How do I automate the calculation the spreading length of a droplet impact

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Challenges

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    I have some images of droplets falling on another droplet. The images are in RGB color and in .tiff format. From the images, I need to calculate the spread length of the coalescence. As the image progresses the spread length varies. I could calculate the spread length of one image by converting it to 8 bit and then to binary. After that, I did the calibration and found out a scale which is 15.5 pixels/mm. After that manually I calculated the spread length. Now how could I automate the entire process so that the spreading length is identified and calculated based on the scale. I have attached some images to make it clear.