases, one does three-dimensional imaging, collecting information on object points not only in one plane.

The simplest kind of optical imaging is achieved with the pinhole camera (camera obscura), requiring only a pinhole and no other optical elements like lenses or mirrors. Because that operation principle is rather limiting, particularly because of a trade-off between resolution and light collection efficiency, other imaging methods are applied in most cases.

The most common principle of optical imaging is that with a single lens, or similarly with a multiple-lens system, also called an objective. This is explained in the article on imaging with a lens.

There are other imaging methods, not requiring lenses, but certain amplitude or phase masks in front of an electronic image sensor. Here, the images need to be computed from the raw data, using sophisticated algorithms.

Some imaging devices work by scanning objects point by point and assembling those data to complete images. In some cases, one does a line scan in one dimension only.

Some imaging methods are suitable for acquiring three-dimensional images. This is possible with holography and with some scanning methods such as optical coherence tomography.

The resolution achievable with optical imaging is in most cases limited by diffraction to the order of half the optical wavelength. However, there are a couple of methods for super-resolution imaging beyond the diffraction limit. For example, there are near-field microscopy methods and certain methods of fluorescence microscopy.

In some cases, imaging is not done with traditional optics like lenses, but based on fiber optics. For example, there are imaging fiber bundles and fiber-optic plates (faceplates) which can produce one-to-one image transfers, sometimes also including some magnification when using tapered structures.

Imaging is not only possible with visible light, but also with electromagnetic radiation in other frequency regions and with other types of radiation:

• Infrared light is widely used for imaging; there are special infrared cameras, e.g. for thermal imaging and night vision.
• In some cases, ultraviolet light is used, for example in the context of laser lithography.
• There are various methods of X-ray imaging. A difficulty in that domain is that it is hard to produce effective mirrors, except for glazing incidence. For example, Wolter telescopes are used in space for X-ray observation of stars.
• Terahertz imaging exploits the penetration of terahertz radiation through substances.

If a couple of spectral bands is used, the term multispectral imaging is common. If a contiguous wavelength band is covered with substantial resolution, one speaks about hyperspectral imaging.

The quality of an imaging system is quantified by several key parameters:

• Spatial resolution: The ability to distinguish closely spaced features, often limited by diffraction (depending on the numerical aperture) or by the pixel size of the image sensor.
• Contrast and MTF: The modulation transfer function (MTF) describes how well contrast is preserved as a function of spatial frequency.
• Signal-to-noise ratio (SNR): A measure of signal quality, influenced by light collection efficiency, detector sensitivity (quantum efficiency), and noise sources (e.g., shot noise, dark current).
• Dynamic range: The ratio between the maximum measurable signal (saturation) and the noise floor, determining the ability to capture scenes with high contrast.
• Field of view (FOV) and depth of field (DOF): The extent of the observable scene and the range of distances over which objects appear in sharp focus.

Some kind of optical imaging is required for a wide range of applications. Some important example cases are briefly explained the following:

• In a photo camera, one uses a photographic objective for imaging objects either onto a photographic film or an electronic image sensor. There is also a wide range of other types of cameras, including video cameras for moving images, used for many applications like surveillance and process monitoring.
• A microscope can image tiny objects to the human eye or into a microscope camera. Beyond simple optical microscopes, there are devices for laser microscopy, more specifically e.g. fluorescence microscopy.
• For small magnifications, loupes and magnifying glasses are often sufficient.
• Various kinds of telescopes, including binoculars and monoculars, can be used for viewing distant objects.
• Endoscopes, borescopes, fiberscopes and videoscopes are used for viewing objects through rigid or flexible tubes.
• Infrared viewers can be used to see based on infrared light.
• Projectors are imaging devices which also contain a light source for illumination of the object. For example, overhead projectors can project images from slides with large magnification onto screens, and laser projectors can do the same for digital images.
• Various types of machine vision devices provide images used by machines, for example autonomous vehicles.
• Optical coherence tomography (OCT) is a scanning method for obtaining microscopic images. Some types of laser microscopes also acquire images by scanning objects.
• Optical profilometers acquire high-resolution images revealing surface profiles.
• Remote sensing systems are used for geospatial imaging, for example; they are used on airplanes and satellites.
• In lithography, for example for the fabrication of computer chips, complicated fine structures are imaged onto wafers. Here, particularly short wavelengths are required for very high spatial resolution.
• Terahertz imaging can be used for security screening, non-destructive industrial inspection and checking of agricultural products.