The human eye's perception of natural light has different feedback at different wavelengths. The human eye produces the same visual effect on monochromatic light with different wavelengths and has different radiation powers. In the visible spectrum, the human eye is most sensitive to the middle part of the spectrum (yellow-green), and the closer it is to the two ends of the spectrum, the less sensitive it is. Under the condition of causing the same visual response, V (λ) = P (555)/P (λ) is defined as the visual function of wavelength λ if the optical radiation power required for light with wavelength λ is P (A) and the optical radiation power required for light with wavelength 555nm (the human eye is most sensitive to this wavelength) is P (555). The response of photodetector usually differs much from the visual function, so a visual function correction filter is needed to correct the spectral response of photodetector, so that the response of photodetector conforms to human vision. Visual function filter is a key element that can be added in front of the detector to correct the spectral response of the detector, so that the response of the detector is related to the light intensity perceived by human eyes, and can be used in the detector of testing instruments such as illumination, brightness, luminous flux and luminous intensity. We can match the selected detector with the appropriate filter to correct the visual function according to the user's needs.