## 1 Introduction  
Grayscale is an important feature which characterizes the lightness and darkness of an image, and grayscale images are widely used in medical, remote sensing, and industrial inspection fields [1; 2; 3; 4]. Color images can be converted to grayscale by weighting algorithm as required [5]. In recent years, it has been applied in the technology of image segmentation, target recognition, and machine vision based on differences and discontinuous changes in image grayscale [6; 7; 8]. Therefore, the standardization of grayscale display is essentially significant for image communication, transmission, and terminal reading.  
The traditional grayscale standardization method is making or selecting a gray image as the display reference and comparing the obtained gray image with the reference to requantize the fixed value [9; 10; 11].  
Obviously, this is limited by the complexity and fineness of the selected gray image for reference. In 2009, a grayscale standard display function (GSDF) was laid down in the Digital Imaging and Communications in Medicine (DICOM) 3.0 Standard by the American College of Radiology--National Electrical Manufacturers Association (ACR-NEMA), which is used to achieve quality control of medical imaging equipment [12]. Specially, it provides a standard model for image display in the luminance range of 0.05-4000 cd/m\({}^{2}\) based on human visual characteristics. However, it cannot meet the needs of a wider range of luminance nor current application areas. Notably, the implementation of the standard model is not mandatory, and it is not traceable [13; 14; 15].  
Besides DICOM, no other research discusses the standard of grayscale, especially the traceability of grayscale. The related works mainly study in two directions. One is the method to identify and distinguish the grayscale from a picture [16; 17; 18; 19], and the other is an improvement on monitor (mostly LCD) performance compared with GSDF based on hardware or PLC [20; 21; 22; 23]. Most research uses grayscale as a relative parameter only, without focusing on the standardization of grayscale. The traceability is a faraway work for them. However, the standardization and traceability of grayscale is crucial for the consistency of image transmission, image display on different instruments, image identification, etc. This research begins with the standardization and traceability of grayscale as it is a key step in image processing, including medical, military, and remote images.  
Establishing equipment that realizes the grayscale standard display with traceability is the key to the grayscale standardization. Considering that grayscale is a discrete value, a digital light process (DLP) technology is used to solve this problem. A digital grayscale generation equipment is established by DLP, which can achieve the metrology of two-dimensional grayscale information and the calibration of a monitor by a GSDF program built into the computer. Furthermore, grayscale-to-luminance conversion measurement is performed at the terminal. Experiments show that the standard uncertainty of the measurement by this equipment has a range of 0.26-65.55 cd/m\({}^{2}\) at the luminance range 0.16-4000 cd/m\({}^{2}\).