- [Instructor] When we describe the brightness
of a pixel using bits,
perhaps using eight bits
to give us a scale from zero to 255,
we're not actually defining how bright 255 is.
It would perhaps make more sense
to describe this as a percentage,
where not percent bright is completely dark
and 100% bright is as bright as the pixel can go.
But this would only give us 100 steps from dark to light,
which we'd see in the image as banding or posterizing,
where there are subtle gradients like shadows on a wall.
Having 255 steps gives us a more subtle transition
from dark to light, which is a good thing.
As mentioned in the lesson on bit depth,
the more bits you use to record a number
the bigger the range of numbers you can make.
However, no matter how big the number you use
to record the brightness of a pixel,
it will still only represent 100%.
In the case of a camera recording an image,
100% is the maximum brightness the light sensor can capture
without losing individual details.
In the case of a display,
100% represents the maximum brightness it can produce.
And the difference between the two,
between the darkest part of the picture
and the brightest part,
is often described as the contrast ratio.
Display manufacturers often advertise
some pretty impressive numbers
when it comes to contrast ratios,
sometimes into the millions to one.
And that's because they're comparing
the brightest image the display can show
with complete darkness,
so the numbers aren't all that useful.
Perhaps a more useful scale
to measure the maximum amount of light a display can produce
is candelas per square meter.
And this is a long name,
so it's more common to use the term nits.
This is the amount of light that would be produced
if the thing generating the light were one square meter.
That's about 20% bigger than a square yard.
As you can probably guess,
one candela is equivalent to one candle,
though it's a candle of particular size and intensity.
So the amount of light produced by one candle
was set as a standard quite a long time ago.
Most consumer displays can produce up to about 200 nits.
200 nits is our 255 setting
for the brightness of an eight bit pixel,
which means normal in most cases.
Because our eyes and brains adapt
so quickly to everything we see,
this amount of maximum brightness works pretty well.
But high-dynamic range displays, or HDR displays,
can display much brighter images,
while still retaining detail in the shadows.
It's a big contrast range
that goes up to perhaps 2,000 nits for consumer displays,
and even 10,000 nits for large outdoor screens.
These screens are so bright
they can cause a physiological reaction.
Your pupils will constrict when bright images appear.
And the impact of this very high-dynamic range is wonderful,
and it opens avenues to creativity for storytellers.