Oh, my word, we are a third of the way through 2022. The expression – “Time and tide wait for no man” was written by Geoffrey Chaucer, author of the well-known Canterbury Tales. Remarkably, he is often referred to as the father of English literature! We will continue our look at the Visual Functions of the human eye.
Brightness Response
When we enter a room or merely move our head in a known environment, the eye tends to seek a state of equilibrium which suits the general brightness conditions. This involves some photo-chemical action within the eye but it is most affected by the action of the iris in front of the lens. The iris opens as general brightness of the scene is low and closes when it is high. But there is a time lag in this re-adjustment, especially when there is a change from high levels of luminance to low (luminance is the physical or photometric correlation of the psychological sensation of brightness). This is an important aspect of lighting design, particularly in a mobile situation such as when driving a vehicle, because immediate dangers can occur without them being recognized when changing from a bright environment to a dark.
Colour Response
Colour is in the eye of the beholder. Nothing on this planet has any colour. You perceive colour through the wonder of the colour photoreceptors in your eyes. The eye is especially responsive to differences in colour under high levels of lighting where the cones in the eye are active, but it is deficient under low lighting levels when the rods alone are activated. It is important to understand the effect of light and colour on the visual scene. For example, colours are not interpreted identically on the retina of the eye. Some studies indicate that a green object is imaged clearly on the retina, a red object of the same size would be interpreted as being slightly larger behind the retina and a blue object slightly smaller in front of the retina. This explains the psychological interpretation of warm colours appearing to advance and cool colours to recede and diminish in size. In some cases it may be important to take into account that 8% of men and 0,5% of women have alternative vision (previously known as colour blindness which today is known to be untrue), the most common being the difference of the red and green sensitivity.
Effects of Glare
The visual response to brightness depends on how the light is distributed over the retina. When there are large differences in the brightness in the visual field at the same time, the greater excitation of a portion of the retina can inhibit the performance of other areas. When this does occur interpretation of lower brightness detail can be seriously impaired. This condition can vary from total disability when, for example, a bright car headlight is shining directly into the eye, to discomfort where the background to the object we are viewing is substantially brighter than the object itself, for example, black printing on glossy white paper.
Detection of Movement
Our eyes are very sensitive to movement in the peripheral area of our vision. These areas are outside the focal cone of our vision but any movement, flicker or changes in intensity will be immediately perceived. Such vision is therefore of extreme use to an individual to maintain a sense of general orientation and relationship to the dynamic activities in the eye in front of and to the sides of his position. This is extremely important for example, when crossing a roadway and the need to be aware of any approaching vehicle from the side and where moving machinery occurs in a work situation.
Next month I will cover the topics of “The ageing Eye”, “The Visual Field”, and “The Fundamental Factors of Vision”. Whilst the magazine features lighting, if anyone who is remotely involved in lighting does not know about the topics that we have and will cover, they will not successfully complete lighting design or lighting projects.
I am prepared to assist any of the readers and give basic advice about lighting to minimise their risk of non-compliance. Until the next episode in the fascinating journey into light and lighting next month, stay safe!
Enquiries: www.bhaschooloflighting.co.za
