You use Thresholds to extract pixels within a certain range. In ENVI 5.3, this is found in the Threshold tab of the ROI tool. In ENVI Classic, it is found under Options->Band Threshold to ROI. After you have the selected pixels, you can export the pixel values and locations to a text file by using the Export function.

Thanks for your quick reply. I am using version 5.2 and what about Segmentation Image function? Thanks again.

What are you asking about the segmentation image function? How to extract an ROI? It would be the same - you set your min/max thresholds in the Thresholding tab of the ROI tool in ENVI 5.2 (new interface).

Sorry. I am getting confused. I used to be a user of Idrisi and am quite new to ENVI. Could you please just tell me how to do this? I have an ENVI image as I mentioned before and I want to create another image which only contains regions with NDVI value

This sounds like you want to create a mask and apply it to your image so that only NDVI values There are different ways to create this mask - with ROIs, vectors or using thresholds for the mask. If you use the Build Mask tool, select your NDVI imge. Then under Options, try using the 'Import Data range'. Set the maximum value to -0.0081 and no minimum. Also under Options choose the 'Selected areas: On' option. This means that pixels value that have a maximum value of -0.0081 will be the 'on' or '1s' in your mask and all other pixels will be 'off' or '0s' in the mask. Output the mask and confirm it is what you intended. You may need to play with this to get the areas you want. Then use Apply Mask to apply it to the original dataset. The output should be only pixels based on your criteria and all others will have a background value of 0, or whatever you set in the Apply mask dialog.

Thank you so much for your kind patience and instructions. The Build Mask tool is really helpful. Again, thanks for your help!