Find the circle and area fraction from ImageJ function

imagej
#1

Hi there, I am a new user to ImageJ and today I obtained the SEM images from my supervisor.

My supervisor wants me to analyse the area fraction of the image containing several circle particles.
However, the colour of the circles and the background are quite similar so when I tried using the Threshold adjustment, the highlighted circle and the background tend to merge.

silicon ball_surface 250x.tif (943.2 KB)

Are there any methods that could analyse the circle and the area fraction with the attached image?

Best regards,

Nathaphon