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TERMS FOR THE DESCRIPTION OF VISION
The amazing human eye can provide a visual sense which operates to provide the majority of the information being received regarding an individual’s surroundings, and to assist in the lighting specification or the design of lighting solutions, this editorial offers an overview meaning to some of the terms generally in use, relating to that of vision when applied to lighting applications. Each given term below is listed by what is deemed as being its common accepted name for everyday use, with a simplified written explanation, not taken from any legislation or official document.
The visual field is a part of an external scene, that is perceived when an observer gazes at some point within the scene, in being the area or extent of physical space visible to an eye or eyes at a given position and direction of view. It should be stated whether the visual field is determined as that of being binocular, the visual acuity of using both eyes together, allowing easier detection of hard-to-see objects or that of being monocular, the visual acuity of using both eyes separately.
The optimum field of vision without undertaking any head or eye movement, is approximately 30° (+/-15°) in both the horizontal and vertical planes from the centre point of reference for the observer, whereas, the maximum field of vision, without undertaking head or eye movement, is approximately 70° (+/-35°) in the horizontal plane and approximately 60° (+25°/-35°) in the vertical plane, providing a slightly greater angle below the centre point of reference than above the same centre reference point.
This field of vision is deemed as being the extent of space in which objects are visible to the eye in a given position and for visual tasks, the maximum field of view for undertaking such tasks, with eye movement only, is deemed as being approximately 170° (+/-85°) in the horizontal plane and approximately 125° (+65°/-60°) in the vertical plane, where there is a slightly greater angle above the centre point of reference, than below. Increasing the field of vision in either the horizontal or vertical planes can then only be achieved through either head movement or actual body rotation.
Where the first image shows the field of vision for an observer in a standing position, the second image shows that for visual tasks which are desk based, the generally accepted field of vision is approximately 38° (+/-19°), however this is not a fixed value, as some tasks may require a somewhat greater field of vision for them to be undertaken efficiently and effectively.
The visual angle of an object is the angle subtended by an object or detail at the point of observation as a measure of the dimensions of the image created by the object as produced within the eye, where the visual angle is dependent upon the distance between the object being seen and the observer, where greater distances provide smaller angles and lesser distances, larger angles.
Visual sensation is the most important sensation for individuals, as it provides them with the greatest amount of information about their local surrounding environment, with the physical stimulus is light where the eye experiences that light and in providing a visual sensation in response to the stimulation of the sensory experience, thereby providing the awareness of objects contained within the field of view, resulting in the creation of an image within the eye of the observer.
Visual perception is the interpretation of visual sensation, which is the ability to ‘perceive’ what is contained within the surroundings of an individual observer, through light entering the eye and providing the ability of the visualise the structure, patterns, and colour of those surroundings, something which is sometimes termed as being ‘vision’.
Visual brightness is attributed to that of visual perception and visual sensation according to which an area appears to emit, transmit or reflect, more or less light, and in being a subjective assessment of the differences in brightness between two or more surfaces which are seen simultaneously or successively.
A visual task is simply the visual elements of the work being done for the required task, as being a term generally given to an activity which requires visual perception, and which is generally undertaken within a defined space, such as that of an office for commercial tasks or a factory for production line task working.
Visual acuity is a measure of the ability of an observer to detect and recognise shapes and the detail of visual objects at a given distance and is considered as being the metric of visual sensation which relies on the refractory power of the eye regarding the structure of the retina and the lens system, which is defined in both qualitative and quantitative aspects. It is the capacity for seeing distinctly fine details that has very small angular separation, and, is the measure of spatial discrimination such as the reciprocal of the value of the angular separation in minutes of arc of two neighbouring objects, as points or lines or other specified stimuli, which the observer can just perceive to be separate.
Visual performance relates to light which allows a person to extract and process relevant information so as to engage in a range of visual tasks, where the performance of the visual system is measured by the speed and accuracy with which a visual task can be accurately carried out. The installed lighting system should therefore provide sufficient light to support and optimise the visual task performance whilst minimising visual discomfort, with regard to minimising direct glare and that caused by reflection.
Visual comfort is a subjective condition of visual well-being induced by the environment in which the person is located, being different for different individuals, in respect of providing the appropriate visual comfort within a space, where consideration of illuminance is deemed as being the main factor to be addressed and this can be made up from the components of natural light and artificial lighting, however it must be appropriate to both the task being carried out and the requirements of user. Other considerations worthy of note relate to lighting quality, visual contact with the exterior, the ability of natural lighting and the concept of glare and is ensured by good colour rendering and harmonious brightness distribution within the space.
Visual discomfort basically refers to discomfort or pain in or around the eyes, and the underlying causes tend to be poor visibility, where a visual task that has stimuli close to detection threshold contains information that is difficult to extract, and overstimulation where discomfort can result from overstimulation of the visual cortex, resulting in eyestrain, headaches, and fatigue, as well as distraction where the visual system has a large peripheral field that detects the presence of bright moving objects, where ignoring objects that automatically attract attention can lead to visual discomfort. Overall, there appears to be a link between visual ecology and discomfort whereby discomfort arises when adaptive perceptual mechanisms are overstimulated by specific classes of stimuli rarely found in nature.
Visual range is the greatest distance at which a given object can be recognized in any particular circumstances, as limited only by the atmospheric transmissivity and by the visual contrast threshold.
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