Panoramic illuminance refers to the average luminous flux per unit area received from all directions in three-dimensional space at a given measurement point. It plays a significant role in lighting quality assessment, particularly in evaluating the overall illumination received by objects and the general lighting level perceived by individuals. However, there has been a lack of simple and effective practical measurement tools. This paper proposes a method for measuring panoramic illuminance based on a regular tetrahedral illuminance meter. This method involves measuring the illuminance on the normal directions of the four faces of a regular tetrahedron, allowing for straightforward calculation of the panoramic illuminance value. This study begins by reviewing existing methods for panoramic illuminance calculation and measurement, followed by a theoretical demonstration of the proposed approach. Subsequently, over 120 000 measurement simulations were conducted using software based on 2 233 indoor and 205 outdoor high dynamic range panoramic images from the Laval database. Results show that approximately 96% of the measurements had an absolute error within 5%, with an average absolute error of 1.7%, validating the method′s accuracy. Moreover, the measurements remained stable under different orientations of the tetrahedral illuminance meter. Finally, we proposed a simple construction plan for the tetrahedron illuminance meter and verified it in four real lighting environments. Experimental results indicate that tools based on this measurement theory are easy to construct, with a straightforward measurement process and calculation, enabling a relatively accurate measurement of the panoramic illuminance values in lighting spaces. The method has opened up a new approach for the measurement of panoramic illuminance in practice.