Laser Scanner NTSC FREQ. camera && projector - v2
---- WHAT IS THIS THING? ---
POISED is an object which captures images using one laser and in real time another laser renders those images. It is a laser scanner, both a camera and a projector.
Presented is a prototype for a larger laser system, one which scans on a human scale and projects on an architectural one. POISED is an object which captures images using one laser and in real time another laser renders those images. It is a laser scanner, both a camera and a projector.
The final form of POISED will be a unique laser projection system for use as an interactive piece of emergent technology.
---- HOW IT WORKS. --- (in short)
A circuit receives reflected light off the camera raster from a photomultiplier tube, a highly photon sensitive component.
This analog circuit sends those readings to an amplifier which increases the gain of that signal. That amplified signal is then sent to the projector raster, altering the brightness of the projection laser. Both the input and output lasers are synchronized in generating their raster scans, thereby scanning and projecting simultaneously.
--- SYNOPSIS of COMPONENTS
WHAT'S INSIDE THE BOX----
532nm INPUT LASER
532nm OUTPUT LASER
RESONANT SCANNER - 15Khz
GALVOS - 30hz
PMT - photomultiplier tube
OPTICS AND IMAGING GLASS
The PMT outputs a bipolar waveform. The Amplifier circuit will have to contend with the output impedance of the PMT, perform a Full wave Rectification, and amplify the signal by a gain of approx. 2 volts.
---SYNOPSIS OF KEY CONCEPTS ---
MATH - NTSC FREQUENCY:
This scanning system scans at NTSC frequencies. As such I have the Resonant Scanner oscillates at 15.75khz - in keeping with the NTSC frequency. The Resonant Scanner reflects both the Input Laser's and Output Laser's Collimated light in opposite angles to their origin Trajectory.
Precisely placed Galvos reflect these horizontal lines vertically. In keeping with NTSC frequencies, the Jones Divider receives the scanner frequency (in tandem with the resonant scanner) and divides the 15.75khz signal from the scanner by 329.97hz.
KiloHertz are units comprised of 1000 Hertz.
To convert hertz to milliseconds, first determine the duration or period of one vibration (in this case - 15750hz) by dividing one second by the frequency.
1 divided by X Hz times 1000 = X Ms
To Check our sanity check the math here.
Basically all of this means:
MECHANICS of RASTERING:
In scientific applications of this kind of imaging, most scientists would use a pre-fabricated mount for these components. I have fabricated mounts for these components as I need to adjust X, Y and Z of these components when setting up for scanning.
I designed a 3 axis gimbal-esque system machined from 7075 aluminum.
With this system I am able to calibrate the angles of the lasers and galvos laterally and horizontally: allowing a fine adjustment of the components in the optical field.
OPTICS & GLASS:
Both Galvos and the Resonant Scanner have first surface mirrors.
The PMT has a 532nm optical bandpass filter which filters all wavelengths except for the 532nm laser light, allowing the PMT to translate into voltages, only the reflected light from the Input Laser raster scan.
I am using this 532nm bandpass filter.
As depicted in the below graphic, the image/signal from the Input Raster Scan will be received as an inverted (upside-down) image/signal. As such, even before determining my focal length and optic type(s), I will need to invert again the incoming image from the input raster scan so that the resulting projection image is not upside-down.
I'm currently using a telescopic 45° focusable assembly from a Wollensak scope.
This lens can capture a wider angle of view. The face of the lens has a concave shape which allows a larger area of light to be focused toward the bulbous shape above.
Another lens focuses this lens, making the resulting image closer.
PHOTON TO ELECTRON - SIGNAL PROCESSING:
When the Input Laser's light, while scanning - hits a surface, the 532nm collimated lightwaves bounce off this surface. Some photons are absorbed by the surface, but some bounce off - the amount of photons which bounce off from any given surface will vary based on the quality of that surface (i.e. shiny surfaces will reflect more and absorb less whereas a matte surfaces will absorb more and reflect less). These photons are received by the PMT through precise placement of the PMT and by filtering what the PMT receives through optical components.
Photomultiplier Tubes are one of the few vacuum tubes to yet be replaced by silicone versions. They are ultra sensitive devices which when photons enter through the front face of the tube, move through a surface which gives the photon an electron-philic behavior. The tube has little fingers of metal preloaded with high voltage (i.e. tons of electrons hanging out on these fingers). As the photon bounces from electron-filled finger to electron-filled finger, it collects electrons, essentially translating photons into electrons.
Once these photons have become an electronic signal, the signal is still weak, and for the signal to be significant enough to translate small variations in surfaces, this signal must be amplified. The Amplifier Circuit then increases the amplitude of the signal from the PMT and translates this to voltages which can modulate the Output Laser.
Here is a catalog of Hamamatsu's Visible Spectrum Line of PMTs.
--- PRODUCTION of Version 2.0 ---
GAIN OF PMT SIGNAL:
Please see this blog post on the process of configuring Gain from The PMT.
MORE THAN GAIN:
The amplifier circuit requires more than gain. As the laser scans the area for imaging, the input voltages - there will be areas which reflect more and areas which reflect less. All areas will have details I wish to image.
In other words, dark patches of an object being imaged still have variances I want to collect. Bright areas are the same. So, a simple threshold value will not be sufficient to tell the Output laser to go ON. That threshold value needs to compare itself not to a stagnant value, but rather a variable value which compares itself to the previous value.
*Click link on graphic below to see logic*
The Jones Divider, is a pic chip programmed for me by the famous Jones Video Synthesizers -used to divide the resonant frequency of the Resonant Scanner to create the 525 lines per frame for the Galvos. As it is a pic chip, I am unable to reprogram the duty cycle (the Galvos are non-responsive because the duty cycle is too high).
I need the Galvos frequency to be synchronized with the Resonant Scanner frequency, I need to sample to the output sine wave generated by the Resonant Scanner and PID the output to the Galvos by checking in with the Resonant Scanner. The Scanner outputs a sine wave at analogRead values from 1-1023, as expected.
The Resonant Scanner resonates, but has some variability in frequency. This means that the two components will eventually go out of phase with one another.
---- Teensy Exploration ---
***Be sure to follow these instructions carefully before attempting to upload to the teensy**
To compensate for this digitally, by sampling the output of the Resonant Scanner, waiting for a rising edge of a wave, and then triggering a 29.97hz at 50% duty cycle waveform. At the end of that cycle, the code should re-sample the Resonant Scanner sine wave, and adjust accordingly.
The code I am currently drafting will wait 6 times the target frequency (15.75Khz) - so that the Resonant Scanner will have enough time to begin to resonate (it takes a few seconds).
Then the code will start a frequency count using this library. Once this frequency has been read, the Galvo signal will be derived from the frequency sampled, and be divided by 29.97. Using this library, I will produce a PWM signal at a specific duty cycle which is a direct derivative of the Resonant Scanner Frequency.
Here is the Code for this:
The fallback of this logic is that I will no longer be producing a 30-frame-per-second video, but a 15 frame-per-second video.
---- OPTION B- Circuit Dividerv& PLL ---
J. M. De Cristofaro has worked with me to develop a Circuit for this by working with a Phase Locked Loop, a Multiplier, and then Dividers with some Logic Gates - incorporating exclusively - IC's from the 4000 Series.
---- PHASE LOCKED LOOP -----
A phase-locked loop (PLL) is an electronic circuit with a voltage or voltage-driven oscillator that constantly adjusts to match the frequency of an input signal. PLLs are used to generate, stabilize, modulate, demodulate, filter or recover a signal from a "noisy" communications channel where data has been interrupted.
See this link for PLL Divider Info:
ADD LINK FOR EAGLE FILES FOR MEANWELL
---SCOPE AND GLASS---
To invert the image coming in from the Input Laser Raster Scan - I went to Surplus Shed, and found an assembly to work with:
The Wollensak 45 degree scope assembly has some crazy ergonomic geometry which created some CAD problems for me.
As I am sure my enclosure is not interested in ergonomics, I decided to remove as much of it as feasible.
--- PROCESS PICS ---
Optical Gimbal Mounts Process Shots...
Electronics Enclosure Process Shots...
PLL/Galvonometer Driver Process Shots...
--- Safety Notes ---
--THANKS AND APPRECIATION --
Eric Rosenthal // Boris Klompus // Entropy Warrior Systems and Research // Matthew Epler // Signal Culture // Dave Jones // New Lab // Simons Foundation // Parsons Making Center // O'Reilly Media // Creative Capital // NYU ITP // J. M. De Cristofaro
This is the second iteration of POISED. The first was one of learning and exploration, and there is a very very long-winded blogpost about that discovery stage here - if anything is unclear here, it will be spelled out on this blog - somewhere ... :