Phototimer


Also found in: Medical.

Phototimer

 

a time-delay relay that automatically shuts off the lamp of an enlarger, reproducing unit, or similar device after a specified time interval—the exposure time—which is measured from the beginning of exposure of the photosensitive layer of a photographic material.

Depending on the method used to regulate the exposure time, phototimers are classified as mechanical (clock-driven), pneumatic, electromechanical, and electronic. Electronic phototimers, in which the exposure time is determined by the charging time of a capacitor, are the most advanced types. The exposure time may be varied over a specific range by using a switch to change the parameters of the circuit to which the capacitor is connected (for example, by means of additional resistors) or to change the capacitance of the capacitor itself. The exposure times of various phototimers range from tenths of a second to several tens of seconds. [27–1774–1 ]

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Proper evaluation of the phototimer on a radiographic system requires the production of 5 to 10 images, depending upon whether or not a table Bucky and chest Bucky are used.
For fluoroscopic units, the phototimer for the spot film device should be checked for thickness/kVp tracking and reproducibility of exposure in different image formats.
For mammographic units, phototimer testing requires tip to 25 films, all of which must be processed and measured for equal density.
The central cell was used as the phototimer domain.
The wide range in numbers for both the AEC device and optical densities is due to the methodology of collecting the calibration data at all density settings of the phototimer, from lowest to highest.
The data in Table 2 were obtained from a Philips high-frequency generator and phototimer at a second location using a different processor but the same film-screen speed (if 400.
Measurements were taken with the chest phototimer in one radiographic room at 90 kVp, 105 kVp and 120 kVp using a Lanex 400-speed system.
The Mammography Quality Standards Act requires that film density values be reported in assessing phototimer performance.
The phototimer in the spot film device in R/F rooms could pose another limitation for the AEC device.
Obviously, this phototimer is well calibrated, since the range of measured optical densities is only 0.
However, phototimers are a fundamental feature of most clinically installed radiographic systems and their performance should be evaluated on a regular basis, whether mandated or not.
This article investigates whether using a commercially-available test device can reduce the time, labor and expense involved in assessing the performance of phototimers on x-ray systems.