Figure 1. Typical Exo Labs camera installation
The Exo Labs camera and iPad app combination makes for an easy to use photo documentation and image analysis system useful in a wide range of applications (Figure 1). However, I recently found a situation where the standard way of calibrating the measurement system didn’t work for what I had in mind.
I wanted to measure the size of mushroom spores expecting the dimensions to be in the 5-10 µm range. I calibrated the system with a calibration slide using the smallest gradations on the slide. I prepared a wet mount of some spores of the common button mushroom Agaricus bisporus and observed them with the Exo Labs camera and iPad app attached to a standard school-grade microscope. The images looked good so I set about measuring the lengths of some spores.
Figure 2. Low resolution measurement of spore lengths
After isolating some fields of spores and capturing images I opened an image in edit mode in the app, zoomed in and started using the measuring tool. To my dismay I realized that I was only able to achieve integer values of the spore lengths (Figure 2). For my application it wasn’t good enough to measure spores to the nearest micrometer, I needed to measure to the nearest tenth of a micrometer. What to do?
I decided to see what happened if I spoofed the measuring system into thinking that the spores were actually 10 times bigger than they actually were, and then dividing the measurements by 10 to get true distances. It worked!
Figure 3. App measuring system calibration set 10x larger than actual
To get sub-micrometer resolution, simply calibrate the system using the small gradations on a calibration slide but tell the app that the distance is actually greater than the true distance by a factor of 10 (see Figure 3). Then, when you measure a small object using that calibration divide the result by 10 to get the correct value, but with higher resolution.
Using the 10x method (that’s what I’m calling it for now) the spores measured somewhere between 60 and 90 µm in length (Figure 4). Dividing each result by 10 produced measurements like 6.4, 6.5, 7.5, etc. (in µm), which were the true lengths of the spores.
It got the job done. Good enough to earn a second prize in the recent Scientific Method Contest at instructables.com. Check it out at Comparison of Spore Lengths in Crimini and White Button Mushrooms.
Figure 4. Divide these distances by 10 to get an accurate measurement to the nearest 0.1 micrometer.
