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January 24, 2019
How bright is your screen?
If you take a handheld computer or a tablet or a laptop outdoors and on the job, it's really important whether you can still see what's on the screen clearly enough to actually use the computer.
Whether you can or not depends on a lot of things, like how bright it is outside, whether there are reflections, the size of the screen, its sharpness and contrast, viewing angles, and so on. Experts calculate the "effective" contrast ratio in bright outdoor light by estimating sunlight compared to the light that's reflected back by the typical computer screen with its various treatments and several reflective layers. How well those internal reflections are controlled determines how readable the screen remains in sunlight.
With current display technology, of equal or perhaps even greater importance is the backlight. A strong backlight is generally better than a weak one, I say "generally" because a super-strong backlight can make a screen look washed out. That happens when the black pixels cannot block a backlight that's too powerful for a given screen technology. And, of course, a strong backlight drains the battery much more quickly.
Nevertheless, backlight strength is very important to outdoor and sunlight readability. But what exactly constitutes a "strong" backlight?
Quantifying light is never easy. Back in the day everyone knew that a 100 watt lightbulb was bright, a 60 watt so-so, and 40 watt was best used where you needed some but not a lot of light.
But that was before the short-lived era of spiral fluorescent light "bulbs" that were "100 watt-equivalent," and before the current era of LED lights that are also still sold by how many watts equivalent to an old incandescent bulb they generate. The brightness of LED bulbs is also stated in lumens and sometimes lux. Figuring out what lumens ("the total quantity of visible light emitted by a source") and lux ("a unit of measurement for illuminance") mean is too complicated to be useful in real life. And so for as long as people remember how bright a "real" 100 watt lightbulb was, newer technology light bulbs will probably sold as so and so many watts "equivalent."
What does all of that have to do with backlights? Not that much. Only that describing the strength of a backlight is just as complex and confusing as it is with light bulbs. So how is it handled?
In essence, the light emitted by a display backlight is given in a unit called candela per meter squared, or cd/m2. Candela is both the average light of one candle and, per Google dictionary, "the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 Hz and has a radiant intensity in that direction of 1/683 watt per steradian." So there. Now take that per meter squared. Auugh.
Given this obvious complexity, someone in the industry at some point suggested calling cd/m2 simply "nit." So 100 cd/m2 became 100 nits. Maybe "nit" is short for "unit." No ones seems to know. Today, many display specs include a nits rating.
Even technically inclined folks often confuse luminance with illuminance. Illuminance is the amount of light striking a surface. Luminance is the brightness we measure off of a surface which is hit by light. So for our purposes, a light source behind the screen lights up the screen. How bright that light makes the surface, its illuminance, is measured in nits.
How bright is a nit? Or 100 nits? Since 100 nits is 100 candela per square meter, imagine a hundred candles sitting underneath a roughly 3 x 3 foot square. How bright would that be? I have no idea. So perhaps it's better to think of things that, on average, generate so and so many nits and go by that. Standard laptops generate about 200 nits. A good tablet or smartphone between 500 and 600 nits. Rugged laptops can generate as much as 1,500 nits, as can modern 4K HDR TVs.
What makes everything more difficult is where we view an illuminated surface. Even a 180 nits laptop can look bright and crisp indoors. Outdoors that same laptop would be barely readable. Outdoors the weather makes a big differences, as does being in the shade or under a blue or cloudy sky. And when it comes to competing with the sun, all bets are off. The sun generates between 10,000 and over 30,000 nits.
So for better or worse, to get an idea how bright the screen of a handheld, tablet or laptop is, look at its nits rating. Which, unfortunately, is listed only in a minority of spec sheets.
That's where a screen luminance meter comes in. The one we use here at RuggedPCReview.com has a range up to 40,000 nits and can show current or peak luminance of a display. We use it in conjunction with a test template to not only record maximum luminance in nits, but also nits readings in steps from black to white.
Until something better comes along, every handheld, tablet or laptop screen spec should include a nits rating. Customers need to know that before making a purchase decision. -- Conrad Blickenstorfer
Posted by conradb212 at January 24, 2019 5:39 PM