Fundamentals of Glare

Glare
In simplistic terms, glare is any light source either direct or reflected that reduces short term or long term visual performance. It is the equivalent of noise in an audio system.
Glare results from an extremely complex set of interactions between the lighting, both electric and natural, the surroundings and the visual system of the body. Although there is a general understanding of the mechanism of glare and some algorithms for prediction of glare, the prediction of the models often, do not agree with the perception of the occupants.
The fact that glare cannot be reliably calculated does not diminish its detrimental effect on the visual environment, nor its need to be addressed in the design process.
At the same time it is not sufficient in designing a lighting installation to say that, as glare is bad, we should minimise it as much as possible. There have been many disastrous installations predicated on this philosophy. Many attempts to eliminate glare have resulted in bland spaces that lack visual interest, sparkle, and areas for visual relaxation. The end result can be uninteresting and often extremely unpleasant spaces to occupy, even for a short period.
Glare control has to be a balance between maximising visual performance and visual comfort. By visual comfort I mean the user satisfaction, rather than a calculated criterion that models some of the glare in the space.

Image Courtesy   lrc.rpi.edu
Glare is generally a combination of five things:
1. The brightness of the glare source. This can be a reflected light source as well as a direct one.
2. The brightness of the background. Note that brightness is relative. As the background luminance approaches the luminance of the glare source, the impact of the glare source is diminished.
3. Location. Generally, the closer a glare source is to the direction of view, the greater the impact.
4. The adaptation level of the eye. The eye is capable of seeing an extremely wide range of luminances, but not concurrently. The eye changes its sensitivity to adapt to the ambient luminance distribution. It is not simply the contraction of the pupil, as this is only a short-term phenomenon. The longer term adaptation is achieved by the change in the sensitivity of the receptors in the retina. The speed of adaptation depends on the incremental change in luminance and the current level of adaptation. An example of this phenomenon is driving into a covered carpark from bright sunlight. Initially, the carpark looks very dark, however once your eyes have lowered their adaptation level, vision is returned. Many glare situations arise when the luminance range in the field of view is too great for the eyes to handle concurrently.
5. The relative size of the object is also important.

There are many types of glare. The relative importance of them will depend on the requirements of the lighting task and the environment.

Disability Glare
As the name implies, it is glare that has a disabling effect on visual performance. This is generally due to the presence of a light source that is significantly brighter than the rest of the field of view. The result is similar to the effect of the sun in a camera lens. The small amount of scatter in the lens is sufficient to wash out the contrast in the remainder of the image. The extreme case of this is the effect of an oncoming car at night on a country road. For a short period of time the whole visual field goes white, because the scattered light is well above the current adaptation level of the eye. This is followed by a period of total blackness as the eye’s sensitivity has risen and is no longer appropriate for the dark environment.



A similar effect is often present in interior lighting. The effect may not be as extreme, as the background luminance is higher. In a mild form, it will scatter light in the eye, effectively reducing
 the contrast of the field of view and causing the eye to adapt at a higher level than the task requires.
Traditional glare theory predicts that the glare will diminish exponentially with the separation from the direction of view. Although there is general acceptance of this, there have been consistent complaints, particularly from people performing screen based tasks, of glare from overhead luminaires that are well outside the area that should contribute to glare and often outside what would normally be considered peripheral vision. Recent studies have shown glare effects resulting from light incident on the eye almost perpendicular to the field of view 4,5. This appears to be flaring of the cornea or internal refracting in glasses. This has significant ramifications for much of the common practice for glare control in offices.

Comments

Post a Comment

Popular posts from this blog

Elements of Building Management Systems

Image
Audio Video System Integration Board Room / Meeting Rooms / Auditorium Public Announcement System Lighting Automation Energy Monitoring and Conservation Curtain Automation Fire Security Systems Surveillance Solutions Access Control Systems Power Trunking Water Management System EPABX, Video Door Phones, Intercom Elevator Control Visitor Management Advantages of Building Automation System (BAS) Centralized control Remote support Reduced complaints Staff optimization Seamless integration Energy savings Increase level of comfort Improved security BAS powers Flexibility & Convenience No location constaint Visualization Parameterization Alarm Monitoring Room Automation Data Point Lights Blinds / Curtains Temperature Sockets / Appliances Motion and Occupation Smoke Doors and Windows contacts Access Control Camera
Read more

Light Color and Color Rendering

Image
True Colors do not exist. People tend to judge colors under what they consider to be natural or true lighting conditions, often  mistakenly using daylight for this purpose. but the colors seen under daylight on a Sunny day can be widely different to those seen on a day with overcast sky. For the spectral distribution of daylight is not constant, but is continually changing from hour to hour and from season to season. The correct thing to do, of course , is to assess the colors under the same type of lighting as that existing in the area where they will be finally seen. For example, an evening dress would be chosen under incandescent lighting, for this is the sort of lighting most likely to be employed at the evening function where it will be worn. Similarly, the color scheme for an office should be preferably be chosen under lighting identical to that which will later be installed there. Image Courtesy Philips Spectral Distribution The spectral distribution of the light de...
Read more

Light & Vision

Image
VISION A simple definition of LIGHT is visually perceived radiant energy. This visible light is a small part of the ELECTROMAGNETIC SPECTRUM and ranges in wavelengths from 380 nm to 780 nm. Light is what energizes our visual system. Light reflected from objects  into our eyes enables us to see.Lightwaves enter the human eye and send signals along the optic nerve.Light enters the eye through the pupil, is focused on the retina, and is transmitted to our brain via the optic nerve. A significant part of our brain is dedicated to the processing of visual information. Of all creatures on this earth, the human visual system is the most impressive. Birds may have sharper vision, felines may be more sensitive to motion, and horses may have almost a 360-degree field of view, but nothing can beat the combination of features humans possess. The human eye allows good depth perception, excellent color perception, reasonable motion detection, and great visual acuity. PERCEPTION...
Read more