Here we are again, another month into the year 2022 and about to reveal what lighting topic awaits you.
I recently read an interesting quote: “Part of what drives us to explore and discover is the intangible: expanding our horizons, feeding our curiosity, finding the unexpected things, and trying to answer those profound questions discussed in previous chapters”. We will continue where we left off last month with our look at the Visual Functions of the human eye.
Effect of Ageing
Unfortunately no part of the human body remains the same as at the time of birth. The eye particularly changes over the life of a person and it is important to recognize this. Age has many effects on the eye. Opacity increases which affects the light transmittance through the eye itself. Typically if we accept 100% transmittance for a person of 20 years of age this can reduce to 50% at the age of 55 and to only 25% at the age of 75. In the same way sensitivity to detail also deteriorates although to a slightly lesser extent. Colour vision becomes slightly modified by yellowing of the lens and central portions of the retina which results in a decreased sensitivity to the blue region of the spectrum.
Changes in the ability to focus the eyes on an object also occur during ageing. Generally the ‘near point’ of vision recedes, which can be detected by the change in the distance at which a book is held to read, as a person ages. The eye also requires longer to adapt to changes in luminance and, probably most important, there is reduced resistance to glare.
As we cannot write off our population when they reach the age of 50 or 60, or 75 for that matter, we must recognise these facts and design our lighting installations accordingly. We cannot set standards for only the younger users but must allow as many people as possible to detect and interpret their visual needs within an adequate margin of safety. It is interesting to note that from an investigation carried out in the USA, the average age of the total population is 57 years.
The Visual Field
Accepting the visual limitation and reactions of the human eye as explained above, we need to understand the human eye and we need to understand the visual responses which can be created by light and colour.
Brightness contrasts in lighting design can create a sense of visual direction and focus in space. A great variation in contrast will create fixed focal centers and thereby generate local attention and even increases circulation of people anxious to see each and every highlighted point. On the other hand, if we produce a more diffuse or low contrast environment all surfaces will tend to become equal in emphasis which will encourage more casual or loosely controlled circulation which will allow individuals to select their own points of interest.
Armed with this knowledge the lighting designer can greatly influence the appearance of a room or an individual’s surroundings. By varying brightness, movement and colour contrasts, it is possible to create suitable modelling of sculptures and architectural features. It can create specific atmospheres of warmth or coolness or it can create moods of relaxation or activity. On the other hand if the positioning and focusing of the light sources are not properly controlled visual clutter can be created. Under these conditions the light sources would be conspicuous not only by their number but also by the contrasting brightness of the source and the controlling reflective surfaces as well as light patterns on or near the object being illuminated.
This aspect of lighting will be amplified later in this course when actual installations are discussed.
Fundamental Factors of Vision
In order to understand the requirements for rapid and accurate sight, it is necessary to recognize that there are four fundamental factors. These are:
- The size of the object to be seen
- The visual contrast between the object being viewed and its background
- The brightness of the object itself
- The time available to see the object
Theoretically if all the above factors were equal we would be at the visibility threshold, i.e. just able to detect the object. However, under most night sight conditions this circumstance is unlikely to occur. To increase visual acuity into the supra-threshold region it will be necessary to increase at least one of the visual factors. Sometimes we would be restricted to achieve this as one or more aspects may be beyond our control, e.g. the time available to see the object. In order to provide the correct type or quantity of light the designer must therefore be able to analyse the visual requirements and design the lighting around the selected factor to achieve the most economical installation. This is particularly applicable under low levels of lighting such as on roadways.
These aspects will also be considered in more detail when we deal with actual lighting design.
Next month I will begin a new series essential to good practise in lighting design. That is the subject of Standards and Compliance. During this series, I will address South African National Standards (SANS) versus the best International Standards covering the same topics and how I will reveal the shortcomings of our SANS. This is one of those not to be missed topics.
Once again, I am always 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: phil@bhalighting.co.za
