Abstract: Pore size of woven geotextile is a crucial parameter in filter design. The digital image method is a precise and repeatable technique. In this study, we used illuminance to examine the impact of light source distance and brightness on textile image quality. Images were collected under different illuminance levels, and the changes in pore shape, pore size distribution curve, and characteristic pore size of two different textile specifications were analyzed. The actual average pore size of the geotextiles was compared with the fitted straight line of the average pore size under different illuminance levels. Experimental results show that with the increase of illuminance, the measured hole area increases, and the hole shape changes. The pore size distribution curve moves towards the direction of the increase of pore size, and the measured values of characteristic pore size (d₉₅,d₅₀,d₃₀) increase. The average measured pore size presents an increasing trend, which can be expressed as a linear regression equation. The pore size obtained by the digital image method differs from the actual pore size. The optimal illuminance for the two textiles falls near 9 lx. Therefore, the best illuminance should be determined through experiments to reduce error.

Key words: woven geotextile, illuminance, digital image method, characteristic pore size