False Colour Image Display and Intensity Scaling

  The basis of all interactive data analysis in FIT2D is the false colour image display for 2-D data, or an X/Y Graph plot for 1-D data. Figure 7 is an example of the false colour display of 2-D data, together with the Z-SCALING menu for changing the manner in which pixel intensity values are displayed as different colours or grey levels.

Figure 7: Example of False Colour Image Data Display

The whole of the ADR is displayed as an image even if there are less screen pixels than data elements, which is often the case. Figure 7 shows the default style, but the style of the graphics may be varied by using OPTIONS menu (see Section 3.10). For general data analysis the style options are generally not important, with the exception of the intensity or ``z-scaling''. It is necessary to understand the manner in which the data element values are changed into displayed pixel colours, to interpret the display images.

By default FIT2D uses automatic full range linear scaling of the ADR data values. This means that the minimum value and maximum of the ADR are found and all values in-between are converted to a colour (black, white and grey's are colours) in the current colour table, using linear interpolation.

With automatic range intensity scaling the whole of the range of data values will always be visible, although some images with a very few very extreme intensity values may not appear very interesting. Full range automatic scaling may be selected using the FULLY AUTOMATIC button. As the ADR changes then so do the current minimum and maximum values, and same intensity values may be displayed using different colours. This can be confusing and undesirable in some circumstances.

``Fixed'' range intensity scaling is also available. With completely fixed scaling the minimum and and maximum data values which correspond to the extremes of the colour table are fixed to some values. Thus, as the ADR changes the intensities displayed with particular colours stay fixed. However, if data values outside the defined range are encountered then they are just displayed using the colour corresponding to the minimum or maximum of the range. Fixed range intensity scaling may be selected using the USER MIN/MAX button, which will prompt for the minimum and maximum values of the displayed range of intensity values. It is also used when the + MAXIMUM, - MAXIMUM, + MINIMUM, or - MINIMUM buttons are used. These increase or reduce the minimum or maximum values of the range by 10%.

In addition to these two scaling modes FIT2D allows the following additional modes:

USER MINIMUM This uses an entered fixed minimum value for the displayed intensity range, but the maximum is automatically defined by the ADR data values.

USER MAXIMUM This uses an entered fixed maximum value for the displayed intensity range, but the minimum is automatically defined by the ADR data values.

WEAK PEAKS This selects reduced range automatic scaling. This is designed to display the weak features of diffraction data. Often full range scaling is not very informative since the intensity range is dominated by a very few strong reflections, and little detail is seen in the majority of the image. In this mode, the average value and standard deviation of intensity values are calculated in several regions of the image. The minimum of the display intensity range is then calculated from the minimum of the mean minus three times the standard deviation, and the maximum is set to the maximum of the mean plus five times the standard deviation from each of the regions.

As well as linear scaling, logarithmic scaling is available through the LOG SCALE button. When logarithmic scaling is enabled this button is replaced by the LINEAR SCALE button. If zero or negative range limits are encountered when logarithmic scaling is being used, a small positive value will be used instead.