This article discusses how polarizing filters work, how to polarize a light source, and how to filter photosensors so that your robot can easily detect two distinct beacons. Additionally, this article describes the steps to machine a test apparatus to align polarizing film.
Most light sources (light bulbs, fire, the Sun, LEDs) emit light that wiggles in all orientations (up and down, side to side, and diagonally). This is called unpolarized light or non-polarized light.
It is difficult to keep in mind that we are only talking about the orientation (or rotation) of the wiggling. This has nothing to do with the direction that the light is aimed, the color of the light, the brightness of the light, or the blinking (steadiness) of the light.
The best analogy that I’ve heard is a jump rope. Imagine you hold one end of a jump rope and I hold the other end. I can wiggle the rope side-to-side or up-and-down (the orientation), but regardless, it is still being wiggled towards you. Likewise, a flashlight can be aimed at you, but the light coming out of the flashlight actually wiggles in a variety of orientations.
A non-polarized light source has an up-and-down component that passes through a vertically-oriented filter, but the side-to-side component is blocked.
Polarizing filters block most of the light except that which vibrates in the filter’s orientation. For example, if a linear polarizing filter is set up vertically, then most of the up-and-down wiggling light will go through and most of the side-to-side wiggling light will get blocked.
Continuing with the jump rope analogy, you can imagine the same would occur if you fed a jump rope through a picket fence and wiggled it up-and-down or side-to-side. The side-to-side waves would get blocked.
(If I were a better artist, the green LEDs would be located in the same place throughout the drawing so that you would see the light was coming from the same direction, and only the orientation of the lightwave had changed. Unfortunately, the wiggling lines clump together when I try to draw them from the same location. Sorry about that.)
A non-polarized light source has a side-to-side component that passes through a horizontally-oriented filter, but the up-and-down component is blocked.
Now change the setup slightly. Use the same light source and the same polarizing filter, but turn the filter 90 degrees (orienting the filter horizontally). The side-to-side wiggling portion of the light will pass through, but the up-and-down portion will not.
Polarizing filter material provides an easy way to alter a light source such that it becomes distinctly recognizable from its natural state. By placing a vertically-oriented polarizing filter in front of a light bulb, the light source will appear to only emit vertically-wiggling light. By placing a horizontally-oriented polarizing filter in front of a light bulb, the light source will appear to only emit horizontally-wiggling light.
To complete this system, we need a sensor that can detect vertically-oriented light and a sensor that can detect horizontally-oriented light. It turns out, this can be accomplished by simply placing polarizing filters in front of photosensors!
As we just discussed, placing a vertically-oriented polarizing film in front of a non-polarized light source will only allow the vertically-oriented light to pass through. In doing so, a polarized light source is created. Let’s call this the “vertical beacon”.
A vertical beacon can be detected by a sensor with vertically-oriented film, but not horizontally-oriented film.
If a piece of vertically-oriented polarizing film is placed in front of a light sensor (such as a photoresistor), then the vertically-oriented light produced by the beacon can pass through and be detected by the sensor.
But, if a piece of horizontally-oriented polarizing film is placed in front of a light sensor, the vertically-oriented light will be blocked and the sensor won’t see anything because the vertical beacon only emits vertically-oriented light.
These same filters and sensors also work on a horizontal beacon.
By having two separate sensors, one with vertically-oriented polarizing film and the other with horizontally-oriented polarizing film, the robot or electronic device can compare the difference in lighting levels.
If one light level is high and one light level is low, chances are that the sensors are facing a beacon that is emitting polarized light. The sensor with the higher light level determines whether the robot is facing the vertical beacon or the horizontal beacon.
Lastly, with the two sensors, the robot can also determine that it is not facing either beacon. Here’s how:
A non-polarized light source produces the same detection levels in both the vertical and horizontal sensors.
If the photo sensors are facing ordinary room lighting, the sensors will see the same lighting levels. This allows the robot to determine that it is not looking at either the horizontal or vertical beacon.
Using this technology, it is easy to mark two different locations that the robot can differentiate from the rest of the environment. The vertical beacon location and horizontal beacon location might be a charging station and destination. Or, they might represent opposing goals or home spaces in a robot competition.
The idea to use polarizing filters for robot beacons and sensors came from Dr. Randall D. Beer, Dr. Hillel Chiel, and Dr. Richard F. Drushel of Case Western Reserve University. (Beer, R.D., Chiel, H.J., Drushel, R.F., Autonomous Robotics to Teach Science and Engineering. Communications of the ACM 42, 6 (1999), 85-92.)
In the robotic egg hunt at Case Western, each team’s home nest is indicated with a polarized beacon. One nest has a beacon that emits vertically-polarized light. The other nest has a beacon that emits horizontally-polarized light.
A robot collects an egg and then deposits the egg in its nest, by locating its polarized beacon using polarized sensors.
A good thing about using polarized visible light for beacons is that it is less likely to interfere with infrared operations. Many robotic sensors use modulated (pulsed on and off) infrared light for obstacle detection, communication, or remote control commands. An infrared beacon would interfere with infrared robotic operations.
But, even better, in a properly-engineered environment, polarized light can be absorbed or randomly oriented (non-polarized) when reflected. That way, a sensor is less likely to be fooled into seeing a reflection as a beacon. A rough, dark surface, such a black-painted wood, is a good choice for significantly reducing polarized visible light reflections.
Let’s find out where to obtain polarizing filter film and how to select it...