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When buying scanners, you will see them described with with terms like "24-bit color" or "36-bit color." That is simply computer jargon (geek-speak) meant to tell you how accurately the machine will sample color.
Let's dispel the mystery.
What is a bit?
A bit is the smallest unit of digital measurement. You can think of a bit as being like a tiny electronic switch. Each bit has only two positions: on and off. There is no in-between.
A single bit cannot do a lot by itself. It can really only indicate one of two possible values such as off or on, 0 or 1, no or yes, black or white, minus or plus, demagnetized or magnetized, etc.
However, by combining bits, you can build larger and larger values that can stand for numbers, letters, sentences, databases and whole libraries. When properly combined, bits can achieve infinite amounts of communication. In fact, bits are the entire ackbone of all computing. Here is how you can combine bits to tell you more than simple on and off.
Let's say you want to scan an ordinary stock certificate and you only have one bit at your disposal. Off equals black and on equals white. If you measure an ordinary stock certificate in that manner, every sampled spot is called either "black" or "white."
Now, imagine that your equipment is good enough to allow you to measure your certificate in small increments. Let's say you divide your certificate into tiny squares, 1/100th of an inch on a side. An ordinary stock certificate would contain about 968,220 squares.
If you then measured the brightness of your certificate with a very simple scanner at each of the 968,200 points, you could portray your certificate with 1-bit accuracy. Every bit would be either black or white, but if displayed on a computer monitor would be good enough to show crucial details.
You can probably imagine that the next logical step is to describe each sample point with two bits.
Amazingly, by merely adding one extra bit, the accuracy of your sampling doubles. With '2-bit' sampling, every point on your certificate can be described as black, white, light gray or dark gray. Displayed on a monitor, 2-bit sampling looks like this.
Why not add even more bits?
You can double your sampling again to 4 bits per pixel. How many description possibilities do you get? Simply multiply 2 x 2 x 2 x 2 = 16. You can see that 4 bits gives you the possibility of measuring and displaying 16 shades of gray. Scanning the same certificate with "4-bit depth" gives you an image that looks like this:
Because so much of the certificate is an in-between shade of gray, we don't see a dramatic improvement over the previous scan. However, look closer. See the four cancellation holes at left? They were missing from the previous example. Also look at Vanderbilt's coat. We can see some of his lapel starting to show. Also not that the background behind Vanderbilt is much smoother than the '2-bit' example.
If you double your bit-depth once again, from 4 bits to 8 bits, you increase your accuracy of rendering grays even more dramatically. With 8 bits, you can show 256 different shades of gray. (2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256 possible combinations.)
Why stop at 8 bits? Why not double the sampling to 16 bits? That would give us a staggering 65,536 shades of gray.
Actually, 16-bit grayscale is used for high-tech uses like digital interpretation of images. HOWEVER, the young human eye can distinguish only about 30 shades of gray, so sampling anything better than 8-bit accuracy is unnecessary if an image is intended for human consumption.
So, how do we measure color?
Measuring gray is simply a matter of measuring brightness and darkness across all of the visible spectrum. If you added colored filters over your certificate, you could measure the amount of light passes through. A yellow filter, for instance, would remove light from the blue part of the spectrum, but would allow green and red light to pass through easily. Understanding the relationship between various colored filters is not trivial and is unnecessary here. Suffice it to say that measuring the amount of light that passes through yellow, magenta and cyan filters would allow measurement of any part of the color spectrum.
Scanners and digital cameras closely measure the amount of red, green, and blue light bouncing off objects with the same degree of accuracy that they can measure gray. In geek-speak, each color is called a channel. Using 8 bit bits of sampling accuracy, scanners can distinguish 256 shades of red in the red channel, 256 shades of blue in the blue channel and 256 shades of green in the green channel. (You could add any number of channels, of course, but you immediately surpass human sensory capabilities.)
Add these bits together (8 bits of red + 8 bits of green + 8 bits of blue) and you have 24 bits of color being measured at each pixel. Here is the result of scanning with 24-bit accuracy.
How many different colors can you measure with 24 bits?
Doing the math (2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2), we find the theoretical capability of 24 bit sampling is 16,777,216 colors. Experts estimate the best human eye should be able to discern about 10 million colors.
But, still, you could add more bits. Right?
Absolutely. In fact, some scanners (but few image manipulation programs) measure 36-bit, even 48-bit color. While the human eye cannot discern such a vast array of color information, engineers can create machines that can. Potential applications for "deep" color information include devices for matching colored paints and dies. Dentistry is greatly dependent on matching the colors of implants to existing teeth. Other applications might include accurately discerning micro color differences in surface coatings that have variable translucency, reflectiveness, and glossiness.
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