The amount of transmitted light can be adjusted by changing the optical density at equal intervals. It can be used for calibration of concentration meter or colorimeter. 11 steps of equal spacing optical density are arranged on a piece of glass, and the approximate optical density value of unknown optical density samples can be known by comparing with eyes. Because it is chrome-plated film, it can also be used to adjust the light quantity of laser beam, and the optical density will not change with the change of wavelength in the visible light region.