The effect of illumination wavelength on the measurement of size distribution of very small particles using a novel imaging based system

This paper presents the results of an experimental investigation into the importance of illumination wavelength on measurement accuracy for an existing digital imaging based particle size analyzer. When the system was originally developed fairly large particles were interrogated rendering the effects of illumination wavelength insignificant, but recent work has suggested that there is an interest in the scientific community in analysing particles down to a few microns in size. At these scales illumination wavelengths and particle size are of similar magnitudes, possibly leading to problems. The work presented in this paper is intended to be a practical investigation into wavelength effects using the existing system. This aim has been met firstly by introducing the basic system, along with some key past results and accuracy, then moving on to explain the wavelength tests and present new results. Two sources of illumination were used - red and blue (625 and 470 nm). Polymer micro-spheres in the 2.5-100 μm range, calibrated to NIST (National Institute of Standards and Technology) standards, were interrogated under static conditions. It is shown that whilst blue illumination does give an increase in accuracy the effect is not marked enough to preclude the use of cost-effective red illumination at these scales.