How Do Cameras Capture Pictures? Functions of the camera obscura and CMOS and CCD technologies

[rayzoredge 2]

In the olden days, cameras were made in the stylish fashion of "camera obscura, " where basically a hole was made in the wall of a box, and on the other side there was a film piece there to "capture" the picture. Of course, we've gotten a lot more advanced than that since then.

 

The basic workings of a film camera rely on that basic mentioned concept, but now we use fancy things like mirrors, photographic film/plates, and other doodads to decrease the capture time, improve image quality, and flip the image vertically (since images are always captured upside-down).

 

With the advent of digital cameras, we introduce two different technologies: the charged coupled device and the CMOS. I'll try to explain both, since even Wikipedia realizes that the explanation of physics of operation (at least for the CCD) is as comprehensive as ancient Greek. (No offense to Greeks, of course. Just a figure of speech.) =p

 

A CCD is an isolated cable made of a semiconductor surrounded by ring electrodes. The low amount of free charge carriers and the finite polarization of the insulator and the semiconductor only disturb the electric field generated by the electrodes weakly. Free carriers in the semiconductor cannot pass the insulator, they are said to be confined transversally. The ring shaped electrodes are used to produce a sine shaped potential along the cable. Electrons drift to the potential hills and holes to the valleys, they are said to be confined longitudinally. An alternating field on the electrodes makes the valleys and hills move along the cable carrying the charge carriers with them.

 

Real CCDs are no round cables due to production issues. There are connections where charge carriers are injected. For readout the small field disturbance generated by the carried charge is sensed (see MOSFET). At the end of the cable the carriers are dropped onto a metal electrode.

 

The photoactive region of the CCD is, generally, an epitaxial layer of silicon. It has a doping of p+ (Boron) and is grown upon the substrate material, often p++. In buried channel devices, the type of design utilized in most modern CCDs, certain areas of the surface of the silicon are ion implanted with phosphorus, giving them an n-doped designation. This region defines the channel in which the photogenerated charge packets will travel. The gate oxide, i.e. the capacitor dielectric, is grown on top of the epitaxial layer and substrate. Later on in the process polysilicon gates are deposited by chemical vapor deposition, patterned with photolithography, and etched in such a way that the separately phased gates lie perpendicular to the channels. The channels are further defined by utilization of the LOCOS process to produce the channel stop region. Channel stops are thermally grown oxides that serve to isolate the charge packets in one column from those in another. These channel stops are produced before the polysilicon gates are, as the LOCOS process utilizes a high temperature step that would destroy the gate material. The channels stops are parallel to, and exclusive of, the channel, or "charge carrying", regions. Channel stops often have a p+ doped region underlying them, providing a further barrier to the electrons in the charge packets (this discussion of the physics of CCD devices assumes an electron transfer device, though hole transfer is possible).

 

One should note that the clocking of the gates, alternately high and low, will forward and reverse bias the diode that is provided by the buried channel (n-doped) and the epitaxial layer (p-doped). This will cause the CCD to deplete, near the p-n junction and will collect and move the charge packets beneath the gates ? and within the channels ? of the device.

 

It should be noted that CCD manufacturing and operation can be optimized for different uses. The above process describes a frame transfer CCD. While CCDs may be manufactured on a heavily doped p++ wafer it is also possible to manufacture a device inside p-wells that have been placed on an n-wafer. This second method, reportedly, reduces smear, dark current, and infrared and red response. This method of manufacture is used in the construction of interline transfer devices.

 

-

 

I'll be honest: I read this and didn't understand a damn word. Attempting to decipher from the technical jargon...

 

CCD output comes from the result of input from the electric field generated by the semiconductor and its surrounding electrodes. This output is then "put" onto the silicon wafer, coated with Boron and phosphorous. (The chemicals are the "designations" where the electric signals become attracted to accordingly.) Through the LOCOS process (this needs further reading), there is a gas reaction that forms the channels of color that we are familiar with: RGB. These channels are exclusive from the electric output that comes from the semiconductor and what we see as captured are these channels of color, "rearranged" by the electric output from the semiconductor, to form a picture.

 

Different CCDs can be made by altering the concentration and application of different chemicals, which will produce varying results, such as infrared.

 

That's what I got from CCD. Anyone a photographer that can correct or verify me on this interpretation?

 

-

 

Now CMOS sensors... gah.

 

It sounds like CMOS sensors work on the same tangent as CCDs, except for the fact that the whole device is actually a chip, or die, consisting of several millions of transistors contained on a single silicon wafer. These transistors seem to work by sending and receiving electric signals via gate logic (on and off, going one way or the other), later essentially doing the same thing as the CCD does with distributing this electric signal across a wafer coated with chemicals to "arranging" channels of color.

 

I also forgot to mention that these devices convert light into electrons in order to "interpret" them as electric signals received by the CCD's semiconductor and the CMOS's transistors. Gah.

 

I also just re-discovered HowStuffWorks.com after having read that whole damn Wiki article. Gah...

 

HSW describes the workings of CMOS and CCD in a few simple descriptions:

 

-

 

* A CCD transports the charge across the chip and reads it at one corner of the array. An analog-to-digital converter (ADC) then turns each pixel's value into a digital value by measuring the amount of charge at each photosite and converting that measurement to binary form.

 

* CMOS devices use several transistors at each pixel to amplify and move the charge using more traditional wires. The CMOS signal is digital, so it needs no ADC.

 

-

 

If I haven't confused you yet, and to conclude, just be appreciative of your digital cameras. I know not a lot of us use film anymore...

 

Oh, and CCD is better than CMOS, but not by far nowadays, with CMOS catching up to par.

 

Quotes are from Wiki and HowStuffWorks... two awesome sites.

[GamerGlitch 0]

Well... That was very confusing.

[rayzoredge 2]

Well... That was very confusing.

Wow. Xisto killed my post? (It didn't post my reply.)

What part was confusing? My part or what Wiki had? (It was hard to translate it into English.)
Edited September 4, 2007 by rayzoredge (see edit history)

[megabytemb 0]

cool i guess the old sayin u learn somthing new every day should definetly be applied herei never new it was so complex

[TikiPrincess 0]

I think this would have been a little more interesting and perhaps less confusing if you could describe it in your own words rather than relying on Wiki and How Stuff Works. My mom took a photography class a couple years ago for fun and learned about the basics of photography. She even made a camera out of a shoebox.

Basically, you poke a little hole in the center of the shorter side.

Next you make sure that all the edges are sealed tight so that no light can come through.

Then you go into a dark room and stick in some photo paper on the other end of the shoe box.

Cover up the hole so that no light can get through it.

Since you can't actually see what you'll be filming, you have to look carefully and try to frame your shot. That's why you'll sometimes see photographers making a square with their hands and looking through it to see what the picture will look like when it's cut off from the surrounding environment.

For some reason, the aperture in the box makes things appear upside down, so your photo will actually be upside down. Our eyes actually work the same way, but different things with the cones and rods and stuff in the retina or whatever fix it so that we see right side up. That's why the cameras use mirrors to fix this anomaly.

You're relying purely on available light to create an exposure, so it's best to take the picture in bright sunlight. But you also want to make sure that there's not too much light. Key times would be late in the mornings and mid-afternoon for most of the year. If it's winter, you might need the full sunlight of early afternoon. Summer light can last well past late afternoon and provide adequate light for most shots.

Once you've chosen your shot and found the right lighting, you need to set your box camera on something stable because any movement will be recorded on the exposure. A tripod doesn't really work very well for this kind of camera, but a stool or any sort of solid surface that won't move is adequate. You'll need to leave the hole open for anywhere between 10 seconds to few minutes based on the amount of light and contrast you want. More time means more exposure, which means a darker picture.

Finally you develop your paper.

The way film used to work is that there were two strips of different material attached. I think one of them was silver and the other was a photosensitive metal that would burn away as it was exposed to sunlight. This created the greys and shades of contrast in the photo. I'm not sure how modern day cameras work, much less digital, but this is what my mom learned while she was taking her class, and she was so excited about making her own camera that she was kind enough to share the information with me.

[auygjhsagy 0]

Well certain Cameras has this feature. You mean to keep it on the camera? Well it's only digital cameras that can do this. You take the picture and it gets saved on there automaticly. If your talking about the other capture all I can say is: ?HUH?

[Forbez 0]

I had a small idea of how cameras took pictures, this clears it up for me. Thank you for this great article, I hope to see more in the furture.

[Matt2 0]

TikiPrincess, that's really interesting, I might have to try it one day

[g3place 0]

Thats pretty cool, I never knew these facts. Its quite interesting how a camera really works. All those parts and functions, and yet not all know what each do. Its quite a shame that most old people think that expensive is good, and they only use a little of the many possibilities in a camera. Sigh.

[Unknown_99 0]

Strange enough I never thought about the mechanics on how a camera actually works XD So yes thanks for giving me some enlightenment on the subject.

[rayzoredge 2]

I think this would have been a little more interesting and perhaps less confusing if you could describe it in your own words rather than relying on Wiki and How Stuff Works.

Well, I tried to describe what a CMOS and CCD processor was and how they worked, because reading it off of Wiki and HSW was way too confusing.

What you described is a dead-basic camera... the camera obscura. Ironic how we can still find that to be wicked cool to accomplish when we have and take for granted 35mm, digital, SLR, and other camera technologies.

[angad619 0]

Good writeup rayzoredge. This sets the base up. Those who didn't understand anything can google it and find more about it

Notice from rvalkass:

Xisto requires you to make quality posts that contribute to the thread. Look through the rules and make sure you follow them.

[TikiPrincess 0]

Well, I tried to describe what a CMOS and CCD processor was and how they worked, because reading it off of Wiki and HSW was way too confusing..

I think that's part of the problem, you were trying to explain something that you're learning off Wiki and How Stuff Works rather than something that you've experienced for yourself. Actually doing it or visuallizing it as with a classroom demonstration or educational video gives you a better understanding of the hows and whys. I firmly believe that experience is a better teacher than a textbook.

What you described is a dead-basic camera... the camera obscura. Ironic how we can still find that to be wicked cool to accomplish when we have and take for granted 35mm, digital, SLR, and other camera technologies.

The topic title states "How Do Cameras Capture Pictures." Having participated in a photography class myself and sharing my mom's experience a little more recently, I offered my understanding of how a picture is captured. While it's a simple method, it actually still works and people might be able to see how minute differences, like exposure time and the size of the aperture, really affects the outcome of the photo.