Microscopy of surfactants
This researcher wants to "analyse images from optical microscopy of surfactants. Those images have a hair of known thickness in order to obtain an idea of the real dimensions involved... The hair's thickness is 0.045 mm."
I did some preliminary work with Pixcavator. First you need to convert pixels into millimeters. I re-analyzed the image. I had to add a black border to the image. This is a way to capture a light object that touches the edge of the image (another is to invert the colors).
The table shows that the thickness of the hair is 23 pixels. So, 23 pixels = .045 mm, or 1 pixel = .045/23 = .002 mm. This is the conversion factor (see calibration). I inserted it at the top the output table. On the right you see two new columns - they are the sizes and the perimeters converted into mm.
One would have to repeat the analysis of the hair thickness for all images though, unless all pictures are taken at the same distance and the hair is the same. If these images are taken from the same distance, you need to do the calibration only once and then use the same conversion factor for all of them. Another good news is that the conversion procedure will work equally well regardless of 10X, 5X etc magnification that comes from the microscope.