How to measure area with intensity over certain threshold in 1 image?

Hi, guys!
For example, if the background noise of a image is 262.47, how can I measure the total area in the image with intensity over 262.47 using ImageJ?

Hello Zonghan -

Basically, you can use the Image > Adjust > Threshold...
and Analyze > Measure commands.

Let’s say that you have an 8-bit grayscale image, and you want
to measure the area of the image with value (intensity) greater
than 172. The specific details are shown in this IJM macro:

newImage("Untitled", "8-bit ramp", 256, 256, 1);
setThreshold(173, 255);
run("Convert to Mask");
run("Set Measurements...", "area area_fraction");

The “Area” column of Results table is the total area of the image
(or of an ROI if you had selected one), and the “%Area” is the
percentage of the image that has value greater than or equal
to the threshold of 173. So the area with value above 172 is
“Area” * “%Area” / 100.

Thanks, mm