Re: Sensor dust Nikon D80

Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:
Chris Malcolm <cam@xxxxxxxxxxxxxxxxx> wrote:

Actually the difference is that the surface of a CRT
screen is well insulated while the terminals of a
battery are usually connected to conductors that drain
off and disperse any static charge instantly.

It's obvious you don't understand the physics,

Yeah, right.

Right, as will become clear.

Because unless the battery is shorted
out,

That would have absolutely *no* effect on static charge.

Try and follow the argument. I'm not claiming that it does.

any circuit it is connected to will keep a potential difference
between the postive and negative terminals even when it is switched
on.

The difference between the terminals has nothing to do
with it. Dust will be attracted by an electrostatic
charge, not by a difference in voltage potential.

If the electrostatic charge is not sufficient to cause a potential
difference of at least hundreds of volts it's not going to be able to
attract a dust particle to it through air.

Same goes for a naked unconnected battery -- a low voltage
potential difference. Yet we do not see a dust collecting differential
between the two terminals of a battery left out in the open air.

Static charge does not equate to the same thing as
voltage difference.

No, but there is a minimum potential difference resulting from the
charge necessary to attract dust through air.

What is more, operability
at the low voltages at which camera sensors work implies a
conductivity which would not permit the development of dust attracting
static charges.

I don't follow that reasoning? There is no implication
of higher conductivity with low voltages.

There is in the sense that some things unconductive enough to be
considered insulators at low voltages are still conductive enough to
leak off static charges, a fact that engineers often exploit in
keeping static charges away from sensitive CMOS circuitry (a
technology often employed in digital camera sensors, and almost
always employed in the circuits connected to it).

So the glass used for the AA filter, or for lenses, is
not a good insulator????

Insulation is relative to voltage. It's usually treated with something
which will provide at least enough conduction to allow static charge
to leak away. That can still be a good enough insulator at the
operating voltages of a digital image sensor.

You might want to look into time constants too. You'll
find that most static discharge circuits (wrist straps,
etc), have a 10MOhm or so resistor.

I don't need to "find" that. I've spent years of my working life
wearing those things.

That does prevent a
high voltage static buildup (which is what will damage
electronic components), but will also prevents a rapid
discharge (which might damage the human using the
device). Think about the time constants for dust
attraction and an open shutter on a DSLR...

Which is why you might need even more than hundreds of volts in order
for static charge dust attraction to be problem on digital camera
sensors :-)

There could be a dust attracting static charge involved, but it could
not be due to the low voltage electronic operation of the sensor. It

Why not?

Check the physics of static attraction for the theory,

You probably should do that.

I will if you tell me something I don't know because you'll probably
have got it wrong :-)

or for a
practical experiment check the differential dust collection between
the termimals on a naked unconnected flashlight battery.

That does not constitute a practical experiment for
electrostatic adhesion.

Because a potential difference of several volts, which is all that is
involved in the operation of a digital camera sensor, no matter with
how much charge, is far too little to attract dust through air.

But if I were to stand the battery on top of a metal plate charged by
a Wimhurst machine it would attract dust, which shows that nothing in
the battery except its potential with respect to the dust is what
normally stops it from attracting dust.

could be due to some kind of transparent protective coating. However,

The coating itself does nothing except perhaps act as a
very good insulator, which is one of the two
requirements for a static build up (the other being a
source of charge).

In order to prevent the coating causing static build up problems, the
engineers have arranged that it is not a good enough insulator to hold
a static charge.

Actually these devices are indeed engineered to reduce
static build up as much as possible, and in relative
terms are only very slightly subject to static build up
as compared to many other objects where that is not
done. But by that token, a CRT screen can suffer
millions of times more dust in a week than would be
reasonable to have on the camera's sensor in several
months.

Which is to say that static build up has been reduced,
but it is not eliminated.

And I'll note that you've argued that that type of
engineering is unnecessary because the CCD does not
produce a static charge when operating... yet there it
is, to help eliminate the effects you say shouldn't be
there.

You haven't been following the argument. My claim was that contrary to
your claim the operating voltages of digital camera sensors are not
enough to cause dust attracting electrostatic charges. But that's not
the only way a surface can acquire a static charge, as you yourself
point out later in this post. Hence the need for static discharge
measures, even though the operating voltages won't cause electrostatic
dust attraction problems.

As far as I know
the camera sensor is protected by a glass screen, and that is what
gets dirty and needs to be cleaned. I can't see why an optical glass
screen should suffer any more from static problems than do your camera
lenses.

The camera lenses don't have a source of charge, other
than interaction with dry air. And indeed, that is
certainly enough to cause them to attract dust if the
air is dry.

That's not how it works -- the dust that glass attracts in dry air is
charged dust. The charge (to begin with) is on the dust, not the
glass. Unless you deliberately charge the glass by rubbing it with an
appropriate cloth, and it happens not to be semi-conductive (which
modern optical coated lenses often are on purpose) Which doesn't
happen on the digital sensor.

If you pass dry air (with or without dust in it) over a
glass surface (a high quality insulator) the glass
surface will in fact become charged.

The verb "pass" is very important there, because of the mechanical
work involved, as is the amount of resultant charge.

And it will in
fact then collect dust if it exists, which will be
attracted by, and adhere to it because of, the static
charge.

The numbers matter. While what you say is theoretically true, you need
a certain dryness of air and strength of "passing" in order to
generate enough charge to attract dust through air. I think you'd find
it rather difficult to subject your camera to such conditions :-)

What is more, I note that some DSLR makers use various sensor
vibration cleaning methods which they claim help to keep their sensors
dust free. These would not operate in the presence of electrical
static attraction.

That would be correct. The sensor is not energized when
the cleaning mechanism of operating.

Energising the sensor does not involve voltages high enough to attract
dust.

Wrong. *Any* amount of static charge will attract dust.

Yes, theoretically in a gravity free vacuum. Not through our everyday
air. Not only are distances involved, which though small are very
important, but at the size of a dust particle the hurly burly of
molecular thermal motion and collision is involved.

Any any amount of voltage can be used to generate a
static charge, plus as noted above, an external source
is not even necessary (though it will greatly increase the
static charge that can be generated).

I think our terminology may be too loose here. A static charge is
simply one that isn't moving. The static charges in CCD sensors are
for example too low to generate enough electrostatic attraction to
pull dust through air, and while static, are far from the minimum
voltages ususally considered necessry to become considered as
electrostatic charges capable of exerting significant electrostatic
attraction. Dust attracting electrostatic forces start somewhere with
effective potential differences in the hundreds of volts.

Nor would the special brushes that use static
charged bristles to attract the dust, since they would transfer charge
to the sensor and make things worse.

Hmmm... I'm not sure that is true or false. Might be!
Interesting thought.

Look up induction and transfer of static charge. It can't be avoided
if you touch an uncharged perfect insulator with a charged one.

Induction???

I suspected you were the one who was a bit light on electrostatic
physics! Look up electrostatic induction in Wikipedia. It's important
and fundamental and quite a different animal from the electromagnetic
induction which I suspect you're confusing it with.

But... that doesn't answer my point. What if it is
true that the brush transfers charge to the sensor? So
what? Is the brush a negative or a positive charge? And
what if the charge is transfered *to* the brush?

Due to the electrostatic induction which you've never heard of that's
rather difficult to arrange :-)

And what about the dust?

If the brush transfers just the right amount of charge,
the dust might just *all* *fall* *off*!

You're technically correct, but the idea is very difficult to put into
practice. There have been some static charge reducers which worked on
that principle by emitting a cloud of charged ions which were selected
by attraction as needed until attraction vanished because attractive
charge had been neutralised, but they didn't always have the intended
effect.

You persist in thinking that the operating voltages of a digital
camera sensor are enough to generate dust attractive static
charges.

Because they are. The typical "contact potential"
between a surface and a particle is less that 1 volt.
It may be true that Laser printers use 500 volts to
generate a charge for electrostatic adhesion, but it is
also true that 10 volts in a camera, producing only
1/50th as much static charge, could still be a total
disaster!

If that is indeed the case, then you ought to be able to describe to
me the experiment I can do with flashlight batteries which will show
ten volts attracting dust particles through air.

although a lot of photographers
who dozed during their school physics lessons do obviously believe that
strongly enough to include the claim in educational materials they

Do you have any experience with electronics, or
electrostatics?

Both. I've built and repaired amplifiers, computers, and electrostatic
loudspeakers. I've still got at least a thousand dollars worth of
electronic test gear lying around the house. In the days before
digital cameras I even built a digital camera using part of a ceramic
mil-spec dynamic RAM chip with the lid off as the sensor. I was
playing with this stuff in labs long before you could look up
miseducated superstitions on the web.

But these are just educated guesses on my part, and I could easily be
wrong. Do you have any good sources of information on this? I note for
example that Wikipedia contains more superstition than science on this
topic.

I hadn't looked at Wikipedia before. I found this
article, and thought it was pretty good, though it only
has two references and the first one is of questionable
value.

http://en.wikipedia.org/wiki/Dust_reduction_system

Where is the superstition you saw?

It mentions dust adhering to the sensor through electrostatic charges,

I see no reason to claim that is a supersition.

a phenomemon (so it claims) that can also be observed on LCD and CRT
screens.

I see no reason to claim that is a supersition.

That, as I have already explained, is a common superstition
amongst photographers without a basic physics education.

As you have already misstated.

So you say. But you're the one who boggled at the elementary and
fundamental concept of electrostatic induction :-)

--
Chris Malcolm cam@xxxxxxxxxxxxxxxxxxxx DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

.

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