The ability of individuals to see or not see particular objects have been determined in the past through experimental investigation. Visual ability is dependent on certain critical, environmental factors. Among these are:
◊ Contrast - The ratio of the illuminance or brightness of the object in relationship to the luminance or brightness of its background.
◊ The size of the object and the corresponding size of the image in relationship to the person seeing it.
◊ Brightness of the object alone.
◊ Background Illumination - The amount of ambient light provided on the object and the viewer.
The inter-relationship of these factors can be seen in Figure 1. Generally, this graph shows that small targets with little contrast to their background are difficult to see or invisible to individuals. Conversely, objects that are highly contrasting with their background can be seen quite easily. A common example which might help to illustrate this phenomenon is the black text of this article on the white background of this page. The text offers a very high level of contrast, despite its small size, with the background that would allow it to be seen in quite low ambient lighting. I however, the text in this article on the white background of this page. The text offers a very high level of contrast, despite its small size, with the background that would allow it to be seen in quite low ambient lighting. If, however the text in this article were light grey it would be difficult to see and, perhaps, even invisible under poor ambient lighting conditions. If, however, the size of the light grey text were increased, it would be more visible under the same poor ambient lighting conditions.
The instrument shown in Figure 2 is a light meter or spot meter. It provides an approximate 1 degree target field that measures the level of luminance of a target. When using this instrument, one would measure the brightness of the adjacent background. This device would be useful for field measurements of a sign illuminated by headlights with a light background. It would, however, be difficult to use this device to measure the difference in luminance of the test on this page with the page itself as the targets would be too small.
The height and size of an image or target as seen by an individual can be determined by measuring its actual size and its distance from the viewer. Using the linear relationship of target size to viewing distance, it can be determined if it would fit in the "no seeing" or "clear seeing" area of the graph shown in Figure 1.
Other important factors that may be useful to consider, especially during nighttime driving, are the age of the individuals as older people require more time and do not adapt as effectively to dark conditions. As well, the amount of time spent at night influences our ability as there is a time over which optimum visual ability occurs with conditioning to sight.
Sometimes, our clients are involved in matters where the concern was, whether an object would be detected. Such objects include warning signs or warning lights, and automobiles. Other times, the concern is that even if these warnings or warning lights had been provided, would they have been visible to individuals given the physical dimensions of the warnings and the environmental conditions at the time of the accident.
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2