Part of the Entertainment Lasers Metanode
Controlling Laser Light
s only emit a single point of light. To change this point into a graphic
on a screen, or a wave or tunnel in the air, laser artists take advantage of persistence of vision
. In simple terms, laserist
s move the single beam of light so fast that our eye does not distinguish the individual points, but instead perceives an entire unbroken line of laser light. The laser projector
consists of the series of devices that control the laser beam allowing it to move, change colors, or add other special effects.
The first component necessary to draw an image or shape with a laser is a pair of galvonometer
s, often called scanner
s. From the housing of each of these small devices extends a tiny metal shaft. A small mirror is attached to the end of this shaft. When electronic signals are given to the scanners, the shaft turns back and forth at very precise distances and speeds, thus turning the attached mirror. The scanners are mounted at right angles to each other so that the laser beam first bounces from one mirror to the other, and then out of the projector to the screen. The first mirror scans the x-axis
, or the left and right motion of the drawn line, while the second controls the y-axis
, or the up and down position of the beam. By coordinating the movements of these scanners, the laser beam can be reflected to any point within the scanfield
. The scanfield is the measure of the height and width of the area the beam can accurately be scanned in. The scanners then rapidly change from position to position, thus causing the laser beam to move.
Extremely high speeds are required to make a line appear rather than just a single point. Even to scan a simple shape such as a circle, the scanners must retrace their steps hundreds of times per second. Most new scanners work at 30 hertz, which means they can theoretically draw up to 30,000 points per second. However, inertia of the mirrors can slow down the scan speed. Like a car entering a curve, the scanners must slow down to navigate harsh changes in direction or location. This means that an image is limited in the number of distinct points that can be drawn. Complex images will require slower scan-rates. The hazard here is that as the scan-rate drops the eye begins to see the movement of the beam and the image appears to flicker. This is also why video images of laser shows tend to be very flickery, as video cameras are more sensitive to the retrace-rate of a laser image than the human eye.
The next device in a typical laser projector is the PCAOM. Short for the polychromatic acousto-optic modulator, this device’s name reveals some of its use. The PCAOM controls the color of the laser beam, and more importantly, what is known as blanking. With scanners alone there is no may to turn off or stop the laser beam. Drawings are then like those created on an old Etch-a-Sketch toy. Without a PCAOM or similar device, there is no way for a laser artist to lift their pencil of light. The PCAOM contains a special crystal. Certain types of crystals have the ability to refract, or break light up into its component colors, just like a prism. This crystal, when exposed to certain frequencies of sound, begins to vibrate. The rate at which the crystal vibrates causes the way it refracts to change. Changing these frequencies allows all of, or part of certain colors of light through to the scanners, while the rest is discarded as a waste beam. Discarding all of the colors is called blanking, and is what provides the “pencil lifts” necessary for scanning line art. Like scanners, the PCAOM reacts very quickly, and is capable of up to a quarter-million color changes per second.
Additionally laser projectors can contain devices for adding special effects to the laser image, or creating effects that do not require a scanner pair and PCAOM. This part of the projector, called the beam table, uses mirrors to direct the beam to various effects. These can be as simple as unmoving beams reflected out of the projector, or the beam can be passed through different types of filters. A diffraction grating breaks the laser beam up into many beams in a fan-like spread, diffusion softens beams, and machidas create fans similar to diffraction grating, but also break up the beam into its component colors. Another effect, called lumia, is achieved by shining the beam through distorted glass wheels. Usually just shower glass, lumia wheels produce haunting, ethereal wisps of light that seam to linger and then suddenly disappear as the laser beam passes through the rotating glass.