Laser projector

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A laser projector is a device that projects changing laser beams on a screen for entertainment or professional use. It consists of lasers, mirrors, galvanometer scanners and other optical components housed in an enclosure. A laser projector can contain one laser light source for single color projection or possibly three sources for RGB (red, green, and blue) full color projection.

Typical components[edit]

Laser Diodes (Direct Injection)[edit]

  • Red: 635 nm, 642 nm, 650 nm, 660 nm
  • Green: 515 nm
  • Blue: 445 nm
  • Violet: 405 nm

Solid State DPSS (Diode-Pumped, Frequency-Doubled)[edit]

  • Red: 671 nm
  • Green: 532 nm
  • Blue: 473 nm, 457 nm

Gas lasers[edit]

  • Red: HeNe (Helium-Neon) @ 632.8 nm, Krypton @ 647.1 nm
  • Green: Argon @ 514.5 nm
  • Blue: Argon @ 488 nm or 457.9 nm
  • Multi-colour (whitelight): Mixed gas Argon/Krypton 647.1 nm, 514.5 nm, 488 nm, 476.5 nm, 457.9 nm

Galvanometer scanners[edit]

Galvanometers, sometimes referred to as "scanners" or "Galvos", are electromagnetic devices that move mirrors which reflect the laser beam and essentially create the patterns, text or animations. The mirror is mounted on the end of a rotary shaft that is deflected when power is applied to the device. Galvos are typically identified by their speed of operation measured in Kpps (kilo points per second). 8k, 12k, 20k, 30k, 35k, 50k, 60k are all speeds that are generally available. The faster the galvos, the smoother and more flicker-free the projected image is. Each galvo moves the beam in one plane, either X axis or Y axis. Placing the galvos close together at 90 degrees to each other allows full movement of the laser beam within a defined square area. The most useful specifications of a galvo pair for laser show use are the speed at which the galvo can draw points and the angle at which this speed is achieved. Galvos come in two main groups, open loop and closed loop (most prevalent) defining the presence of a feedback signal. They are driven by amplifiers that are not too dissimilar to audio power amplifiers, that in the case of closed loop devices have the ability to take a feedback signal from the scanner to make an accurate, quick and repeatable placement of the mirror position.

Controller (DAC)[edit]

In the case of using a computer to control a laser projector, a Digital-to-analog converter (DAC) is needed to convert the digital control signal from the computer into analog signals that control the scanners in the laser projector. Typically, 2 channels are used for x-y position control and 3 channels are used for controlling the RGB values of an RGB projector. In the case of a single color projector, the intensity channel is used instead of the RGB channels. Most commercially available projectors and DACs are compatible with the ILDA standard that specifies the channels and pinout for the 25-pin D-SUB input connector on the projector.

DMX[edit]

Many laser projectors and galvanometer sets include Digital Multiplexing (DMX) input. DMX was originally designed to control theatrical lighting, but has spread to laser projectors over the years.

DMX allows the user to control the inbuilt patterns of the projector. A few of these features are Size, pattern, colour and rotation. However, DMX Does Not let you design and display your own graphics/animations, it is simply just a way of controlling the patterns included in your laser projector. A Digital to Analog Converter (DAC) is required for custom graphics/animations.

Dichroic Mirrors[edit]

A dichroic mirror is a mirror with different reflection or transmission properties at two different wavelengths. Typical dichroic mirrors used in laser projectors pass red light and reflect green and blue, or pass green light and reflect red and blue. Dichroic mirrors are required for combining laser beams of different colors, e.g. to combine the red, green and blue beams into a single white-light beam. The individual red, blue and green lasers are then controlled in brightness (modulated) to produce any desired color in the final beam. A typical analog-modulated RGB projector has 256 brightness levels for each laser. This gives (256 x 256 x 256) 16,777,216 different available colors (the same as a modern computer monitor).

Typical terminology[edit]

Blanking[edit]

Blanking is a state in which the laser beam is turned OFF while the mirrors are changing position during the creation of animations or text. Blanking is an ultra fast operation happening typically hundreds of times per second. New technology solid state lasers use direct electronic control of the laser source to provide the blanking. With gas lasers such as argon or krypton, this was not possible and the blanking would be carried out by use of a third galvanometer that would mechanically interrupt the beam. As new technology was developed, a Poly-Chromatic Acousto-Optic Modulator, or PCAOM was used, that allowed high-speed electronic blanking, intensity control and color selection of a multi-color laser beam.

Modulation[edit]

Most DPSS lasers used in laser projectors support modulation. Modulation has to do with blanking but is a slightly broader term. A DPSS laser will support either 'analog modulation', 'TTL modulation' or both. Modulation is usually specified in terms of kHz. 2 kHz can be considered low and 30 kHz can be considered high. Manufacturers do not specify an exact relationship between this number and the behavior of the laser.

Analog modulation[edit]

An analog signal is used to control the intensity of the output beam. This signal is usually a voltage in the range of 0 V to 5 V. With an RGB laser and analog modulation you have, with an 8 bit system, 16.7 million colours at your disposal.

However, since most laser show software uses a 0-100% control for laser brightness modulation (so 100 steps instead of 255), the total of available colours at disposal is 1'000'000. Furthermore, usual laser sources start lasing at a voltage between 1-2 volts and reach their full brightness at voltages between 3.5-4V, and the power/voltage curve between these points are usually not perfectly linear. Consequently the dynamics of the color palette in a real lasershow use is decreased to only a few thousands of different colors.

TTL Modulation[edit]

TTL modulation indicates that the laser does not support analog modulation of the output but only ON / OFF control. See blanking. With an RGB laser and TTL blanking you have seven colours at your disposal. Red, Green, Blue, Cyan, Magenta, Yellow, White.

ILDA[edit]

The International Laser Display Association. A trade association dedicated to promoting the use of laser displays.

Scan Angle[edit]

This term defines the optical angle that can be achieved by a set of scanners normally at a given rate of points per second. The wider the angle the larger the area covered by the scan will be but the more difficult it will be for the scanner to accurately track due to physical limitations of the mechanical nature of a scanner. As an example a 20 degree angle will provide a 3.5 metre scanned area at a distance of 10 metres from scanner to screen. Scan angles can be easily calculated using standard trigonometry.

See also[edit]

External links[edit]