The values in TM4SAT5 should not be used when creating spectral response curves, because it is a composite image of HOW87TM1, HOW87TM2, HOW87TM3 and HOW87TM4. This composite image has been created to increase the contrast of the display image, the values have been stretched and no longer represent the true reflectance values of the pixels, therefore only the original images should be used in creating a spectral reflectance curve.
Image enhancement tools can be very useful in redisplaying an image so that it provides more visual information. However, when an image is altered in such a way, the true data is altered and is no longer accurate, so it cannot provide much quantitative meaning. For example, when constructing spectral reflectance curves like in the exercise above, you would need to determine and differentiate between the different area types like water and urban. In the original image it is very hard to do so because of the low contrast of the displayed image. The enhanced image can provide you with the information that allows you to differentiate. However, the reflectance values cannot be used from the enhanced image, so the original image will be able to provide more meaning.
The DESTRIPE image enhancer like most image enhancers alters the reflectance values of the pixels. The operation calculates the means and standard deviation for the entire image and then for each individual detector. Afterwards, the output from each detector is scaled to match that of the entire image.