CCD versus CMOS:

Which is Better?

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CCD Versus CMOS: Which is Better? | 2

The Charge Coupled Device (known as a CCD) has dominated astronomy and consumer
electronics for nearly ve decades. But that is changing. Today, the Complementary Metal
Oxide Semiconductor Active Pixel Sensor (CMOS APS, also known as CMOS) has matured
to the point where it is replacing CCD detectors in all but the most specialized applications.
So which sensor is best for your needs? This ebook provides an overview of CCD andCMOS APS sensors—including how both types work, and a side-by-side comparison offeatures and advantages to help you make your decision.

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CCD Versus CMOS: Which is Better? | 3First, A Brief
History Lesson
Invented in 1969, the Nobel prize-winning CCD became a mature technology after about 20 years. CCD cameras gainedwide acceptance for still imaging, video, and photometric measurements, replacing the previous generations of bulky
vacuum tube equipment. The Hubble Space Telescope, launched in 1990, famously uses CCD technology to produce its
stunning vistas and science data. On the home front, consumers bought CCD-based handheld camcorders to record
family life, and businesses used them for security cameras and inspection equipment.
In the mid-1980s, CMOS Active Pixel sensors were produced as a low-cost alternative to the dominant CCD technology.
First proposed in 1968 by Peter Noble at Plessey in the UK, these sensors were updated in the early 2000s to use the nowstandard CMOS transistor technology.
Early CMOS was only used in low-performance applications. However, with the advent of smartphones, security cameras
and car safety systems, manufacturers were able to drive the development of low-cost CMOS at multi-million-unit
volumes—driving prices down, and driving performance and innovation up. By 2007, CMOS had achieved market parity
with CCD sensors, and by 2019 the rst sensors capable of surpassing CCD performance appeared.

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CCD Versus CMOS: Which is Better? | 4

How Both Sensor Types Work
Camera sensors use picture elements known as “pixels” to detect light. A common
analogy when talking about pixels is to imagine an array of buckets collecting
rainwater. You could determine the shape and density of the cloud overhead by how
much water ends up in each bucket.

CMOS and CCD both use arrays of silicon pixels (“buckets”) to detect light. When aphoton of light hits a silicon atom, it knocks an electron into a higher energy
state. This frees the electron to move through the material. It is now referred to as aphotoelectron (“rain drop”).
The big di erence happens when you read out the sensor.

CCDCMOSIn a CCD, special electrodes attract and repel electrons,shu ing them out one-by-one to a corner of thechip. Using the analogy above, water is poured from one
bucket to the next, like an old-fashioned re brigade, until
it reaches a corner of the array where it is measured. In areal sensor, a couple of on-board transistors make thismeasurement by converting the number of electrons
from a pixel into a voltage. It then goes to someelectronics outside of the sensor, which include ananalog-to-digital converter. The result is a number foreach pixel, describing how much light was detected. Since
all the pixels are measured by the exact same electronics,
CCD cameras can be made very consistent and accurate.

Most modern electronics are built using Complementary
Metal Oxide Semiconductor (CMOS) technology, which is
a combination of NMOS and PMOS. By using CMOS, it ismuch easier to build complex electronics right into thesensor itself. This can be a major cost and space savings,
especially for a miniaturized cell phone camera.
In a CMOS detector, there are transistors at every single
pixel. They convert the signal to a voltage, which connects
via internal wires to some complex on-board
electronics. Typical CMOS sensors have one or two
analog-to-digital converters for each column in thesensor. Instead of a couple of transistors on board, there
can be millions.

CCD sensors are built using either NMOS or PMOS
technology, which was popular in the 1970s but is rarely
used today for other semiconductor devices.

CCD sensors have one readout in corner, while CMOS sensors have readout at each pixel.

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CCD Versus CMOS: Which is Better? | 5Readout
By incorporating all these electronics into the sensor, the chip itself is made much more complex, but the camera is greatlysimpli ed.
CCD sensors only have one, two, or sometimes four readouts—potentially one in each corner. CMOS sensors havethousands. This means that CMOS cameras can read out incredibly fast, even 100x faster than a comparable CCD. For longexposure applications this is not so important—but it is especially important for video cameras.
These thousands of readouts in a CMOS sensor have a huge speed advantage, but there is a high price to be paid in terms ofampli er glow and pattern noise. CCD users have seen a little glow in the corners of the sensor; early users of CMOS sensors
were overwhelmed by the glow and long exposure problems of these new sensors.

Side-by-Side Comparison
In the last few years, the best CMOS sensors are nally approaching or even exceeding CCD performance levels, but not in every
aspect. This table compares CCD to the highest-performing CMOS sensors available today.
As you can see on the following pages, CCDs still have some signi cant advantages for high-performance, low light level imaging—although these advantages are slowly being chipped away at by new CMOS technology:

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CCD Versus CMOS: Which is Better? | 6Parameter

CCD

Scienti c CMOS

WinnerAvailability

Some major CCD sensor
lines are becoming
obsolete. Very expensive
specialty sensors made bycompanies like Teledyne e2v
are here for the foreseeable
future.

Companies are making
major investments, and thetechnology has been
improving rapidly. Newsensors appear all the time.

CMOS is the future for mostapplications. CCD willcontinue to serve specialty
niches such as scienti cinstruments.

Both the sensor and thecamera itself

Large CCD sensors areexpensive, and external
analog and digital camera
electronics are complex.

Large CMOS sensors are
similarly expensive. Analog
electronics are eliminatedbut digital electronics are
more complex.

For simple cameras, CMOS
is much cheaper. For cooled
low-light imaging cameras,
there is little to nodi erence.

Sensitivity(Quantum E ciency)

60-95%, though high QE
sensors are very expensive.

75-95%CMOS provides more bangfor your buck.

1 to 40 MPS

100 to 400 MPS

CMOS

5-10 electrons for standard
CCDs, 1 electron for more
complex electron
multiplying devices(EMCCD).

1-3 electrons is common formodern CMOS sensors, andthis continues to improve.

CMOS or EMCCD

CoolingHigh cooling is relatively
easily achieved.

Sensors generate a large
amount of heat and cannot
operate at extreme cold
temperatures.

CCD

Electronic ShutterInterline and frame transfer
sensors only.

Rolling shutter is lesscomplex but pixels expose
at di erent times; global
shutter is more expensive.

No major advantage of one
over the other.

Mechanical Shutter

Required for full-framesensors; very helpful forimage calibration.

Very helpful for imagecalibration.

No major advantage of one
over the other.

CostSpeed
Readout in megapixels per
second (MPS)
Read NoiseHow much noise inelectrons is produced ateach pixel when thesensor is readfi

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CCD Versus CMOS: Which is Better? | 7Parameter

Pixel Size

CCD

Scienti c CMOS

Winner3 to 25 microns

2 to 9 microns

Larger pixels are a bettermatch for long focal length
telescopes. Most CMOS
sensors have small pixels,
but some larger pixel
models are appearing.

40,000 to 200,000

30,000 to 75,000. Can be
mitigated via stacking given
low read noise.

CCD, but stacking can give
CMOS the advantage.

16 bits

Usually 12; some chips now
use dual gain to create 16bit images but with some
pitfalls. 14- and 16-bit
sensors are becoming
available.

CCD

Easily achieved at an analog
level with zero added noise,with extremely high binninglevels possible.

On-chip analog binning isextremely limited; mostavailable sensors can only
perform 2×1.

CCD

Easily mitigated by powering
down readout transistors.

This is a bigger problem with
CMOS, since there can be
millions of on-boardtransistors.

CCD, though CMOS has
improved substantially.

Deep depletion sensors canachieve high QE at 650 to1000 nm.

Currently not possible withCMOS sensors based onsilicon.

CCD

Occasional hot columns,easily mitigated.

Fixed pattern noise can be asigni cant problem, but
technology is improving
rapidly.

No major advantage withnewer sensors.

Techniques for CCDs arewell-established ande ective.

Can be more complex, e.g.
HDR modes, lack ofoverscan data; techniques
are still being perfected.

CCDWell Depth
How many electrons can eachpixel hold—very important forphotometry

Analog-to-Digital (A/D)
Converter Bits

Binning
Combining pixels forsensitivity or resolution
matching
Amp Glow
On-board electronics create
some light via LED e ect

Infrared Imaging

Fixed Pattern Noise

Calibration
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CCD Versus CMOS: Which is Better? | 8

Which Sensor is
Right For You?
If you need to detect extremely faint light sources, you will require either hour-long exposures or very high binning
factors to achieve su cient signal-to-noise ratio. For these applications, CCD sensors have a massive advantage over thenewer CMOS technology—they have far less “amp glow” and have far better analog binning capabilities. CMOS sensors
simply don’t work in these applications.
So why, then, are major companies switching to CMOS now? The reality is that most (non-scienti c) imaging applications
require video or short exposures. In those situations, CMOS is superior in both cost and performance. This has
undermined the economic proposition for manufacturing CCD sensors in volume.
As a result, ON Semiconductor discontinued the former Kodak / Truesense devices in 2019. But this is not the end of CCD
technology. Certain Sony CCD sensors will be available until 2026. For high-end astronomy and spectroscopy markets,companies like Teledyne e2v will continue to manufacture extreme-performance CCD sensors for years to come.
Serious astronomical applications such as photometry and spectroscopy or life sciences applications such as
bioluminescence and uorescence will continue to need CCD technology for the near term. Less demanding imaging orthose needing higher speed imaging will all switch to CMOS sensors.
The technology, and the imaging market in general, continues to change rapidly. As such, it is likely that the state-of-theart in CMOS will soon be used in ever more applications.

To meet your needs for today and tomorrow,
Diffraction Limited’s SBIG® line of cameras
includes both high-performance CCD andmodern CMOS APS sensors.

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CCD Versus CMOS: Which is Better? | 9Questions about Diffraction Limited solutions?

Contact Diffraction today.
+1 613 225 2732 | diffractionlimited.com
59 Grenfell Crescent, Unit B, Ottawa, ON, K2G 0G3, Canada

For more than 25 years, Di raction Limited has been the global leader in Astronomy andScienti c Imaging Solutions—continually delivering unsurpassed products, software andservices to a multitude of industries including research, education, spectrometry,
astro-imaging, unique optical laboratory applications, and more.

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