Keeping Your Eyes Healthy During a TV Marathon

Sometimes nothing's better than a Saturday morning, afternoon and night spent in front of the tube catching up on those intense TV drama plot lines and find out who's slapping who on VH1's Celebreality shows. But before you plop down with a soft throw and a pint of ice cream, you might want to take a few precautions to keep your eyes as healthy as they can be...

The two things that matter most to keeping your eyes from over straining are keeping proper lighting in the TV-watching room and keeping a safe distance from the TV screen.

The Canadian Association of Optometrists did a study that proved it's equally as bad for your eye to watch TV in a bright room as it is in a dark one. Your everyday light is the best atmosphere for marathon movie sessions, and the CAO says that watching TV in optimal light can even have less strain on your eyes than reading a book.

The advice on a proper viewing distance for a certain size TV varies depending on where you look. But keep in mind that any figures you find on a manufacturer or retailer's Web site are probably skewed a bit to get you to pick up the biggest and most expensive TV they can.

The CAO recommends that a person watch TV from a distance of 5x the width of the picture (note that's the actual width of the screen—not the diagonal screen measure that's typically used to compare screen size), but anything more than 3x the width should be sufficient.

Image from SFB579 on Flickr with a Creative Commons License

hmm, I'm not sure I buy the 5x distance figure. The reason they give is that you have a sharper image at this distance, but this figure is probably based on a standard definition television since the television tips are from 1999. Since HDTV pack more pixels a person with normal vision can sit closer and still have a sharp image because the individual pixels are still below one's visual threshold.

A person with 20/20 vision can distinguish a spatial pattern separated by a visual angle of 1 minute of arc. (http://en.wikipedia.org/wiki/Visual_acuity) For a proper image we want the pixels to be just below this figure for an optimal picture.

The 5x rule means that you have a 11.6 degree viewing angle (tan(1/5)*(180/pi)).

Which translates to 696 (11.6 x 60) minutes of arc.

With a standard definition television each pixel has 1.08 (696/640) minutes of arc with the 5x rule which I agree is completely appropriate.

But with an HDTV each pixel accounts for .3625 minutes of arc (696/1920), which is fair below threshold and essentially means your losing resolution because your not able to perceive it.

To find the appropriate distance for a 1080i/p television we just have to solve the following equation:
tan(1/x)*((180/pi)*60)/1920=1

which yields x = 1.96

So in conclusion, I think 2x is a better figure for an HDTV.

(On a side note, I'm a vision researcher.)

posted by roger_lew on June 2nd 2009 at 8:23pm
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