Take image of ruler with different magnification of your microscope.

then transfer measurement to equivalent from micron in each magnification.

thank you, it is a good idea.

I agree, you can use both: take a photo of micrometric slide at the the magnification you need or with imajeJ is also an easier way.

Thank you Mohamed, thank you Francesca for your valuable information.

 is it necessary to use ruler bar for microscopic image..?? why.???