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
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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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