Measuring Leaf Area with Adobe Photoshop CS3

     Author: Zach Jarou

This video tutorial demonstrates how to use Photoshop to measure leaf area, an important metric for use in plant growth analysis. The purpose of this tutorial is to demonstrate how to use Adobe Photoshop to determine the exposed surface area of plants leaves for use in growth analysis research. Let's get started.

Using the marquee tool, located to the left of the image viewing area, make a rough selection of the area of interest. In our case, we will be selecting three plants of the same phenotype for which we would like to obtain an average size. Next, open the color range tool, located under "Select" on the Main Menu. Once the dialogue box is open, ensure that the radio button is on "Selection" and the Selection Preview drop-down is set to "Black Matte." Begin by making an initial selection of the plant using the eyedropper tool in either the image viewing window or the color range dialogue box. Notice how the selection gradient changes depending upon which part of the plant is selected first. To capture the full area of the leaf, hold down shift & click the darker areas of the leaves. Notice how the eyedropper has changed to include a plus sign. The selection area can also be increased by dragging the "Fuziness" bar to the right. Do this slowly so that you can continue to add hues along the way. Finally, to ensure no tones are left out, increase the fuziness as much as you can, until the point where unwanted regions of the image are added to the selection, such as the growing container or soil artifacts. To finalize the selection, simply click the "OK" button. Now that the leaves have been selected, it is necessary to find the number of pixels within the selection. This can be done using the histogram located in the palettes toolbar on the right hand side of the screen. Ensure that the view is in "Expanded" mode with the "Show Statistics" option selected. The number of pixels can be found in the bottom left hand corner of the statistics display. In this case, there are ______ pixels selected. Occasionally, the histogram will display an error message, indicating that it needs to be regenerated with uncached data. This is easily remedied by selecting the refresh above the histogram & must be done to ensure that an accurate pixel count is obtained. Finally to convert pixels into physical units, we must measure the number of pixels of an object within the image for which the real-world size is already known; in our case this is a white block with a size of 4 square centimeters. By selecting this square with the marquee tool, we can see that it is _______pixels.