Those of us that have not thought about the process of seeing might be surprised at how complicated vision is. Most of the time we take vision for granted but when we when we look at a computer screen, indeed anything, each of our eyes is invoking a total of ten muscles and six cranial nerves to position the eyes, and adjust the lens and pupil to accommodate focus on the retina which detects and then processes visual information (one of my areas of research).
Most of the systems involved in our visual pathways are designed to be relaxed when looking at distant objects at optical infinity, technically defined as farther than 6 meters from the cornea or outermost portion of the eye that light passes through. In order to view objects closer than optical infinity, muscles inside the eye called ciliary muscles actually change the shape of the lens to bring objects in focus on the retina. Additionally, each eye has muscles that position the eye and even turn the eyeball inward so that the image in both eyes falls on the same place of the retina and is properly aligned. For more details on eyes, the vision process and retinal anatomy and physiology, see one of the original online textbooks, Webvision.
As with most physiologic processes, the effects of long-term use or even abuse vary depending on the individual. However, given enough abuse, all computer users will experience at least some degree of computer related eyestrain. It is also important to note that this abuse can come from working in graphic design applications such as Photoshop, office productivity applications like Microsoft Office, or even by playing games. Additionally, we commonly think of computer related eye strain as being the province of adults, but children can also be susceptible. In fact, a recent study by UC/Berkeley indicates that as many as 30% of children unduly stress their eyes through the use of computers at school and home.
Therefore whatever you use computers for, be mindful of fatigue issues and potential problems with extended computer use. For the time being, we will set aside musculoskeletal issues of computer related stress and keep our focus on the problems related to computer displays and vision. Visual dysfunctions of extended computer use include symptoms such as headaches, dry or burning eyes, dizziness, blurred vision and difficulties maintaining focus particularly when combined with poor ergonomic work environments. Therefore, any means by which one can reduce unnecessary visual strain will allow us to either work longer or work more efficiently and most importantly, without pain and discomfort.
The problem that we are having with computers and stress injuries is that computers have arrived and advanced so fast over the past twenty or so years that the consequences of their chronic use are only now beginning to be discovered as computers are becoming more widely adopted into our everyday lives. Repetitive stress injuries (RSI) are commonly prevented by the use of ergonomic keyboards and proper positioning of said keyboards and monitors. However, the monitors and display technologies themselves are also the source of many problems, but LCD technology and developments at Apple are helping out here as well.
Recently liquid crystal displays (LCD) have started to become viable as replacements for cathode ray tubes (CRT) both in performance and cost. LCD’s are currently still more expensive than CRT’s, but their cost differential is offset by savings in physical space, electricity, cooling and most importantly, potentially health care costs. Fundamentally, these factors result in increases in productivity both for individuals and business and lower health care costs related to vision related dysfunction which according a survey appearing in a 1992 issue of the Journal of the American Optometric Association, cost nearly $2 billion per year to diagnose and treat eye problems related to vision problems related to the use of computers. This was ten years ago and its fair to say that costs are considerably higher today given the more pervasive nature of computers in society and the spiraling health care costs of the past decade which is another discussion entirely.
We should probably cover some of the technology behind displays to enhance the discussion and make computer related eye strain a little more relevant. To start out with, CRT screens are essentially linear accelerators that we sit in front of. It’s scary to think of sitting in front of something called a linear accelerator, but until all of our CRT computer screens and televisions are replaced by other technologies, we’re stuck having electron guns pointed at our heads. At any rate, as the electrons are accelerated, they pass in between charged plates that deflect the beams of electrons to different places on a phosphor screen lining the back of the glass. When electrons strike the phosphors, they illuminate for a short period of time creating a visual point of light. However, the phosphors decay after a short period of time (milliseconds), so the beam has to be refreshed or passed over the same point again to maintain the phosphor so we can perceive the image. Because different folks have varying visual abilities, we perceive the flicker of the image and the decay of the phosphors at inconsistent rates. Some folks absolutely cannot stand any refresh rate of say, 75Hz or below as it appears the whole screen is flickering and moving to them and they require higher refresh rates. This is where LCD displays begin to show their advantages. With an LCD, there is no refresh rate. Rather, the LCD uses constant illumination to shine through gated transistors and there is no perceived flicker or any variability in focus due to images not being formed on the plane of the phosphor. Additionally, there is no phosphor aging so brightness and color remain constant over the surface of the display as the display ages. LCD’s are not necessarily new technology per se as anyone who has worn a digital watch starting back in the seventies can attest to. Additionally laptops have used LCD’s for some time due to weight and power issues, but the performance of the LCD until recently has not been suitable for desktop use and replacement of the CRT. Current LCD’s rival CRT’s in terms of brightness and response and my current 22″ Cinema Display is actually brighter with contrast and more accurately rendered colors than the high end 21″ CRT it replaced.
So, pure hardware technology aside, the software developed by Apple in OS X is the much larger factor in alleviating eye strain. The issues of presenting information on a computer screen are not trivial and can vary depending upon the type of information presented. Surprisingly, presenting graphic information in many ways is more complicated than presenting text based information, however, OS X includes a number of new features and technologies such as Quartz and now Quartz Extreme that allow for operating system wide support of text anti-aliasing making it easier to spend time in front of the screen for extended periods of time. We first found out about Quartz back in May of 1999 at MWSF, but not many really understood what was to come of it and how important it would become to OS X and the new graphic user interface. Through the implementation of a superset of the .pdf standard, and Quartz imaging in the operating system, Apple has allowed for a resolution independent rendering engine capable of creating and managing graphics and text system wide, with the text anti-aliasing making text displayed on the computer screen more resemble traditionally printed text on paper. This is important as our eyes can more easily focus on printed text on paper due to the smooth appearance with well defined borders, whereas traditional text display on the computer screen becomes ill defined around the edges with sometimes indistinct borders. These differences in contrast and sharpness can make reading text on the traditional computer screen much more difficult than reading text from a printed source. However, the improved text handling in OS X results in a much more printed text on paper appearance alleviating loads on the ciliary eye muscles and thus limiting eye strain.
Given that computer use is a requirement for many of us, aside from using OS X and having a good LCD screen, what else can we do to limit eye strain? There are things that we should all be doing regardless of our choice in operating system platforms such as giving your eyes a rest every half hour for half a minute or so by looking out a window or at something farther than 6 meters away being a good start. Additionally, cleaning your screen from time to time can also help along with limiting glare on your computer screen by proper positioning of lighting and the display. Speaking of lighting, ensure that the lighting within your work environment is as even as possible as lighting with high contrast changes can force your eyes to adjust constantly as you change your gaze from point to point within your work environment. Finally, the position of the monitor plays a prominent role in ones posture which can be important for other musculoskeletal complications. Our skull is positioned by the musculature of the head and neck so that the subject of our concerns is parallel to the plane of our face and perpendicular to our direction of gaze. By considering where your head is most comfortably positioned, you should have some clue as to where to place the monitor so as to minimize any strain. For most users, an imaginary line coming straight out from the top of the computer display should contact your head approximately right in the center of your forehead. For wearers of bifocals, since bifocals are tilted anywhere from ten to fifteen degrees from the point of horizontal gaze, the monitor should be placed a corresponding number degrees lower to minimize the anterior posterior tilting of the head required to bring images on the display into focus. Also ensure that there is no glare of reflection in your display that could result in unnecessary strain. As far as distance is concerned, the distance from the eye to the display should approximate fifty one to seventy six centimeters or twenty to thirty inches and again, if you wear bifocals, one should ensure that distance is considered in the placement of the display. And finally, since many wear contact lenses, it should be noted that typically wearers of contact lenses tend to blink less than those who either wear glasses or have normal vision. Blinking more often and possibly the use of artificial tears can help to reduce dryness, irritation and the accompanying redness of the eyes that can occur.
So, along with a little help from Apple technology and good ergonomic configurations in our work environments, we should be able to minimize the effects of computer related eye strain in our lives. Be careful out there, these are our eyes we are talking about.