June 10, 2004
False color, or the use of the LUT (Look Up Table) is an extension of
the idea of brightness and contrast adjustment. Here, we look at other intensity
scales and custom LUT's.
- Load Sample Images / Filter Samples / face.tiff. This is an
example of a three-dimensional object that we interpret using shading.
SEM images are often of this nature. (Another good example of 3-D shadowed
objects is Sample Images /Filter Samples / scaled 06 2000.tiff.)
On the PC with versions earlier than Scion Image
Beta 3b, I have found that if an optional LUT (other than gray scale) is
applied to an image, subsequent images (.tiff files, anyway) are not read
correctly. The pixel values of the image are corrupted so that the image
looks 'solarized' (the contrast is much too great). To correct this, close
the corrupted image (by clicking on the "X" at the right end of
the title bar of the image), temporarily apply the gray scale to any images
that are already open (use the Options -> Grayscale menu), and
open the image again.
Image with normal gray scale.
In ImageJ, the Map window and the LUT are not there.
- Invert the pixel values with the Edit -> Invert menu.
| This is a 'negative' of the original. Note that the LUT has not changed.
The pixels values have been subtracted from 255.
The image has been edited by this menu - the LUT was not changed. The same
appearance can be obtained by inverting the LUT.
- Restore the image with either the Edit -> Undo Editing Edit
/ Undo menu or the Edit -> Invert menu, or the File
/ Revert to Saved File / Revert menu.
- Invert the LUT
- with the Options -> LUT Options... menu, clicking on the Invert
- Using Image / Lookup Tables / Invert LUT
A 'negative' image like the one above, but this time the LUT is inverted
(note that black is now at the top on the LUT window), and the pixel values
of the image have not been changed.
The Map window and the double-arrowed LUT tool can now be used to adjust
this inverted LUT.
The thermal color scale (or 'Hot Body' color scale) is the most natural
looking and most easily interpreted color scale (other than the gray scale).
This color scale sometimes renders dark image features more visible, without
degrading the bright features. (This is not the case for this example).
- Apply the Options -> Color Tables -> Fire-1 Image
/ Lookup Tables / Fire menu.
Image with thermal scale.
In ImageJ, the fire1 lut, without the blue/purple
tones between black and red, is not available directly from the menu.
This is the fire lut, which is the same as the Fire-2 LUT in Image, shown
This image resembles sepia prints of black and white images. The colors go from
black through red, orange and yellow to white, without any greens, blues, purples,
thus the ease in interpreting the image. The ImageJ Fire LUT
has the blues and purples, making the image less "natural".
If this color table had been applied after the Edit -> Invert step above,
the image would have not been 'restored' before application of the thermal scale.
The result would be the same as applying the thermal scale (as we have just done
above) , and then inverting the thermal scale using Options -> LUT Options...
menu, and clicking on the Invert box as was done with the gray level
image above, or in ImageJ, using Image / Lookup
Tables / Invert LUT.
Adding any colors at all to the thermal scale degrades the interpretation
of shapes of three dimensional objects, although the extra colors may be
used to highlight areas of interest or visualize some phenomena that otherwise
would appear as a slight change in gray level.
Here are examples of other color scales:
Options -> Color Tables -> Fire-2
ImageJ: Image / Lookup Tables / Fire
Purple added between red and black. The steps in the LUT window show that
the number of gray levels has been reduced to about 30.
Not available in ImageJ directly..
Fire-1, with LUT inverted.
8 Grays The 'contoured', 'pixellated', or 'banded' effect is due
to the reduced number of gray levels. Images sometimes appear this way
when the Monitors or Screen control panels
of the computer are set incorrectly.
- Process / Math / Divide /
- Image / Adjust / Brightness
& Contrast / Auto
- To see a similar effect in ImageJ:
- Click the Apply button
in the Brightness & Contrast dialog.
- Image / Lookup Tables / Spectrum