Designed for researchers and scientists, the measuring capabilities in Photoshop CC are quite powerful. You can measure just about anything and count the number of whatevers in a technical image, perhaps from a microscope or telescope.

If you know the exact size of any element in an image, you can then discover just about anything you want to know about anything else in that image. The key is to set the measurement scale, as shown in this figure. The Measurement Scale dialog box is opened through the Image→Analysis menu or the Measurement Log’s panel menu with the command Set Measurement Scale→Custom.

Drag a known distance; set the measurement scale.
Drag a known distance; set the measurement scale.

In this example, you know that the knee is 2.5 inches wide and that the Ruler tool (nested in the Toolbox with the Eyedropper) was Shift+dragged over 138 pixels. You can, therefore, set the scale to 138 pixels = 2.5 inches. You can add the measurement scale to the image as an editable layer group (visible in the top center) with the Image→Analysis→Place Scale Marker command.

Using any selection tool, you can isolate any part of the image, click the Record Measurements button in the Measurement Log panel (which you open, like any panel, through the Window menu), and you’ll find out more than you ever wanted to know about that particular selection and its content.

In addition to the fields visible in the figure, the Measurement Log can also track (among other things) the height, width, area, and perimeter length of the selection, as well as the minimum, maximum, mean, and median gray values within the selection.

After you have made and recorded all the various measurements you need, you can select all the lines in the Measurement Log (or only a few) and click the third button in the upper-right corner of the panel to export the data for use in a spreadsheet program.