A visual phenomenon that causes discomfort or reduces the ability to observe details or targets due to inappropriate brightness distribution or range, or the existence of extreme brightness contrast, is collectively referred to as glare. If human eyes are exposed to glare, they will feel irritated and nervous. Working under such conditions for a long time will cause boredom, irritability and fatigue, which will have a great impact on people’s production and life.
Glare can be divided into discomfort glare and disability glare according to the degree of impact on vision.
(1) Uncomfortable glare
It only causes visual discomfort, which can cause distraction, but does not necessarily reduce the visibility of visual objects in a short period of time. This kind of glare is called discomfort glare.
(2) Disability glare
Since the light source is located close to the line of sight, the edges of the retinal image appear blurred, thereby hindering the observation of nearby objects and reducing the visibility of visual objects. Suppressing it sideways also makes visibility of these objects worse. Such glare is disability glare.
Glare is divided into four categories according to the formation mechanism: direct glare, indirect glare, reflected glare and contrast glare.
(1) Direct glare
Direct glare is the glare produced by luminous objects that exist in the direction of the object or close to the line of sight. In the built environment, sunlight passes through glass, light sources in luminous ceilings and lamps, etc. When these light sources are too bright, direct glare occurs.
(2) Indirect glare
Different from direct glare, there is a high-brightness light source in the field of view, but not in the direction of the object being observed. The glare it causes at this time is indirect glare.
(3) Reflected glare
Glare caused by reflections, especially glare caused by reflections close to the line of sight. Reflected glare can be categorized based on the number of reflections and the mechanism of glare formation. It encompasses primary reflection glare, secondary reflection glare, and light curtain reflection.
Primary reflected light refers to the specular reflection phenomenon or diffuse specular reflection phenomenon formed by the reflection of strong light onto the object being viewed due to the surface gloss of the target object. For example, if you hang a mirror on the wall opposite the window, when sunlight enters through the window, a light spot will appear when we look at things inside the mirror frame. This light spot is actually the image of the side window.
Secondary reflected light occurs when the brightness of the human body itself or other objects is higher than the surface brightness of the object being viewed, and their reflected images just enter the person’s line of sight. At this time, the human eye will see the reflected image of the person or object on the surface of the object, making it impossible to see the target object clearly. For example, when you stand in a glass display case and want to see the exhibits clearly, you see yourself instead. This phenomenon is secondary reflection glare.
Light curtain reflection is the specular reflection of visual objects, which reduces the contrast of visual objects, making it difficult to see part or all of the object details clearly. For instance, consider a scenario where light illuminates the surface of a document printed on glossy paper, causing a significant portion of the light to reflect directly into the viewer’s eyes.If the text of the article is black and bright, it will also be reflected in the eyes of the viewer. There will be light curtain reflection, making the viewer unable to see the text clearly.
(4) Contrast glare
The reason why people feel uncomfortable is not only light stimulation, but also the brightness of the environment plays a big role. The greater the difference between the ambient brightness and the light source brightness, the greater the brightness contrast, and the easier it is to form contrast glare. For example, if a street light is on, pedestrians will not notice its existence during the day; but at night, pedestrians will feel that the street light is very dazzling. Because the background brightness at night is very low, the street lights appear very bright, forming a strong contrast glare.
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