Laser projectors


Laser projectors

A laser projector is a collection 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 3 sources for RGB (red, green, blue) full color projection.

Typical components


=Laser Diodes=

* Red: 635nm, 650nm, 670nm
* Blue: 450nm
* violet 405nm


=Solid State DPSS=

* Red: 671nm
* Green: 532nm
* Blue: 473nm & 457nm

Gas lasers

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

Galvanometer scanners

Galvanometers, sometimes known as "scanners" or "Galvos" are the 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, 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 ot 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.

DAC

In the case of computer control, a Digital-to-analog converter (DAC) is needed to convert the digital control signal form 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

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=

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

;BlankingBlanking 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.

;ModulationMost 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. 2kHz can be considered low and 30kHz can be considered high. Manufacturers do not specify an exact relationship between this number and the behavior of the laser.

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

;TTL ModulationTTL 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.

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

;Scan AngleThis 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.


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