This DIY Instrument Can Affordably Measure the Size of Red Blood Cells

Most of us just take it for granted that scientists and lab techs can quantify our physiological and anatomical traits. For example, you get some blood drawn and a couple of days later, they tell you about anything abnormal. But how do they actually determine that? And how do they do it so quickly and affordably that it is practical on a mass scale? If you’re curious, then you’ll want to see HaloPi, which is a DIY red blood cell measurement instrument designed by DaniloR29.

Many different conditions and diseases can result in red blood cells that are abnormally large (called macrocytosis) or abnormally small (called microcytosis). So, being able to measure the size of a person’s red blood cells is pretty useful. You can do so with a powerful microscope (1000x magnification seems ideal), but that process is time-consuming.

A more economical method works using light diffraction and HaloPi makes that method accessible. It works by shining a laser through a slide with a thin blood smear. When the laser hits a red blood cell, it diffracts. That diffraction can be seen as a kind ring-shaped “halo” on a projection screen and the halo’s size is inversely proportional to the red blood cell’s size, so a large red blood cell projects a small halo. A simple formula that takes into account the attributes of the laser and projection can yield an approximate average red blood cell size.

HaloPi is an affordable and accessible way to use this diffraction technique. The design includes mostly 3D-printed parts for mounting an off-the-shelf laser pointer and a projection screen.

More importantly, HaloPi includes software to perform the analysis for measurement on a Raspberry Pi. Written in Python, that provides a GUI for capturing a clear image of the projection screen. It then uses computer vision to process that image and analyze the halo, popping the relative measurements into that formula to spit out an average red blood cell size.

Notice that I’ve said the measurements are for the average size of the red blood cells in the sample. That is the primary limitation of this method. Results also depend on the sample’s preparation, so there is room for error there.

But despite the limitations, HaloPi is perfect for student experiments and maybe even citizen science.