Light field camera
A light field camera, also known as plenoptic camera, is a camera which captures information about the light field emanating from a scene; that is, the intensity of light in a scene, and also the direction that the light rays are traveling in space. This contrasts with conventional cameras, which record only light intensity.
One type uses an array of micro-lenses placed in front of an otherwise conventional image sensor to sense intensity, color, and directional information. Multi-camera arrays are another type. Holograms are a type of film-based light field image.
The first light field camera was proposed by Gabriel Lippmann in 1908. He called his concept "integral photography". Lippmann's experimental results included crude integral photographs made by using a plastic sheet embossed with a regular array of microlenses, or by partially embedding very small glass beads, closely packed in a random pattern, into the surface of the photographic emulsion.
In 1992, Adelson and Wang proposed the design of a plenoptic camera that can significantly reduce the correspondence problem in stereo matching. To achieve this, an array of microlenses is placed at the focal plane of the camera main lens. The image sensor is positioned slightly behind the microlenses. Using such images, the displacement of image parts that are not in focus can be analyzed and depth information can be extracted.
Standard plenoptic camera
The "standard plenoptic camera" is a mathematical model used by researchers to compare different types of plenoptic (or light field) cameras. By definition the "standard plenoptic camera" has microlenses placed one focal length away from the image plane of a sensor. Research has shown that its maximum baseline is confined to the main lens entrance pupil size which is small relative to stereoscopic setups. This implies that the "standard plenoptic camera" may be intended for close range applications as it exhibits increased depth resolution at very distances that can be metrically predicted based on the camera's parameters.
In 2004, a team at Stanford University Computer Graphics Laboratory used a 16-megapixel camera to demonstrate that pictures can be refocused after they are taken. The system used a 90,000-microlens array, yielding a resolution of 90 kilopixels.
Focused plenoptic camera
Lumsdaine and Georgiev described the design of a type of plenoptic camera in which the microlens array can be positioned before or behind the focal plane of the main lens. This modification samples the light field in a way that trades angular resolution for higher spatial resolution. With this design, images can be refocused with a much higher spatial resolution than images from the standard plenoptic camera. However, the lower angular resolution can introduce aliasing artifacts.
Coded aperture camera
A type of plenoptic camera using a low-cost printed film mask instead of a microlens array was proposed in 2007. This design overcomes several limitations of microlens arrays in terms of chromatic aberrations and loss of boundary pixels, and allows higher-spatial-resolution photos to be captured. However the mask-based design reduces the amount of light that reaches the image sensor compared to cameras based on microlens arrays.
Lytro's light field sensor uses an array of micro-lenses placed in front of an otherwise conventional image sensor; to sense intensity, color, and directional information. Software then uses this data to create displayable 2D or 3D images. Lytro trades maximum 2D resolution, at a given distance, for enhanced resolution at other distances. Users are able to convert the Lytro camera's proprietary image into a regular 2D image file, at any desired focal distance. The maximum Illum 2D resolution is 2450 × 1634 (4.0 megapixels), The 3D light field resolution is 40 "megarays". The first generation camera has a maximum 2D resolution of 1080 × 1080 pixels (roughly 1.2 megapixels),
- Variable depth of field and "refocusing": Lytro's "Focus Spread" feature allows the depth of field (depth of focus) of a 2 dimensional representation of a Lytro image to be adjusted after a picture has been taken. Instead of setting the focus at a particular distance, "Focus Spread" allows more of a 2D image to be in focus. In some cases this may be the entire 2D image field. Users also are able to "refocus" 2D images at particular distances for artistic effects. The Illum allows the "refocus-able" and "Focus Spreadable" range to be selected using the optical focus and zoom rings on the lens. The Illum also features "focus bracketing" to extend the refocusable range by capturing 3 or 5 consecutive images at different depths.
- Speed: Because there is less need to focus the lens before taking a picture, a light field camera can capture images more quickly than conventional point-and-shoot digital cameras. This is an advantage in sports photography, for example, where many pictures are lost because the cameras auto-focus system cannot precisely track a fast moving subject.
- Low-light sensitivity: The ability to adjust focus in post-processing allows the use of larger apertures than are feasible on conventional cameras, thus enabling photography in low-light environments.
- 3D images: Since a plenoptic camera records depth information, stereo images can be constructed in software from a single plenoptic image capture.
Lytro was founded by Stanford University Computer Graphics Laboratory alumnus Ren Ng to commercialize the light field camera he developed as a graduate student there. Lytro has developed consumer light field digital cameras capable of capturing images using a plenoptic technique. After Lytro ceased operations in March 2018, few options exist for purchasing light field cameras.
d'Optron and Rebellion Photonics offer several plenoptic cameras, specializing in microscopy and gas leak detection, respectively.
Pelican Imaging has thin multi-camera array systems intended for consumer electronics. Pelican's systems use from 4 to 16 closely spaced micro-cameras instead of a micro-lens array image sensor. Nokia invested in Pelican Imaging to produce a plenoptic camera system with 16-lens array that was expected to be implemented in Nokia smartphones in 2014. Pelican moved to designing supplementary cameras that add depth-sensing capabilities to a device's main camera, rather than stand-alone array cameras.
The Adobe light field camera is a prototype 100-megapixel camera that takes a three-dimensional photo of the scene in focus using 19 uniquely configured lenses. Each lens will take a 5.2-megapixel photo of the entire scene around the camera and each image can be focused later in any way.
CAFADIS is a plenoptic camera developed by the University of La Laguna (Spain). CAFADIS stands (in Spanish) for phase-distance camera, since it can be used for distance and optical wavefront estimation. From a single shot it can produce several images focused at different distances, depth maps, all-in-focus images and stereo pairs. A similar optical design can also be used in adaptive optics in astrophysics, in order to correct the aberrations caused by atmospheric turbulence in telescope images. In order to perform these tasks, different algorithms operate on the raw image.
Mitsubishi Electric Research Laboratories's (MERL) light field camera is based on the principle of optical heterodyning and uses a printed film (mask) placed close to the sensor. Any hand-held camera can be converted into a light field camera using this technology by simply inserting a low-cost film on top of the sensor. A mask-based design avoids the problem of loss of resolution, since a high-resolution photo can be generated for the focused parts of the scene.
The modification of standard digital cameras requires little more than the capacity to produce suitable sheets of micro-lens material, hence a number of hobbyists have been able to produce cameras whose images can be processed to give either selective depth of field or direction information.
In a 2017 study, researchers observed that incorporation of light field photographed images into an online anatomy module did not result in better learning outcomes compared to an identical module with traditional photographs of dissected cadavers.
Plenoptic cameras are good for imaging fast moving objects where auto focus may not work well, and for imaging objects where auto focus is not affordable or usable such as with security cameras. A recording from a security camera based upon plenoptic technology could be used to produce an accurate 3D model of a subject.
Stanford University Computer Graphics Laboratory has developed a light field microscope using a microlens array similar to the one used in their light field camera. The prototype is built around a Nikon Eclipse transmitted light microscope/wide-field fluorescence microscope and standard CCD cameras. Light field capturing ability is obtained by a module containing a microlens array and other optical components placed in the light path between the objective lens and camera, with the final multifocused image rendered using deconvolution. A later version of the prototype added a light field illumination system consisting of a video projector (allowing computational control of illumination) and a second microlens array in the illumination light path of the microscope. The addition of a light field illumination system both allowed for additional types of illumination (such as oblique illumination and quasi-dark-field) and correction for optical aberrations.
- Angle-sensitive pixel
- Compound eye
- Integral imaging
- Light-in-flight imaging
- Photo finish
- Streak camera
- Strip photography
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