you can use simple scale and measure the width and length

hi Emmanuel, 

my team  has developed a simple software for measuring irregular area using images as input.

please have a look at our paper at:

QIMS: An integrated image measurement and management system for engineering and scientific application, 10th IEEE  International Colloquium on Signal Processing & its Applications (CSPA), 2014 , Malaysia.

its quite simple to use.

regards

-hanif-

Yes, you will need a scale bar first. How are you creating the images for your leaves? I'm assuming you are collecting leaves from either the ground or off of the plant, otherwise LAI might be your better measurement.

Does your department or greenhouse have an area meter -- a Li-Cor 3100? That would be the easiest. Of course, images have some advantages as they can be saved and used in multiple ways. If you are using a camera then just make sure that your ruler (scale bar) is on the exact same plane as the leaf you are photographing or your measurement will be off. In the past, I have used a flatbed scanner with a ruler on it -- fresh leaves are best (they can be kept in a refrigerator/cold room for a few days to prevent water loss) but dried, curled leaves can be rehydrated by soaking them in warm water for ~2-5 min and then gently flattening them out. Depending on the size of your scanner, you can fit more than one leaf in an image as long as they don't overlap (you lose area in overlapped places). This is useful for subsamples of multiple leaves, bundled leaves (e.g., pine needles), and compound leaves. I would suggest including all petioles as they are technically leave parts.

If you are measuring multiple leaves (or one very large leaf) in the same image: 1) set the scale and then remove it by cutting it out (so you don't end up measuring it too), 2) convert the image to grayscale or binary, 3) adjust the threshold, and 4) measure area by Analyze Particles. Another option is to adjust the Color Threshold instead of taking it through the binary conversions and measure individual leaves in an image using the ROI Manager (see links below). The variability in dried, spotty, or damaged leaves can give you some problems when thresholding so come up with a set of rules as to how you will handle all of your leaves (to decrease subjectivity).

https://youtu.be/WIrY-egQSS8

https://youtu.be/idsrN2NKtJ4

You can easily measure leaf area with a white background and a scale.

Open the image, measure the scale with the function line. Next, go to Image > Adjust > Color Treshold and make changes to Hue and Saturation so that you  will obtain a white background with the leaves in red. Then with the tool "wand" select the leaves, one by one, and type Ctrl + M to take measures. You can select for the ImageJ to give you the area and the largest vertical and horizontal distance.

At my article we've used it with Basil leaf area:

Basil leaf area by allometric relations. Journal of Medicinal Plants Research (2014).

And also in my Master's thesis.

Article Basil leaf area by allometric relations

I've developed an iOS app for measuring leaf surface area. See my project below.

https://www.researchgate.net/project/Leafscan-iOS-app-for-measuring-leaf-surface-area

If you want to measure leaf area, I recommend LeafByte. It's a free and open source app for iPhones and iPads. It measures leaf area and herbivory levels much more quickly than ImageJ and related software. It also automatically saves your data for you in a spreadsheet.

You can download it in the app store or read more about LeafByte at the website below. You can also check out the FAQ page for ways to measure leaves non destructively and in the field.

https://zoegp.science/leafbyte

https://zoegp.science/leafbyte-faqs

Disclaimer-I was involved in making this app, but since its free I make no money from its use.

This blog post has a guide for using Image J

https://rookieecologist.wordpress.com/2016/11/21/how-to-measure-leaf-area-

in-imagej/

Here is an Adobe photoshop tutorial

https://www.youtube.com/watch?v=E3O-V6WLw0g