Backlit cellphone keys – not such a good idea!

Have you had the experience, as a child, of discovering the optical properties of oiled paper?

I remember it well… I was an avid reader as a kid, and when a smudge of oil from some snack would get onto my books I’d notice that the translucent oily spot that looked darker than the paper would become lighter when you held the page up to the light. Before long it occurred to me that if the shade of oiled paper can go from darker to lighter as you raised the page, there must exist an intermediate position where the spot would become invisible as its shade became identical to the dry paper’s. Sure enough, so there was!

We met the hard to read keyboard on my Nokia 6230i before. Well, apparently the keyboard designers at Nokia were also trying to emulate the oil/paper experiment from my childhood…

Nokia 6230i keys in daylight

Like many handheld devices these days, the keyboard on this phone is backlit. The numbers are translucent, and a strong white light can shine through. Seemingly a great idea, for night time use. Now in bright daylight this works fairly well. The backlight gets washed out in the sunlight and although the transparent digits are of a lower contrast than a black ink would have provided, you can see them well in dark gray on silver (since the keys are silver and reflect ambient light in unpredictable ways, the contrast admittedly varies, as seen in the photos; I’ve already commented on the advisability of plain black on white for optimal viewing). But once evening starts to fall…

Nokia 6230i backlit keys in low ambient lightThe third photo shows what happens in lower ambient light. There is an intermediate light level when the transition from dark numerals on bright silver to white backlit numerals on dark silver just evens out, and the numbers become almost invisible. In fact, since the backlight source is localized, different keys reach this point at different light levels; in the photo the “7” is in day mode, the “0” is in night mode, and the 5 is barely visible – just like the oil spot on those old books.

Now imagine trying to distinguish these keys while driving, with the phone in a hands-free cradle, where you can’t touch-type and you need your attention on the road.

So, what can we do about this? Well, we could prefer black keys with white digits – then the day and night contrast would be the same so there would be no crossover point of invisibility. Another good idea (which I recall seeing on some devices) is to make the backlight come on only in truly low light conditions. And of course, we should test the devices we buy in all situations, not just in the bright showroom where we normally make buying decisions…

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