Tongue diagnosis is an important part of Chinese medicine diagnosis, and it is a common diagnostic tool to explore the operation of qi, blood, yin and yang in the body and changes in the function of internal organs by using the state of the tongue and tongue coating. Tongue diagnosis has been recorded since the Yin and Shang dynasties, and has developed into a systematic system of “identifying the tongue and using medicine”. The four diagnoses together, tongue diagnosis occupies an important position in the identification system of Chinese medicine. “The liver is …… and the veins are connected to the tongue.” “The heart is open to the tongue, and the tongue is the seedling of the heart, which must influence the changes of the essence and reflect on the tongue. “The disease of the meridians and viscera, Ying, Wei, Qi and blood, Yin and Yang, cold, heat and deficiency are all shaped in the tongue. “The tongue is outside the spleen, and the moss is produced by the gas” “Foot Taiyin spleen meridian …… connects to the tongue and disperses under the tongue” “Kidney foot Shaoyin’s pulse …… holds the tongue”. The essence of the internal organs can reach the tongue through the meridian connection and play its role in nourishing the tongue and maintaining the normal functional activities of the tongue. “The lesions of the internal organs, qi and blood, yin and yang, cold and heat, deficiency and reality are all shaped in the tongue.” Look at the tongue can understand the vitality of the internal organs, the depth of the disease, the nature of the evil, the progress of changes in the disease, to guide the clinical prescription of drugs and determine the prognosis of the disease is of great significance, which is the clinical significance of “identify the tongue with drugs”. In fact, the key point of “looking at the tongue” is “looking”, which is a practical operation with certain subjective bias and cannot be understood at a glance like blood routine and biochemical indicators, so the operation specification of looking at the tongue is particularly important. The correct operation specification is to observe with the naked eye in natural light or under fluorescent light, excluding interfering factors such as food, medicine and moss scraping. Here we analyze the accuracy of light on the tongue. The method of tongue observation under sufficient outdoor light and indoor light of different colors is adopted, which are: outdoor natural light; indoor without light (relatively dark light); with light on (relatively suitable light); with white light under light, and 5. five scenarios with yellow light under light. Due to equipment limitation, only ordinary cell phones can be used to take pictures of the tongue surface and subungual Due to the limitation of the equipment, only ordinary mobile phones can be used to take pictures of the surface of the tongue and the subungual ligatures respectively. The following is a description of the tongue images taken in the five scenes, and three clinicians were consulted. The volunteers’ tongues are normal and are not analyzed here The tongue color of the five is basically the same under natural light, indoor and white light exposure, with a greater difference under indoor open light and yellow light; the tongue shape of the five differs little, but is clearer under natural light; the tongue coating appears less without open light, where the coating is clearer under white light; the sublingual veins are not visible under yellow light and normal under white light, and the rest of the sublingual descriptions differ little and It was clearer under natural light. In terms of tongue color, the brightness and color of the light will have an effect on the observation of the tongue. The luminance affects the visualization of color, as outdoor light is relatively dark compared with indoor light, and the tongue color appears darker; the luminance increases the clarity of color, as outdoor light is brighter than indoor light, and the observed tongue color is naturally more specific, which includes the observation of the color of each tongue partition; due to the reflection, the concentrated light unexpectedly increases the interference compared with the diffused light, for example, only a light red tongue is seen under white light, while natural light can be observed in the tongue tip red; tongue shape, with the impact of the brightness of the light on the meticulousness of the observation, the light is appropriate, the observation is clearer, the natural tongue texture, tongue edge teeth marks will be clear at a glance, see more, describe more. Yellow light can make people visually deepen the sense of color, tongue light red by the yellow light of the observer looks reddish, white light observation tongue color more clear, but need scattered light, the purpose is to enhance the brightness. Secondly, in terms of moss, due to the influence of light and shadow, the contrast between places with shadow and places without shadow will make the thin white moss more obvious, so the moss color of indoor open light and indoor torch light is more obvious, compared with the two, the brightness is enhanced, making the torch light to see more clearly. The moss color of the yellow light bulb becomes ambiguous under the light color. Finally, the sublingual veins, the visual effect of yellow light affects the observation of color, the contrast of light intensity can strengthen the depth of the veins, so indoor observation of the sublingual veins more clearly, and even their lines of tortuous can also be observed, while the sublingual veins under yellow light is not seen. This shows that “color is the objective existence of color light as the main body”, “there is light to color, light arises from the light source.” Light is vital to the accuracy of looking at the tongue, we usually look at the tongue should be selected under natural light, such as the situation does not allow or night, you can choose parallel light for the observation of tongue color, in order to make the tongue tongue shape observed more detailed and subtle, you can choose to use a white flashlight.