Feasibility of detecting ‘tracheid effect’ on tropical hardwood (light red meranti, Shorea spp.) using 650 nm (red) and 808 nm (near infrared) lasers, utilizing industrial colour camera

Abstract

Tracheid effect is a well-known phenomenon, widely studied and industrially applied for softwoods. However, studies of this effect on Malaysian tropical hardwoods are scarce to non-existent. This paper investigates the light scattering characteristics of light red meranti (LRM) (Shorea spp.), Malaysia’s most common commercial timber species. Diode laser dot projectors at wavelengths of 650 nm (red) and 808 nm (near infrared, NIR) were used, imaged by a Hikrobot MV-CE200-10UC colour industrial area-scan camera. The laser dots on the wood were imaged at 30° intervals. Eccentricities and angles of the ellipse generated by the tracheid effect were obtained using principal component analysis (PCA). Red channel of the images scored the best results for both NIR and red laser for LRM. Angle detection accuracy and linearity for NIR laser on milled LRM surface (root mean squared error, RMSE = 2.892°, coefficient of determination, R² = 0.9996) were comparable to those of pine (RMSE = 1.5330°, R² = 0.9998). For rough sawn LRM surfaces, NIR performed poorly with large errors (RMSE = 11.4948°, R² = 0.9887). The red laser fared mediocrely with LRM milled surface (RMSE = 7.8922°, R² = 0.9947) compared to pine (RMSE = 1.1582°, R² = 0.9997), and was practically unusable on rough LRM surfaces. Approximation of grain directions using NIR lasers on rough surfaces, or 650 nm red lasers on milled surfaces, is also possible albeit with substantially degraded accuracy. This study informs future studies of the viability of using tracheid effect to research on related characteristics and properties of LRM using visual methods.