Need to calculate surface area of the picture

[natashab]

Dear all,

I am trying to calculate total area coverage of the slide by cells adhered to collagen-coated surface. I have used "find edges" tool and adjusted threshhold for detection to 87. However, the software does not seem to include the internal structures of the highlighted areas and gives me a number of 30% coverage, although it is clearly around 50-60%. It must be some default setting that need to be changed, but I don't know which.

Would somebody help be figure out how to calculate area coverage?

Thank you very much!!!!!
Happy New Year to everybody!
Natasha