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Tuesday, December 10, 2013

How does a digital camera work - Part 3

In Part 1 we saw how millions of tiny bits of silver can make up an image. In Part 2 we explored color filters and their role in seeing some things and not others. Today let's take a baby step into the electronic world.

Everyone knows about photo-voltaic (solar) panels and how they generate power. Claims that they will save the planet are, of course, ridiculous since I found they couldn't even charge my mobile phone reliably. We would have to pave the planet with them to generate enough power for all our needs.

That rant aside, they do create a voltage when exposed to light. Best of all it is roughly proportional to the amount of light falling on them. That is, at high noon they produce some volts, at midnight on a moonless night they produce zero volts, and in between they produce some value between the two.

Ambient light like… Ambient light (lux) Photocell resistance (?) LDR + R (?) Current thru LDR+R Voltage across R
Moonlit night 1 lux 70 KΩ 71 KΩ 0.07 mA 0.1 V
Dark room 10 lux 10 KΩ 11 KΩ 0.45 mA 0.5 V
Dark overcast day / Bright room 100 lux 1.5 KΩ 2.5 KΩ 2 mA 2.0 V
Overcast day 1000 lux 300 Ω 1.3 KΩ 3.8 mA 3.8 V
Full daylight 10,000 lux 100 Ω 1.1 KΩ 4.5 mA 4.5 V
From Using a Photocell

If we took a photo cell and hooked it up to a meter we could see the voltage it produced. Then we could point it at things and see if they were bright or dark by looking at the meter. Not very practical since we could just look at the object we were pointing at but stick with me for a minute more.

Suppose you were blind and couldn't see visually. I could try to describe something but it would be very difficult. However, you do understand warm and cold from touch. You have a vocabulary for warm and cold. What if we were to translate what I saw into that warm/cold vocabulary so you could create a representation you could understand.

I might say "Upper left corner, cold zero. Upper right corner, warm 5. Lower left corner, warm  5. Lower right corner, hot 10". This would describe a box that was black, grey, and white.

Black Gray
Gray White

For more precision I would point my photo cell at the object to get exact readings to help my friend see the best image possible. To make an even better picture I could increase the locations I was pointing the photo cell at and get a better defined picture for her.


spac spac spac spac spac spac spac spac spac spac
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .

Can you now see a tree, a cloud, and a stream in a meadow?

Since I like my friend a lot and don't want her to be dependent on anyone I could hook the photocell directly to a small heating element and give it to her in a box. She could point it to anything she wanted and create her own vision of the surrounding world by putting her finger on the heating element.

Think about all this and I'll be back soon.



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Monday, December 2, 2013

How does a digital camera work - Part 2

In the last post I talked about how a film image is composed of millions of bits of silver blocking the light so that what could be seen could be reproduced although as a light-for-dark negative image.


But what about colors? We see colors and know that color photographs exist, so how do they do that?

There are two ways to do it. One is reflected and one is transparent. Think of the first as looking at a painting using colored pigments and the second as looking through colored pieces of glass.

I'm going to talk about color in a very simplistic way. Remember those 3-D books and movies where you wore cheesy cardboard glasses (technical term - anaglyphic) with one blue lens and one red lens? The images on the screen or page were very confusing when viewed with the naked eye but jumped off the page through those lenses.

The principle is simple. The lens passed the color that was the same and blocked the color that was opposite. Thus, the red lens passed the red color which made it seem the same as the background but blocked the blue color which made it seem black. The blue lens did just the same but for the other color. That meant that left eye looked through the red lens and saw the blue (now black) image and the right eye looked through the blue lens and saw the red (now black) image. The brain interpreted these slightly different images as the same but at different distances (parallax).




Back to our film with millions of silver bits that can only produce black and white. If we were to coat some of the bits with a filter agent so they could only see red, then we could tell where the blue in the image was. The same would be true of yellow and blue filters. Combine red and blue to get purple, look across the color wheel and you see yellow. Just the red filter and you can see green. So imagine taking three pictures of the same scene, one each through a red, blue, and yellow filter. When looking through the three images laid on top of each other all of the colors would be recreated.


I cheated a bit here. The real colors are red, green, and blue but you get the idea (hopefully). The filters over the sensitive silver bits are what make the difference between the colors.


See you soon for how this applies to digital devices.




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