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Joint Photographic Experts Group  (JPEG)
This international group, a joint effort of the ISO and TSS, has developed standards for still image compression. Motion JPEG applies JPEG compression to each frame of a video clip.

Key Frame
 
A video frame in which all of the video information is recorded in compressed fashion. If the clip has a large amount of motion, better playback will occur with every frame being a Key Frame. If there is very little motion, such as a narrator, a higher number of Delta Frames will give satisfactory playback. In general, making every 3rd frame a Key Frame is a good choice with the current Indeo technology algorithm. Also see Delta Frame.

Key System
A telephone system in which multiple phones share a set number of outside lines and a currently open line must be selected by pushing the corresponding button on the phone.

Laboratory Information System (LIS)
An integrated system for the processing of laboratory data. While electronic lab reports are becoming more commonplace, the proprietary nature of commercial lab systems has limited the changeover to a paperless, patient record.

LAN connectivity
The ability to connect the video system to a LAN within the health care facility. This can allow access to and sharing of patient records, test reports, demographics, etc. during a video conference. Currently few institutions are capitalizing on this potential.

Laser Card
A storage device similar to an optical disk but in a card form.

Laser digitizer
 
Laser scanner. Employs a laser to capture image information to digital form. Very high resolution and wide gray scale range are possible. Because it uses coherent light, the laser scanner can penetrate thick films (an ability measured in units of optical density) and extract information better than CCD scanners. Serves the same function as CCD X-ray scanners, but generally at higher resolution, better contrast, and much higher price.

Latency
In a LAN used for videoconferencing for example, latency refers to delays which may be experienced by some data, causing intermittent cessation of the audio and/or visual communications. Similarly, the sound might arrive before the image or vice versa under some high-latency conditions. The cause is often poorly designed networks with utilization greater than 30 per cent or single users accessing or originating multi-megabyte-sized files.

Lempel-Ziv-Welch (LZW)
a data compression algorithm named after its developers.

Leveling
A software manipulation technique, using mathematical algorithms, to compensate for a teleradiology monitor's inability to provide the same contrast and bit depth as the original hardcopy X-ray. The great variation in the density of X-ray exposure within an image can lead to seriously over- and underexposed portions of a monitor (screen) image. Thus, a chest film may have very high density (white) portions, such as the spine, and very low density portions (such as the lungs). Using locally adapted histogram leveling, bit depth (contrast; gray scale) can be selectively adjusted for different portions of the image, or for different pixel depths. This yields much more useable clinical information.

Lip Sync
The maintenance of sound (i.e., speech) exactly in step with movement in a visual image (i.e., faces). Because the processing time for the video portion of the signal in a low-speed codec is about 100 times longer than the audio processing time, codecs usually incorporate adjustable audio delay circuitry to delay-equalize the two signals, to regain lip sync.

Liquid Crystal Display (LCD)
A very low-power device capable of displaying characters, words and symbols, often built into a codec or videoconferencing room control panel.

Local Area Network (LAN)
A computer network linking computers, printers, servers, and other equipment within an enterprise. Can support audio, video, and data exchange. Typically runs at 10-100 Mbps.

Lossless compression
See compression ratio.

Lossy compression
See compression ratio

Low bit rate or low data rate video
Video that we are doing over the satellite at 56 kpbs. It is live video, but not FULL MOTION Video. There is now technology to do low bit video at 16 kpbs...

Low pass filter
A filter for leveling out the borders in the screen display of a radiology image. The image is altered by placing a value on each pixel which is the weighted sum of the pixels in a small square surrounding the pixel in the original image. The weights or filter coefficients determine how the new image will look, and the borders of the image can be leveled out ("low pass filter") or sharpened ("high pass filter").

Luminence
That portion of a composite video signal which represents the monochrome or brightness part of the image.

Lux
A unit of illumination. Generally, lighting levels of 1000-3500 lux are satisfactory for telemedicine applications, while newer "low lux" cameras produce quality pictures at levels as low as 750 lux. Having a lux level above 1000 enables the camera to keep more of the image in focus, since the trade-off for low light conditions is lower depth of field.

 


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