VQiPS: Capture

Guide to Defining Video Quality Requirements

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Capture
Capture is the process of recording data, such as an image or video sequence. The capture function converts the information (light) from a real scene into a stream of information that is suitable for the remaining links, via a photographic or electronic medium.

In the case of modern video, the chain is modified slightly. The camera is in front of a scene and it has focus optics (usually just a lens, but it could be a night vision system). The lens presents focused light to the internal workings of the camera, a projection of the information from the scene. The camera converts the projected information into a stream of electronic data which can support subsequent processing, storage, and viewing. For digital images, the capture process converts light into a digital form via a sensor and digitization.

Equipment: Camera
The video camera houses the capture function. The following features can affect the quality of the image or stream the camera captures.

Resolution
The act, process, or capability of distinctly capturing two separate but adjacent parts or stimuli, such as elements of detail in an image, or similar colors.

Frame Rate
The rate at which frames of video data are captured.

Color Fidelity
The extent to which camera accurately captures image colors.

Recording Medium Dynamic Range
The difference between the maximum acceptable signal level and the minimum acceptable signal level of the image or stream capture.

Bit Depth (digital cameras)
The number of levels that a pixel might have, such as 256 with an 8-bit depth or 1024 with a 10-bit depth.

Noise (analog cameras)
Random spurts of electrical energy or interference. In some cases, it will produce a “salt-and-pepper” pattern over the image or stream.

Infrared Capability
Image or stream capture wavelengths range from about 700 nm (nanometer) to about 900 nm. Usually an “infrared filter” is used, which lets infrared (IR) light pass through to the camera, but blocks all or most of the visible light spectrum (the filter thus looks black or deep red).

Fast Shutter Speed
Fast shutter speed is the length of time that the camera sensor is exposed to the image it is capturing for each of the still-photos that comprise the video. If there’s enough light to do so, it’s better to have fast or “short” or “high” shutter speeds (measured in fractions of seconds) so that while each image is being recorded, the scene being recorded or the camera have less of a chance to move and thereby blur the image. One way to decrease the need for the environment to be bright is to use a wide-aperture lens.

Large Imaging Sensor
Sensor size is measured in fractions of inches. The sensor is to digital cameras what film is to conventional cameras. Large sensors like a 1/2-inch sensor can capture more light than, say, a 1/3-inch or 1/4-inch sensor.

Lux Rating
The ability of video systems to work in low light is measured in their “lux” rating. Lux is a measure of the amount of light present; cameras that have a lower lux rating can work in darker environments.

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