The emissivity of a groove

In order to check the performance of any temperature measuring device, which is dependent on radiation, a standard radiator should be available which possesses a thermal emissivity as close to unity as possible. One way the emissivity may be increased is to use a grooved target and in this paper an estimate is made of the improvement to be expected by using such a target. The variation of the effective emissivity with sighting angle is determined. It is shown that a grooved target behaves as a Lambert surface when the angle of sighting is within a groove apex angle, but that this is no longer true for other sighting angles. The mean effective emissivity over all sighting angles is calculated using the relaxation method for a graphite target having a surface emissivity of 0.64 before grooving. After grooving it becomes 0.77, 0.82 and 0.82 for groove apex angles of 60°, 30° and 15° respectively. The corresponding emissivities when the angle of sighting is within the groove apex angle are 0.78, 0.88 and 0.94 respectively.