International hydrographic organization specifications for chart content and display aspects of ecdis

S, Edition 6.0 March 2010 80


ANNEX Cb b

PROCEDURE FOR MAINTAINING THE CALIBRATION OF DISPLAYS

C.1
Introduction The application of ECDIS to practical marine use necessitates the development of a greater understanding of monitor limitations as they relate to an accurate colour display and a marine environment. The factors that effect colour accuracy and the life of the monitor are discussed and an approach for onboard monitor testing and calibration is described in this Annex.
C.2
System Requirements Monitor calibration is necessary for accurate colour reproduction. Several factors are important to review about the monitor system
1. When the monitor is initially installed in the system, it should be calibrated and the system matched to the IHO colour tables.
2. In use, the monitor should not let the colours shift beyond a certain perceptible relative limit. Each colour must be perceived as it was intended, and in a relative sense, one colour must not be mistaken for another.
3. When a colour shift occurs that is beyond limits, it should be possible to identify that the shift is beyond limits without the need for special instruments and completely dark environments.
4. Monitor's will develop calibration errors with time, requiring a periodic recalibration of the system. C Display calibration-drift processes Monitors may lose calibration through a number of mechanisms
-
Wearout:
For example overtime, the electron gun structure of the CRT type of monitor wears, reducing the efficiency of the gun. This causes a relative diminishing of the output of the colour that is wearing fastest, and causes a colour shift toward the other colours. The wearing is 'coulombic', so depends on the total number of electrons that are emitted by the gun overtime. Ina practical computer graphic CRT, this effect will start to become noticeable (if you are looking for it) after 10,000 to 20,000 hours of operation. Usually it is roughly equal in each gun (because the dominant colour displayed is generally grey/white) and results in very little colour shift. Ina situation where one colour dominates overtime, colour shifts may appear.
- Drift Drift occurs as components age. Drift will result in a change in the white point of the monitor. Fora CRT type of monitor this change in white point will usually result in a shift in the colour of grey over the luminance range. For CRT's that are used in computer graphics situations, this drift is usually not a serious concern. In shipboard applications, drift will be a greater concern because of the constant vibration causing internal setup controls to change, as well as ECDIS

S, Edition 6.0 March 2010 81 being a more colour-critical application. It is expected that recalibration due to drift will be required after several thousands of operational hours.
- Component Failures Component failure in the monitor or image generator component can cause problems ranging from drift to complete failure. Failures can result in a display shifting its colours or in a complete failure of one of the colours or even the complete picture. It is possible that the shift will be proportionally different at different drive levels. High quality monitors are designed with meantime before failure (MTBF) of 50,000 hours or more which results in along life of the imaging system. The shipboard environment will degrade this due to the vibration and salt air. In practice these mechanisms will cause adrift in the white point. This means that the specific colour of white will drift to cause the picture to appear a little more pinkish, bluish, or greenish. Usually, this can be 'touched up, without requiring a completely new calibration.

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