Pixel intensity from different images

I have multiple stem cross sections images from each I need to calculated the percentage of the internal white area in relation to the rest of the cross section area, which is generally greenish. I have created a macro that was able to calculate this ratio, but it only work for a single stem cross section. When these different images are splitted into RGB channels there is a great variation for pixel intensity between each individual cross section. This complicates my analysis based on threshold values, since a unique threshold won’t work for every internode cross section. Is there a way to standardize the pixel intensities between this images? or any suggestions?

Thank you very much!!!

Try Image > Adjust > Color Threshold and threshold the stems on the hue. A range from 18-67 worked for me to get all the stems. There is a button on the Color Threshold window to generate macro code.

If you want to correct the color before analysis, the most accurate methods use a calibration standard, but there is a method (Colour Correct) by Landini that uses white and black areas within the image as an internal standard.

Hope this helps.