Apple Watch Display Technology
Shoot-Out
Dr. Raymond M. Soneira
President, DisplayMate Technologies Corporation
Copyright © 1990-2015 by DisplayMate
Technologies Corporation. All Rights Reserved.
This article, or any part
thereof, may not be copied, reproduced, mirrored, distributed or incorporated
into any other work without
the prior written permission of DisplayMate Technologies Corporation
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Apple Watch
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Introduction
The smart watch revolution is now fully underway, with
the Apple Watch rounding out the offerings from
all of the current major players. An advanced display is what makes a smart
watch possible by providing high resolution high pixel density bitmap graphics
and text images sharply displayed on a tiny screen. In fact, the displays
needed for smart watches are right at the cutting edge of display technology,
and they determine how good everything will look on the watch – the display is
literally the crown jewel of the smart watch.
The display on the Apple Watch
rises to the challenge with an excellent state-of-the-art Flexible OLED display
covered by a sapphire crystal, or by an Ion-X strengthened cover glass for the
Sport model. Also very significant, it is Apple’s first OLED display, which are
often more challenging to engineer than using long established LCDs. We will
examine, lab test, and analyze the Apple Watch
display with a sapphire crystal below…
But why are the displays for smart watches so
challenging? First of all, they are tiny displays, typically around 1.5 inches
(38 mm) across, yet they still should have at least 250x250 pixels or more,
with many providing 320 pixels and even reaching up as high as 480 pixels.
Plus, the display is expected to fill almost the entire watch face so there
isn’t much room allowed for a bezel, so the display itself must have very
narrow borders along all the outside edges. Plus, it must be very bright in
order to be readable outdoors, be very thin because the entire watch has to fit
nicely and unobtrusively on your wrist, and be very power efficient because the
display is the single biggest power drain for the watch and determines the size
battery that is needed to keep it running for at least a full day.
Smart Watch Display
Performance
One especially important point to bear in mind when
selecting any smart watch is that while the software and apps will be updated,
refined and improved, the display on a smart watch will remain the same and can
not be updated – so pay particular attention to the quality and performance of
the display on any smart watch you are considering!
Surprisingly, Apple has said very little about the
display on the Apple Watch. For their other
products, like the iPhone 6 for example, Apple provides detailed display specs
including the screen size, pixel resolution, brightness, contrast ratio, color
gamut, and viewing angles on their website. For the Apple
Watch they mention none of these, not even the display technology, just
that it’s a “Flexible Retina Display.” We’ll fill you in on all of these
together with in-depth lab performance tests.
In this article we will cover everything related to the
display on the Apple Watch in-depth, but we
won’t review its processor or communication functions, its apps, user interface
and software, or consider its fashion and design aspects as stylish jewelry.
You can read about all of these elsewhere…
Traditional Mechanical Watch Displays
All traditional mechanical watches use a motor to move
the hands and accessory widgets (like the date and day of the week). Their
display simply works by reflecting ambient light off the watch face – so they
are very easy to read in bright ambient light, but invisible in the dark. The
electronic display on a smart watch can show the time in a zillion different
ways, but on many smart watches the display still simulates a traditional
mechanical watch face, however, an unlimited number of designs and styles are
possible.
While the traditional mechanical watch display can show
the time in only one way, it does that extremely well – the display is always
on, it has very sharp and often elegant watch face graphics, it is easy to read
in high ambient light, including direct sunlight, has great viewing angles, can
fit inside a fairly thin case, and the electric drive models have very low
power consumption. In fact, my Citizen Eco-Drive watch is 9 years old and has
never needed a battery replacement because it is automatically recharged by a
built-in photocell in ordinary ambient light.
Display Technologies for Smart Watches
Given the extent that traditional mechanical watches have
been refined over the last century, the displays on smart watches have some
pretty challenging performance criteria to match or beat. For some performance
issues they are still not as good as traditional watches, but overall the
latest and best smart watch displays can already deliver excellent performance
that we will examine in detail below, plus they are continuing to improve.
So which are the most attractive display technologies for
a smart watch? There are currently two that are very well suited for smart
watches – LCD and OLED. Other display technologies are also being used,
including e-ink (Seiko Spirit watch), e-paper (Pebble watches actually use a
transflective memory LCD), and mirasol (Qualcomm Toq watch). These displays all
work by reflecting ambient light, which provides major screen readability advantages
in bright light, plus they require very low power, so they have much longer
running times on battery. However, due to major advancements in LCD and OLED
displays they are no longer mainstream in the current market. Our discussion
below will focus on the latest generation of watch displays with at least
250x250 pixels.
LCD displays have
been in digital watches all the way back to the 1980s, but the first LCD smart
watches were introduced in 2014, including (alphabetically) the LG G Watch, the
Motorola Moto 360, and the Sony SmartWatch 2 and 3. Manufacturing small and
thin LCDs for a smart watch is very challenging because they are made up of
many layers including a backlight, two light polarizers, various optical films
in addition to the two glass sheets that sandwich the active Liquid Crystal
layer. As a result, LCDs for watches tend to be much thicker than OLEDs,
typically over 2.0 mm, and also have a wide rim because the LCD drive signals
must enter along the outer edges since the visible part of the LCD display must
transmit the backlight.
OLED displays are
an ideal technology for smart watches because they are thin single layer
solid-state displays with high pixels-per-inch (ppi) that can be manufactured
and cut into small smart watch sized screens with resolutions of 320x320 pixels
and above. They offer much better black levels, viewing angles, and power
management compared with LCDs. In 2013 Samsung introduced the first OLED smart
watch, the Galaxy Gear, with 320x320 pixels and full 24-bit color. The Galaxy Gear 2
was introduced in 2014, and performed very well in our Display Shoot-Out
tests. These OLED displays are made on a rigid substrate like almost all
smartphone displays. They are fairly thin at just 0.9 mm and can be made with a
very narrow rim and bezel, which are major advantages over LCDs. But the newest
Flexible OLED displays have evolved way beyond this…
Flexible OLED displays are the latest advancement in OLED technology.
They are made on a flexible plastic substrate, so they can be used to make
curved displays. But they also provide major advantages for flat screen watches
because they are extra thin, typically less than 0.5 mm, much lighter in
weight, plus the flexible substrate makes it possible to enclose them into a
smaller space. LG and Samsung are the leaders in this display technology. In
2013 both introduced Flexible OLED smartphones (the Samsung Galaxy Round and
the LG G Flex) and in 2014 both introduced Flexible OLED smart watches (the
Samsung Gear S, which has a curved display, and the LG G Watch R, which has a full
circle round display, which is only possible using an OLED plastic substrate).
Note that on the Apple Watch the Flexible OLED display is bonded to the
sapphire crystal or cover glass, so it isn’t flexible from the outside.
Sapphire Crystal
The
Apple Watch Sport model has its display cover made from “Ion-X” strengthened
scratch resistant glass, the same as the iPhone 6 and 6 Plus (and very similar
if not identical to Corning Gorilla glass). However, the Apple Watch and Watch
Edition instead have a sapphire crystal, the same as on most high-end
traditional watches, because it is extremely scratch resistant and almost
scratch proof under normal use.
But
a major issue for sapphire is that it reflects about twice as much ambient
light as glass (due to fundamental principles of physics and optics). That is
not a problem for traditional watches because their displays actually work by
reflecting ambient light, so the extra light reflected by the sapphire crystal
on top is a minor issue. On the other hand, it is a major issue for smart watch
displays because doubling the amount of reflected light will significantly wash
out the display’s own image light, reducing both contrast and color saturation,
and making it harder to see the image. So for smart watch displays, but not for
traditional mechanical watches, sapphire comes with a significant display
performance cost. We’ll measure and discuss this in detail below.
The Apple Watch is being promoted by Apple as having a “Flexible Retina
Display” which is an OLED with high pixels-per-inch (ppi), and is most likely
being made by LG Display. It is very interesting and exciting that Apple has
chosen to use a true cutting edge state-of-the-art display in their first
watch. We’ll cover all of these issues and much more, with in-depth
comprehensive display tests, measurements and analysis that you will find
nowhere else.
The Display Shoot-Out
In our
first Smart Watch Display Shoot-Out article we tested and compared the OLED
display on the Samsung
Gear 2 and the LCD display on the Sony SmartWatch 2.
For this Display Shoot-Out article we test the Apple
Watch display in-depth. To examine the display performance for the smart
watches we ran our in-depth series of Mobile Display Technology
Shoot-Out Lab tests and measurements in order to determine how the displays
performed. We take display quality very seriously and provide in-depth
objective analysis based on detailed laboratory tests and measurements and
extensive viewing tests with both test patterns, test images and test photos.
Results Highlights
In this Results section we provide Highlights of the
comprehensive DisplayMate Lab tests and measurements and extensive visual
comparisons using test photos, test images, and test patterns that are covered
in the advanced sections. The Display
Shoot-Out Comparison Table summarizes the Apple
Watch and iPhone 6 Lab measurements in
the following categories: Screen
Reflections, Brightness
and Contrast with Ambient Light, Color Gamut
with Ambient Light, Viewing
Angle Variations. You can also skip these Highlights and go directly to the
Apple Watch Conclusions.
Overview of the Apple Watch Display
The Apple Watch comes in 3 models and two sizes – we
tested the larger 42 mm Apple Watch model, which has a sapphire crystal. All
models and sizes have a Flexible OLED display, which is most likely being made
by LG Display. The resolution for the 42 mm size is 312x390 pixels. The
sub-pixel arrangement is RGB Stripe. The screen Aspect Ratio is 4:5=0.80 so it
is a portrait style display rather than perfectly square or round like most
watch displays.
A Retina Display
Apple hasn’t published the actual display size, so we
carefully measured it with a digital caliper. The display screen is 30.8 x 24.6
mm (1.21 x 0.97 inches) with a diagonal of 38.7 mm (1.53 inches). Note that the
display corners are all slightly rounded off. That works out to 322
pixels-per-inch (ppi), which is virtually identical to the iPhone 6 (and could
be identical due to the measurement error). That falls right in with Apple’s
Retina Display classification, which means that for normal 20/20 Vision the
pixels are not resolved and the display appears perfectly at typical viewing
distances. For 322 ppi, this applies for viewing distances greater than 10.7
inches (27 mm), which is applicable for a wrist watch.
Watch Color Gamut and Color
Management
OLED displays, like on the
Apple Watch, typically have a rather wide native Color Gamut due to the
properties of the materials used to make OLEDs. That results in some highly
saturated colors, which are actually helpful for viewing under bright ambient
light, but it then introduces noticeable color errors when looking at standard
content (particularly photos and videos) under typical indoor lower lighting
levels.
The Lab measurements indicate
that Apple decided that the Apple Watch should have an appearance and Color
Gamut close to the iPhone 6, so the watch has Color Management implemented in
order to adjust the OLED Color Gamut, particularly the Green primary (by adding
the proper admixtures of Red and Blue to each Green sub-pixel). Samsung uses
the same approach for its color accurate Basic and Cinema screen modes. The
Apple Watch and iPhone 6 Color Gamuts are shown in Figure 1 along with the
sRGB/Rec.709 Standard that is used for almost all current consumer content. As
we discuss next, their color agreement is very good, which is important because
the Apple Watch and iPhone 6 will often be used side-by-side, so they should
look the same. See the Color Gamut
section and Figure 1 for
measurements and details.
Watch Color Accuracy
The Apple Watch provides a very
good color match to the iPhone 6 as shown in Figure 1. For the most important
Red and Green primaries the agreement is excellent. The Blue Primaries differ
somewhat, but as we have explained in our Absolute Color
Accuracy Display Shoot-Out article, color errors in Blue are much less
important and noticeable than for Red and Green. Another important issue for
high image quality is the color and intensity bit depth – and the Apple Watch
delivers Full 24-bit color, like the iPhone 6.
Both the Apple Watch and iPhone
6 have slightly Bluish White Points with about 7,200K Color Temperature instead
of the standard D6500. As discussed above it’s important that they be
consistent, so that’s fine, however it reduces their absolute color accuracy.
Another issue, which we discuss below is that the Bluish White Point decreases
display power efficiency of the watch OLED display. Overall the Apple Watch
delivers comparable Very Good Absolute Color Accuracy like the iPhone 6. See
our Absolute
Color Accuracy Display Shoot-Out article for details.
Ambient Light Sensor
Most mobile displays come with
an Ambient Light Sensor that samples the ambient light falling on the screen
and then automatically adjusts the display brightness up and down to try to
improve viewing comfort (so it’s not too dim or too bright) and also to
regulate the power used by the display. This Power Management Function is
especially important for the watch in order to maximize the running time on
battery. In fact, it is so important that Automatic Brightness can not be
turned off like on iPhones and iPads to prevent a fixed manual brightness
setting, which would run down the battery more quickly. Apple does provide 3
settings: low, medium and high settings, but Automatic Brightness can not be
overridden, so if you find the screen too dim for your liking you’ll just have
to shade the watch somehow to see the screen.
On most mobile displays the
Ambient Light Sensor is located on the outer bezel near the top center. Apple
has taken an innovative approach and placed the sensor behind the display,
which is possible with a plastic OLED display. One side effect is that the
sensor is affected by the current image content appearing on the display, which
introduces some variability in the resulting Brightness settings. However, as
the ambient light level increases this variability is much less of a factor.
Watch Display Brightness
With running time on battery
being such an important issue, it’s not surprising that Apple has implemented a
rather aggressive Brightness management strategy. At lower indoor ambient light
levels the display’s Brightness (Luminance) is satisfactory. For the 3 level 1/2/3 user Brightness settings
for the Apple Watch the display has 15/60/80 nits at 0 lux, which are good for
very low light. As the lux level increases the display Brightness then
increases at different rates, but all three settings eventually reach a maximum
of 482 nits at very high ambient lux light levels, which is impressive for a
watch. But for intermediate lux levels it can be iffy: for example, for the
highest (3) user setting, the display reaches 203 nits at 500 lux, 203 nits
also at 1,000 lux, and 290 nits at 2,000 lux. In some cases you will need to
position your wrist into your shadow in order to see the display well. Indoors it’s
fine, outdoors it can be iffy like on most mobile displays. See the Brightness
section for measurements and details.
Sapphire Crystal and Reflectance
The Apple Watch Sport model has its display cover made
from “Ion-X” strengthened scratch resistant glass, the same as the iPhone 6.
However, the Apple Watch and Watch Edition instead have a sapphire crystal, the
same as on most high-end traditional watches, because it is extremely scratch
resistant and almost scratch proof under normal use.
However,
sapphire reflects about twice as much ambient light as glass (due to
fundamental principles of physics and optics). Since any reflected ambient
light decreases image contrast and color saturation, displays with sapphire
can’t perform as well as the lower Reflectance glass in ambient light. While
Anti-Reflection coatings are used to lower the Reflectance of camera lenses and
other optical components, the coatings scratch relatively easily, so they can’t
be used for a watch because they would defeat sapphire's tremendous scratch
resistance. A specially treated Enhanced Sapphire
will soon be available that delivers both high scratch resistance and low
Reflectance, which we will discuss further in the Conclusions.
We measured the Reflectance of the Apple Watch with a
sapphire crystal to be 8.2 percent, (0.5 percent greater than the single
surface Reflectance of sapphire in air), indicating that Apple has done an
excellent job in optically bonding the sapphire to the OLED display without an
air gap. However, the Reflectance of the iPhone 6 with Ion-X cover glass is
only 4.6 percent, so the Apple Watch with a sapphire crystal reflects 178
percent of the amount of light reflected by the iPhone 6. We didn’t test the
Apple Watch Sport with Ion-X glass, but expect similar results to the iPhone 6.
Note that many of the
measurements for the Apple Watch depend to varying degrees on whether it has a
sapphire crystal or Ion-X cover glass, which we have identified with a Sapphire label in the Display
Measurements Comparison Table. See the Reflections
section for measurements and details.
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Watch Performance in Ambient Light
Smart watches are likely to be used more often in higher
overall ambient light than smartphones, so how the screen visibility and
readability are affected by ambient light is extremely important. Reflected
ambient light washes out the screen colors and image contrast. There are a
number of ways to improve display performance in ambient light: the two best
known are increasing the screen brightness and reducing the screen Reflectance.
Another is to use extra saturated primary colors and dynamic image contrast to
counteract the image washout. But high ambient light will at some point
overpower all emissive displays like OLEDs and LCDs. One additional important
viewing strategy that we all do automatically is to adjust the angle and
position of our wrist to improve watch visibility, and if necessary also rotate
so the watch is in our shadow. That works quite well in most circumstances,
except in places like the beach.
With running time on battery being such an important
issue, Apple has implemented a rather aggressive Brightness management strategy
for the Watch, which holds down the display Brightness as much as possible.
This significantly reduces the Contrast Ratio and Color Gamut as the ambient
light level increases. For example, at 500 lux, which corresponds to bright
indoor lighting, the Apple Watch Contrast Ratio has fallen to 17 and the Color
Gamut to 69 percent. For the iPhone 6 set to Maximum Manual Brightness, at the
same 500 lux the Contrast Ratio is 76 and the Color Gamut is 90 percent.
However, when the iPhone 6 is set to Fully Automatic Brightness (without any
manual slider overrides) it performs about the same as the Apple Watch. But the
iPhone 6 allows the Automatic Brightness to be pushed up or down, providing
much greater user control in adjusting the Brightness. See the Brightness
and Contrast with Ambient Light and Color Gamut
with Ambient Light sections for details.
Watch Viewing Angle Performance
Almost all displays and display technologies look best
when viewed straight on with a zero degree viewing angle – and that’s how most
people try to view their smartphones, tablets, notebooks, monitors, and TVs.
However, a watch is attached to your wrist, which can only move in a
constrained manner, so most of the time it’s easier, more convenient, and more
comfortable to hold it at an intermediate viewing angle like 30 degrees.
At that 30 degree Viewing Angle the Brightness of most
LCDs falls by over 55 percent, while OLEDs typically experience only a 20
percent decrease. However, the Apple Watch with a sapphire crystal experiences
a larger 32 percent Brightness decrease and also larger Color Shifts than other
OLED displays because sapphire has much higher optical depth than glass. The
Brightness and Color Shifts become larger with Viewing Angle and can be quite
noticeable at 45 degrees and above. The Brightness and Color Shifts will be
much smaller with the Ion-X cover glass for the Apple Watch Sport model, but we
did not test that model. See the Viewing
Angle Variations section for details.
Viewing Tests
The Apple Watch provides very nice, pleasing and accurate
colors and picture quality, and is a very good side-by-side match to the iPhone
6. Even on its small 1.5 inch display, the very challenging set of DisplayMate
Test and Calibration Photos that we use to evaluate picture quality looked Beautiful, even to my experienced hyper-critical
eyes. Even the Test Patterns were accurately reproduced and artifact free with
excellent calibration and rescaling to the Apple Watch’s native 390x312
resolution. All of the images appeared very sharp on the relatively high
pixels-per-inch (ppi) screen. Well done!
Watch Display Power
Management
One major advantage of OLED
displays is their per pixel power management. With most LCDs the entire
backlight must be turned on even if only a single sub-pixel needs to be activated.
On the other hand, for OLEDs each individual sub-pixel is independently
powered, which provides tremendous flexibility in display power management. An
all black OLED display uses no power, and a screen with a black background will
use only a small amount of power compared to an white background. That’s why
it’s better to have a black background on mobile OLED displays. In addition, a
black background improves screen readability in high ambient light compared to
a white background.
All OLED and LCD mobile
displays need to carefully manage their power by controlling display brightness
and by turning off the display when it is not needed or being watched. This is
especially important for smart watches with their tiny batteries. We will
discuss these power management strategies further in the Conclusions
section, below..
Apple Watch Display Conclusions
The primary goal of this Display Technology Shoot-Out
article series has always been to publicize and promote display excellence
so that consumers, journalists and even manufacturers are aware of and
appreciate the very best in displays and display technology. We point out which
manufactures and display technologies are leading and advancing the
state-of-the-art for displays by performing comprehensive and objective
scientific Lab tests and measurements together with in-depth analysis. We point
out who is leading, who is behind, who is improving, and sometimes (unfortunately)
who is back pedaling… all based solely on the extensive objective careful Lab
measurements that we also publish, so that everyone can judge the data for
themselves as well… See the Apple Watch Display
Shoot-Out Comparison Table for all of the Lab measurements and testing
details, and the Results Highlights section above for
a more general overview with explanations.
An Excellent Smart Watch Display!
The key element for a great smart watch is its display…
and Apple has done a great job with the OLED display on the Apple Watch. It
provides very nice, pleasing and accurate colors and picture quality, and is a
very good side-by-side match to the iPhone 6. It is very nicely implemented and
an Excellent Smart Watch Display!
Even the challenging DisplayMate Test Patterns were
accurately reproduced and artifact free with excellent calibration and
rescaling to the Apple Watch’s native 390x312 resolution. All of the images
appeared very sharp on the relatively high pixels-per-inch (ppi) screen. Well
done!
A major issue for all smart watches is the limited
display Brightness, the performance in high ambient light, display power
management and the running time on battery. The Lab measurements show that
these are also major issues with the Apple Watch as well. For details see the Lab Tests
and Measurements Comparison Table.
First: Apple has implemented a rather aggressive display
Brightness and display power management strategy using a mandatory Automatic
Brightness Ambient Light Sensor that significantly restrains the display’s
Brightness. While satisfactory for low indoor ambient lighting it significantly
reduces the Contrast Ratio and Color Gamut as the ambient light level increases
and there is no way for the user to make any adjustments other than just shading the watch somehow. Another form of aggressive
display power management is turning off the display after 6 to 17 seconds,
which is rather inflexible and can be very annoying, and it interferes
with some uses of the watch. More on power management below…
Second: the premium Apple
Watch models have a sapphire crystal rather than using a cover glass like on
the iPhone 6 and most smartphones. That is the same approach used on premium
traditional watches, but there is one very significant difference between them.
The much higher Reflectance of sapphire compared to glass almost doubles the
reflected ambient light, which is fine for traditional watches that work by
reflecting ambient light, but significantly washes out the image contrast and
color on emissive smart watch displays. It’s an interesting compromise between
the luxury and scratch resistance of sapphire versus optical performance. More
on sapphire below…
The Apple Watch is Apple’s first OLED display, which is a
very interesting development, and who knows where it might lead in future
displays for Apple smartphones, tablets, and monitors. While LCDs make
excellent displays for smartphones, tablets, monitors and TVs, there is
absolutely no question that for smart watches, OLED is the only way to go for a
great display, and it’s very significant that Apple has chosen an OLED for its
first Apple Watch display!
Enhanced Sapphire
While sapphire is incredibly hard and scratch resistant,
the Lab measurements have demonstrated that it significantly affects the
optical performance of (emissive) smart watch displays compared to glass,
particularly in high ambient light and for large viewing angles. This behavior
is very different from sapphire as it is used in traditional watches, which
work by light reflection and not by display emission. In order to increase the
use of sapphire for displays, the sapphire industry will need to modify the
optical properties of sapphire without significantly affecting its scratch
resistance and other mechanical properties. It can’t be done using traditional
Anti-Reflection optical coatings which scratch easily. A
specially treated Enhanced Sapphire
will soon be available that delivers both high scratch resistance and low
Reflectance. We will provide more information on Enhanced Sapphire in
our upcoming 2015 Innovative Displays and Display
Technologies article. Here is a link to the 2014 Edition.
Watch Software will be
Updated and Improved but the Display will Remain the Same
One especially important point to bear in mind when
selecting any smart watch is that while the software and apps will be updated,
refined and improved, the display on a smart watch will remain the same and can
not be updated – so pay particular attention to the quality and performance of
the display on any smart watch you are considering!
Apple Watch Comparison with
the iPhone 6
Since the Apple Watch is often used right next to the
iPhone 6, Apple has gone to considerable lengths to give them the same colors,
color calibration, and color accuracy. The two most significant differences
(other than size) are the much higher Reflectance of the Apple Watch with a
sapphire crystal (8.2 percent compared to 4.6 percent), and the aggressive Brightness
control using a mandatory Automatic Brightness Ambient Light Sensor in order to
increase the running time on battery for the tiny watch.
Apple Watch Comparison with
other Smart Watches
The only other OLED smart watch that we have tested so
far in our public Display Technology Shoot-Out article series is the Samsung Gear 2,
which also has a very impressive and excellent display with Full 24-bit color,
but with the following principal differences with the Apple Watch: the Color
Gamut is similar to the wider Adobe RGB rather than sRGB, it has Gorilla Glass
with a very low 4.7 percent Reflectance rather than sapphire with 8.2 percent
Reflectance, it does not have an Ambient Light Sensor or Automatic Brightness,
which allows the user to set the desired Brightness as high as they like,
resulting in much higher Contrast Ratios and Color Gamuts in ambient light, and
also better Viewing Angle Performance. To compare them in detail click the
following two links in a tabbed web browser: Samsung Gear
2 and Apple Watch.
Recommendations for Enhanced
Watch Displays
4 of the 6 Recommendations
below can be implemented in software for existing smart watches.
· The best way to improve screen readability in ambient and
also increase the running time on battery at the same time is to significantly
reduce the screen Reflectance for both sapphire and glass as we have discussed
above.
· Enlarging the Color Gamut for high ambient light is another way to
improve display performance in ambient light without increasing display power,
and then using Dynamic Color Management at lower ambient light levels.
· Implement Automatic Brightness in a way that allows some
degree of manual adjustment as on the iPhone 6 as discussed above.
· For
OLEDs, Blue is by far the least power efficient primary drive color, so
it is desirable to reduce the Blue drive levels whenever possible. Just
lowering the White point down to the Standard 6,500K from the Bluish 7,200K can
save almost 10 percent on display power.
· For OLEDs, going a step further and turning off the least
efficient Blue drive entirely, producing a Red-Yellow-Green color space, would
produce a 35 percent display power savings for the same Luminance. Going even
further and in addition turning off the Red drive and just using the most
efficient Green primary would produce a 55 percent display power savings for
the same Luminance. For some situations and applications this would be very
helpful and can be used to improve both screen Brightness and running time on
battery.
· Allow users to set the length of time before the display is
turned off. For the Apple Watch simply fixing it to 6 seconds for wrist motion
and 17 seconds for interactive actions is inflexible and can be very annoying,
and it interferes with some uses of the watch. Both permanent and temporary
time extensions should be allowed under user control. This applies to most
smart watches.
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Apple Watch
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Display
Shoot-Out Comparison Table
Below we
examine in-depth the display on the Apple Watch based
on objective Lab measurement data and criteria.
We
have also included results for the display on the Apple iPhone 6 – although considerably
larger it is the primary display that will often be near the Apple Watch, so it’s important to
know how well they compare and perform.
Note
that all measurements for the Apple Watch differ to varying degrees for the
Sapphire crystal and Ion-X Glass.
We have
marked the most significant variations with a Sapphire
label.
For
additional information on OLED display measurements see our Galaxy S6 Display
Technology Shoot-Out article.
For
comparisons with Samsung and Sony Smart Watches see our Smart Watch
Display Technology Shoot-Out article.
For
comparisons with the other leading displays including LCDs see our Mobile Display Technology
Shoot-Out series.
Categories
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Apple Watch 42 mm
with Sapphire Crystal
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Apple iPhone 6
with Ion-X Glass
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Display Technology
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Flexible OLED
RGB Stripe
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IPS LCD
RGB Stripe
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Display Shape
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4:5 = 0.80
Portrait Only
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9:16 = 0.56 or 16:9 = 1.78
Portrait or Landscape
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Display Size
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1.53 inches Diagonal
1.21 x 0.97 inches
with rounded corners
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4.7 inches Diagonal
4.09 x 2.30 inches
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Display
Area
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1.2 square inches
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9.4 square inches
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Relative
Display Area
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100 percent
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802 percent
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Display
Resolution
|
390 x 312 pixels
|
1334 x 750 pixels
|
|
Total
Number of Pixels
|
122K pixels
|
1,000K pixels
|
|
Pixels
Per Inch
|
322 ppi
|
326 ppi
|
|
20/20
Vision Distance
where
Pixels are Not Resolved
|
10.7 inches
|
10.5 inches
|
|
Appears
Perfectly Sharp
at
Typical Viewing Distances
|
Yes
Apple Retina Display
|
Yes
Apple Retina Display
|
|
Text
and Graphics
|
Very Sharp
|
Very Sharp
|
|
Display
Color Depth
|
Full 24-bit color
|
Full 24-bit color
|
Screen Reflections
The
Screen Reflects Ambient Light, which washes of the Image Contrast and Colors.
The
Average Reflectance is measured with an Integrating Hemisphere and
Spectroradiometer.
The
Mirror Reflections are measured with a highly collimated beam of light and
Spectroradiometer.
The
Reflectance of Sapphire is much higher than Glass.
|
Average Screen Reflection
Light
from All Directions
|
with Sapphire
8.2 percent
Good
|
4.6 percent
Excellent
|
Relative Brightness of the
Reflected Ambient Light
|
178 percent
|
100 percent
|
Mirror Reflections
Percentage
of Light Reflected
|
with Sapphire
9.2 percent
Good
|
6.0 percent
Very Good
|
|
Contrast Rating for
High Ambient Light
The Higher the Better
|
with Sapphire
Up to 59 Good
Automatic Light Sensor
|
121 Excellent
Maximum Brightness
Setting
|
|
Apple Watch 42 mm
|
Apple iPhone 6
|
|
Brightness and Contrast with Ambient Light
At
0 degrees Viewing Angle.
But
the typical Viewing Angle for a Watch is actually 30 degrees or more. See
below.
|
Maximum Brightness
|
Up to 482 cd/m2
Automatic Light Sensor
|
558 cd/m2
Manual or Automatic
Sensor
|
|
Black Brightness at 0 lux
at
Maximum Brightness
|
0 cd/m2
Increases with Ambient
Light
|
0.35 cd/m2
Increases with Ambient
Light
|
Contrast Ratio
Measured
in the dark at 0 lux
|
Infinite at 0 lux
Decreases with Ambient
Light
|
1,592 at 0 lux
Decreases with Ambient
Light
|
|
|
Auto Brightness Level
Measured
in Ambient Light
Maximum
Brightness Setting
|
with Automatic Light
Sensor
203 cd/m2 at 500
lux
203 cd/m2 at 1,000
lux
290 cd/m2 at 2,000
lux
482 cd/m2 at High
lux
|
with Automatic Light
Sensor
110 cd/m2 at 500
lux
137 cd/m2 at 1,000
lux
180 cd/m2 at 2,000
lux
558 cd/m2 at High
lux
|
Contrast Ratio
Measured
in Ambient Light
Maximum
Brightness Setting
|
with Automatic Light
Sensor
17 at 500 lux
9 at 1,000 lux
7 at 2,000 lux
|
with Automatic Light
Sensor
16 at 500 lux
11 at 1,000 lux
7 at 2,000 lux
with Maximum Manual
Setting
76 at 500 lux
39 at 1,000 lux
20 at 2,000 lux
|
|
Apple Watch 42 mm
|
Apple iPhone 6
|
|
Color Gamut with Ambient Light
The
Image Colors depend on the Ambient Light and Viewing Angle.
|
Color of White
Color
Temperature in degrees
Measured
in the dark at 0 lux
|
7,145 K
Slightly Too Blue
Lower OLED Power
Efficiency
|
7,241 K
Slightly Too Blue
for Accurate Image Colors
|
Color Gamut
Measured
in the dark at 0 lux
See Figure 1
|
104 percent at 0 lux
sRGB / Rec.709
Decreases with Ambient
Light
See Figure 1
|
101 percent at 0 lux
sRGB / Rec.709
Decreases with Ambient
Light
See Figure 1
|
Color Gamut
Measured
in Ambient Light
|
with Automatic Light
Sensor
69 percent at 500 lux
49 percent at 1,000 lux
39 percent at 2,000 lux
|
with Automatic Light
Sensor
62 percent at 500 lux
50 percent at 1,000 lux
38 percent at 2,000 lux
with Maximum Manual
Setting
90 percent at 500 lux
82 percent at 1,000 lux
68 percent at 2,000 lux
|
|
Apple Watch 42 mm
|
Apple iPhone 6
|
|
Viewing Angle Variations
The
typical Viewing Angle for a Watch is actually about 30 degrees or more from
exactly face on.
See this Figure for an explanation and visual
definition of JNCD.
The
Viewing Angle Variations for Sapphire are much higher than for Glass.
|
Brightness Decrease
at a 30
degree Viewing Angle
|
with Sapphire
32 percent
Medium Decrease
|
44 percent Portrait
57 percent Landscape
Very Large Decrease
Typical for all LCDs
|
Black Level Change
at a 30
degree Viewing Angle
|
0 percent
No Change for OLED
|
8 percent
Slight Decrease at 30 degrees
|
White Point Color Shift
at a 30
degree Viewing Angle
|
Small Color Shift
Δ(u’v’) = 0.0102
2.5 JNCD
with Sapphire
|
Small Color Shift
Δ(u’v’) = 0.0037
0.9 JNCD
|
Color Shifts
Primary
Colors and Mixtures
at a 30
degree Viewing Angle
|
Medium Color Shift
Largest Shift Δ(u’v’) = 0.0375
9.4 JNCD with
Sapphire
|
Small Color Shift
Largest Shift Δ(u’v’) = 0.0052
1.3 JNCD
|
Categories
|
Apple Watch 42 mm
with Sapphire Crystal
|
Apple iPhone 6
with Ion-X Glass
|
About the Author
Dr. Raymond Soneira is
President of DisplayMate Technologies Corporation of Amherst, New Hampshire,
which produces display calibration, evaluation, and diagnostic products for
consumers, technicians, and manufacturers. See www.displaymate.com. He is a research
scientist with a career that spans physics, computer science, and television
system design. Dr. Soneira obtained his Ph.D. in Theoretical Physics from
Princeton University, spent 5 years as a Long-Term Member of the world famous
Institute for Advanced Study in Princeton, another 5 years as a Principal
Investigator in the Computer Systems Research Laboratory at AT&T Bell
Laboratories, and has also designed, tested, and installed color television
broadcast equipment for the CBS Television Network Engineering and Development
Department. He has authored over 35 research articles in scientific journals in
physics and computer science, including Scientific American. If you have any
comments or questions about the article, you can contact him at dtso.info@displaymate.com.
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