The relative intensity-angular distribution of surface reflectance of one kind of non-circular cross-section (NCCS), herein referred to as octagonal cross-section, polyester monofilament and its yarn was investigated. Using a goniophotometer (GP), both the horizontally and vertically mounted substrate geometries were used, with the fiber or yarn at various, but known, tensions. An angle of incidence of $45^{circ}$ with respect to the fiber axis was used for the horizontally mounted fiber. The results indicate that the (specular) maximum appears at the expected surface reflectance angle around $40^{circ}$, with the fiber held straight under applied load. The maximum in the reflection profile for the vertically mounted fiber or yarn, under the same conditions, was around $85^{circ}$. It was also found, as is to be expected, that the calculated luster of the octagonal cross-section fiber and its yarn, using a Gaussian fitting model, was affected significantly by the load applied for both the horizontal and vertical geometries of the substrate. Further, it was observed that the relative reflection intensity of vertically mounted single octagonal fiber/monofilament was higher than that of horizontally mounted one under the same value of applied tension, while it was the reverse for the yarn from these filaments. With increased tension, the shine values of both horizontally and vertically mounted single fiber increased while those of its corresponding yarn decreased. Further, for both the horizontally and vertically mounted fiber, the difference in shine values for the two cases of tension and no tension was large, whereas such differences were much smaller for the yarn.