Fast, high precision autofocus on a motorised microscope: automating blood sample imaging on the OpenFlexure Microscope

TL;DR

This paper introduces a fast, precise autofocus method for a low-cost, motorized microscope, enhancing automated blood sample imaging crucial for malaria diagnosis in resource-limited settings.

Contribution

It develops and evaluates new autofocus techniques for the OpenFlexure Microscope, improving speed, accuracy, and reliability in high-magnification imaging.

Findings

Autofocus speed increased significantly

High precision achieved in focus detection

Reliable autofocus verification method implemented

Abstract

The OpenFlexure Microscope is a 3D printed, low-cost microscope capable of automated image acquisition through the use of a motorised translation stage and a Raspberry Pi imaging system. This automation has applications in research and healthcare, including in supporting the diagnosis of malaria in low resource settings. The plasmodium parasites which cause malaria require high magnification imaging, which has a shallow depth of field, necessitating the development of an accurate and precise autofocus procedure. We present methods of identifying the focal plane of the microscope, and procedures for reliably acquiring a stack of focused images on a system affected by backlash and drift. We also present and assess a method to verify the success of autofocus during the scan. The speed, reliability and precision of each method is evaluated, and the limitations discussed in terms of the end users' requirements.