O.K. I am assuming (I shouldn’t do that) that the attached image is as you aquired it with no preprocessing yet. So, the following is what I did.
Since any analysis in the future will vary in characteristics this first part was done by hand. Obtain the Polynomial Shading Corrector and Polynomial Fit plug-ins (available from the ImageJ/Fuji plug-in site) and firstly shade correct the image to flatten out the background. You can even do this twice if there’s any doubt and it’s quick, Then create a Poly fit image. Now the following is for the image and is completly variable to any images you may have in the future.
Use Process > Math > subtract(150) all pixels from here down are not providing any data so it’s useless. Then Math > Divide(16) the highest value of pixels remaining, and then Multiply >(100) this gives you an extreamly good contrast. Better then the Macro which would be v=((v-150)/16*100)
This you can do and then undo if it does not improve the image, but subtract the Fit image from the Prime image.
Your done and can color threshold and analyze particles.
This I done just because it didn’t seem that you needed the Blue channel so I split the channels and then merged only the Red and Green channels.
Hope this helps now and in the future , any questions just ask.