wikipedia article on Image scanner - Wikipedia discusses “charged coupled device”, “contact image sensor” and high resolution digital cameras as dominant technologies in modern scanners. The word “laser” is not mentioned at all in the article.
So why are lasers good for barcode scanning but don’t even show up as an also-ran secondary importance technology for general purpose scanning? E.g. if the digital camera scanner is described as good for nondestructive book digitization since apparently it can work at an angle, could a laser based technology also achieve that benefit while providing higher precision in capturing black-and-white text?
If you look at the wiki for bar code scanneryou’ll see that laser scanners aren’t the only method of reading bar codes anymore. Their advantage is they are less affected by ambient light. But they require a mechanical system to scan the laser. That’s simple when reading a bar code. It would be slower and more costly than any array device to scan an arbitrary image. I suspect the rapid improvement in ccd and similar technologies will make laser bar code readers obsolete soon.
To scan a randomly aligned 2D barcode at various ranges (up to meters in some environments) a bright point source with low dispersion moving in a pattern that guarantees a single crossing path across the target. For this, a laser is ideal and the known frequency nature of a laser additionally allows filtering on the sensor to avoid ambient light interfering with the signal. The fixed path of the laser also allows a fairly simple mechanical solution.
A laser based 2D scanner would have to scan every element of the target at the desired resolution. This will be very slow and far more complex mechanically (remember, you need mirrors to move the laser). Also, the fixed frequency nature of the laser light would not be useful for a general purpose solution - black and white barcodes are fine, but you could lose information if you scanned a target that did not sufficiently absorb/reflect your monochromatic source.
For this reason, 2D cameras or 1D CCD scanners are preferred - a 1D scanner only has to move in one plane and moves a consistent (both intensity and frequency distribution) light source with it (but requires flat targets). A 2D camera has more flexibility and speed (capturing the entire target in one go) but has far more issues with lighting and arrangement.
So, for 2D scanning, a laser is slow and complex compared to a camera. For 3D, a laser is slow and accurate, but systems like Kinect and new Light Field cameras will probably change that rapidly.
Barcodes don’t require really good alignment – you can scan approximately perpendicular to the code and get a goos reading. Plus, especially in the early days, you wanted an intense source. Lasers were perfect, because they were really bright and it didn’t matter what the scan direction was. But they did freak people out.
With 2D coding, the direction is very important. With a camera-based system, you can snap a shot into a frame0grabber and perform all the needed calculations to orient the code properly inside the computer. I’ve worked at a machine vision company – they did this efficiently and rapidly. They even had a system that snaped a picture of a can and could “unroll” the label for you. Using a laser scanner to repeatedly 'raster-scan" the spot takes longer than taking a single photo, even if you then use similar software to “re-orient” the image.
plus, as I note, “laser” still freaks a lot of people out. At one of the companies I worked at a big selling point of our system was that we did not , unlike our competitors, use lasers.
Laser barcode scanners were invented way back when imaging detectors were expensive and cumbersome. A laser is reflected off a rotating or oscillating mirror, so it sweeps across the target area repeatedly. A non-imaging light sensor (photodiode) measures the overall brightness of that whole area. So when the laser sweeps across a bar code, the brightness variation corresponds exactly to the black/white pattern. This is a very simple system that eliminates the need for an imaging detector, and also eliminates alignment difficulties: the laser just needs to hit somewhere on the barcode, straight enough that it sweeps from one end to the other.
If you try to use the same technique to read a 2-D barcode, you would have to sweep the laser in 2 dimensions (like the electron beam inside a CRT), which requires a mirror with 2-axis (tip/tilt) motion. And unless the laser is aligned perfectly with the barcode (i.e. the laser hits and scans across each row of dots on the barcode precisely), you don’t get an electrical signal that corresponds directly to the barcode pattern.
got it, so laser scanning would take a long time and maybe involve more expensive equipment.
Question - would laser scanning of black-on-white text have any benefit for whatever niche use if the problem of low speed of the aiming device were somehow resolved? Would such a hypothetical system have any advantage over the established competing systems, or would it still be at best a replication of what other systems do perfectly well?