New technology in the pixel design of the sensors in digital cameras suppresses dark current.
That means cleaner low light images. For astrophotographers, it means there is no
longer a need to measure dark frames and subtract the dark signal in post processing.

On long exposures with a digital cameras, electrons collect in the sensor
due to thermal processes. This is called the thermal dark current. As
with photon noise, the noise from dark current is the square root
of the signal. One of the big contributions of noise in long exposure
digital camera photography is noise from dark current. A significant
problem in older sensors is the variation of the dark current from pixel
to pixel creating fixed patterns. Dark current doubles every few degrees
increase in temperature, so if there is a temperature gradient across
the sensor, the dark current is not uniform. Electronics in the camera
heating one side of the sensor produces varying dark current, leading
to the commonly seen "amp glow" (Figure 1) in cameras without the new
suppression technology. Fortunately, a new technology has been developed
that suppresses the dark current eliminating amp glow and other patterns,
such as banding.

The traditional
method to deal with dark current effects is to measure dark
frames separately and then subtract them in post processing.
Camera manufactures added "long Exposure Noise Reduction" (LENR)
to some cameras where after the camera makes the exposure, it
closes the shutter, makes a second exposure and subtracts the
dark frame from the exposed image. This doubles the amount of time
needed to get an image.

First some facts about dark current.

Dark current doubles every few degrees increase in temperature.
Typically, the doubling in CMOS sensors is every 5 to 6 degrees C.
To be really precise with dark current subtraction, one needs the
the dark current to be measured at the same temperature to within
a fraction of a degree, and have the same temperature gradients.
That is not practical.

In practice, making master dark frames for later subtraction is
an approximation of the dark current that actually occurred during
the imaging session as the temperatures are not always the
same.

On the internet, some have argued that on sensor
dark current suppression technology is just a software implementation. The camera would need
to store many dozens of dark frames as a function of temperature and thermal gradient
in order to effectively subtract the dark current. That idea is incorrect;
Dark current suppression technology is part of the hardware electronics design of each pixel.

Figure 1. Comparison of the dark current from a modern sensor (left, model from 2014)
with older technology (2005). The 2014 camera has on-sensor dark current technology
which blocks the accumulating dark current level, but not the noise. What is important
is the fact that the 2014 model shows a uniform image with very little variations or patterns.
Imaging with such modern cameras allows one to skip long exposure noise reduction
or post processing subtracting dark frames. Besides uniformity and very low patterns,
the dark current is around 100 times lower in the left (2014 camera) than in the
older technology resulting in impressively lower noise.

On Sensor Dark Current Suppression

On sensor dark current suppression is an electronics technology
in each pixel that reduces dark current during the exposure.
On Sensor Dark Current Suppression is not long Exposure Noise Reduction
software. It is not software. It is not firmware.
It is part of the hardware pixel design
and is not software or firmware that can be turned on or off.

On sensor dark current suppression is a technology that reduces
dark current during the exposure of your subject. There are several technologies
and patents that are online describing the methods, and the
technology is constantly being refined.

"An apparatus and method that reduces the dark current in each
pixel of an image sensor, Where each pixel has a pinned
photodiode. A negative potential barrier at the transfer gate of
each pixel is raised when the photodiode is integrating (When
the transfer gate is "off") to thereby eliminate dark current
generation in this region. The potential barrier is applied via
a "triple Well" transistor circuit structure that is capable of
handling a strongly negative voltage. The circuit structure
also serves as a conduit for conducting a strongly positive
voltage to minimize the potential barrier during signal transfer
and readout, thereby reducing image lag."

"1.3 Dark current suppression technology:
Dark current is produced by microcrystalline defect or leakage
current on CMOS and pixel charge is accumulated and increased in the
case of long exposure or temperature rise, which causes CMOS to
produce noise. For this reason, Canon Company adopts the
architecture of "buried photodiode" to reduce probability of noise."

All this means that by suppressing dark current inside the pixel during the
exposure on the subject, there is no problem of mismatching temperature
of dark frames made at other times that may have been made at slightly
different temperatures. This technology results in the amazing smoothness out-of-camera of the 5-minute
dark frames shown Table 4 of my
Canon 7D Mark II review, especially when compared to older cameras without
the technology (Figure 1, right panel), and in the impressive smoothness
of the 10-minute dark frame shown in Figure 2.

Figure 2. Dark current image with 70 minutes of exposure with the sensor at room temperature, on
an absolute scale. The noise is Gaussian and extremely low, only 5.8 electron standard deviation.
That means just a few photons in 70 minutes could be detected if the source was a few pixels in size.

signal from dark current = DC * time and the noise is (DC * time)0.5,
and DC is also a function of temperature (Figure 3).

Figure 3.
Dark current as a function of temperature for 6 cameras are compared.
Lower is better as noise from dark current is the square root of the dark
current multiplied by the exposure time. The temperatures are the camera
temperature reported in the camera's EXIF data and was 2 to 10 degrees
higher than measured ambient temperature due to internal electronics
heating the camera. The more massive 1D cameras tended to have a larger
difference between internal camera and ambient temperature. The
Canon 7D Mark II digital camera
sets new and impressively low levels making this the current
top long exposure low light camera in the canon line.
Sony sensor data are from commercial data sheets found online.

So the dark current suppression technology blocks the DC * time component, leaving only the random
noise. In order to make dark current suppression technology effective, manufacturers have had to
refine manufacturing methods so that the sensors are very uniform, reducing pattern noise (banding).

Measuring dark frames has the main purpose of subtracting an offset caused by accumulating
dark current level (DC * time) and pattern noise (including
banding and amp glow) from the pixel to pixel variation in (DC * time),
but dark current subtraction can't remove random noise.

Dark current suppression technology does the same thing, but in the pixel
with the hardware architecture of the pixel, and does so during the
exposure on your subject (while the camera is taking the picture). So
there is no need for dark frame subtraction in cameras where the on
sensor dark current suppression technology is well implemented (most
recent camera models from the last few years). With such sensors, dark
current is already subtracted DURING the light integration. There is no
need to do it again in post processing (which will result in more noise).

If you subtract dark frames, the equation for random noise at the darkest signal level in the image
is:

where r = read noise, DC = sensor dark current, t = time, DCF = dark frame dark current, N = number of
light frames, M = number of dark frames. If you measure the dark frames at the same temperature as the
light frames, then DCF = DC.

The equation reduces to the noise is proportional to:

noise = ( 1 / N + 1 / M ) 0.5

Common in astrophotography is the take many exposure on the subject as one has time for,
then fewer dark frames.
For example, if you did 220 exposures on your subject (called "lights"), then N = 220 and
if you did only 10 darks (M = 10), the noise in your image is proportional
to sqrt ( 1 / 220 + 1 / 10 ) = 0.323. Skip the darks and the noise reduces to: sqrt ( 1 / 220 ) =
0.067, or 0.323 / 0.067 = 4.8 times better! How much is actually achieved is dependent on how much
noise from the sky signal is present.

CCD versus CMOS sensors

The typical read noise in a CCD is on the order of 6 to 16 electrons.
The best CMOS digital camera sensors are under 1 electron.
Dark current in good CMOS sensors are under 0.02 electrons/second at 10C (circa 2014-2016).
Compare that to the SBIG STF-8300C CCD camera, which has a dark
current of 0.02 electrons/second at -10 C, and read noise of 8 electrons.
Note the CCD must be run 20 degrees C colder to achieve the same performance
as the best current DSLRs. At 8 electron read noise of the CCD, read noise
is still a significant factor in faint signal detection. At read noise under 2 electrons,
read noise becomes a non-issue. A 1 electron read noise versus 8 is a factor of
8 fainter detection in favor of the CMOS in short exposures. CCDs must be exposed
longer than CMOS so that read noise will be less of a factor in the total noise
from faint objects.

Which Cameras Have On-Sensor Dark Current Suppression Technology

On-sensor dark current suppression started to be introduced in camera
models circa 2008 and has been refined ever since, with dark current
levels being reduced, the suppression technology improved, and uniformity
improved (less pattern noise issues like banding).

To test if your camera has on-sensor dark current suppression, make two exposures
in a dim room with the lens cap on: one short, like 1/100 second and one
long, like 10 minutes. Use ISO 1600 and be sure no light is entering the camera from the
viewfinder.

Next convert the raw files from the above two exposures with the same settings.
Adjust the black point so that the peak of the histogram is well separated
from the left edge. One some cameras it is not possible to completely separate
the histogram because low end data values are clipped.

Next use the levels tool, moving the right slider to the left to increase
brightness. Do the same with both images. You can apply the levels tool multiple times to magnify the
differences if desired. Look for patterns, like that shown in Figure 1, right panel.
If you do not see any patterns in the long exposure image, you have a good
sensor and likely do not need to subtract dark frames in your astrophotography.

Calibrating your images to photoelectrons.
One first needs the gain. Gain for any modern digital camera can be
estimated within about 10%. The max signal is about 2000 to 2100
electrons per square micron at ISO 100, or 2000/16 = 125 electrons at
ISO 1600. So a camera with 4 micron pixels would have 4 * 4 * 125 =2000
electron range at ISO 1600. The 14-bit gain is then approximately 2000
electrons / 214= 0.12. In practice the digital range is a
little less, more like 14000, so gains more like 0.14 to 0.15.

Next convert the raw data to a 16-bit tif with a program like dcraw
with flags to not change the data or demosaic it into a color image.
dcraw -D -4 -j -t 0 -T rawfilename
The data range is still 0 to 214 -1.

With the gain (eg. 0.12 e-/DN, DN = data number in the file), +/-20
electrons would have a DN range of 40/0.12 = 333 DN. So now find the
mean of the data and select +/- 166 DN about that range, scale it to 0
to 255 range and make a jpeg for viewing. You can do this in any photo
editor that displays 16-bit values. If it only displays 8-bit values,
first zoom in on the image by 10. For example, in the curves tool, move
the upper right point from 255 to 25 and click OK. Bring up the curves
tool again. Then the range would need to be +16.6 to -16.6 about the
mean (or -17 to 17, a range of 34). Move the lower left slider in the
curves tool and the upper right slider so that the range between them
is 34 and the histogram peak is in the middle between those two values.
You now have an estimated +/- 20 electron range.

The gain and measured standard deviation allows one to find the noise in electrons.
Using the above example after making the data range -20 to 20 electrons about the
mean, and if that image is displayed in an image range of 0 to 255, the gain for that
image is 40 / 255 = 0.157 electron / DN. Next measure the standard deviation.
Let's say it is 30 (the histogram box in photoshop shows the standard deviation). Then
the noise is 30 * 0.157 = 4.7 electrons.

Not all new cameras have on-sensor dark current suppression technology.
Lower end, entry consumer cameras may have cheaper sensors that do not
have this technology.

Conclusions

More and more newer cameras have improved sensors with lower dark current, lower pattern noise
(e.g.banding) and on-sensor dark current suppression, allowing for beautiful long
exposures in low light conditions without the need for long exposure dark frame subtraction
in the camera or post processing subtracting dark frames. This speeds
image acquisition in the field and simplifies post processing.

See the section on dark current, page 39 left column: "Negative bias on TG during signal integration
can help draw holes to under the PPD edge of TG and suppress dark current generation from Si-SiO 2
interface states [83]–[85]."

Note too that the author states on page 35:
"widespread adoption of the PPD in CMOS image sensors occurred in the early 2000’s and helped CMOS APS
achieve imaging performance on par with, or exceeding, CCDs."

1.3 Dark current suppression technology: Dark current is produced by microcrystalline defect or
leakage current on CMOS and pixel charge is accumulated and increased in the case of long exposure or
temperature rise, which causes CMOS to produce noise. For this reason, Canon Company adopts the
architecture of "buried photodiode" to reduce probability of noise.