Hard data on a hot topic: Studying Climate Change With Lasers

Performing the experiment

Overview and equipment

The experiment aims to measure the specularity of a laser beam reflected from an ice or water surface, and is described in detail here. A guide to all the equipment you will be using can be found here. Before starting work on the experiment, you should read the safety notes

Laser Setup

The laser you will use to perform your investigation is a 532nm diode laser. The light emitted from the laser is powerful enough to severely damage your eyes if not used correctly! Never shine the laser into anyone's eyes or allow stray beams to escape into the surrounding area. You have a safety surrounding to block any stray reflections.

Tasks:

Water reservoir to reflect from: When filling the `freezing reservoir' you will need enough water to freeze such that the light does not reflect from the bottom surface of the metal box. However, care must be taken not to cover the top surface of the thermistor with any water. (Aim to make the water level just touch the lower surface of the thermistor using the pipette.)
Mirror: Metal mirrors rotate the polarization of light but, importantly, do not scramble it. The mirror needs to be positioned such that light can be reflected down onto the water/ ice surface and then back up to the photodiode.
Photodiode and rotatable polarizer: Align the photodiode such that the light reflected back from the mirror is collected on its surface (Hint: You can use the screw adjustments on the mirror to steer the beam some way but be careful not to move the mirror so much that the spot on the ice no longer hits where you want it). The polarizer must be positioned somewhere in the path of the beam between the mirror and the photodiode and perpendicular to the propagation direction of the light.
Once you have your apparatus assembled (see Figure below) you can align the laser beam onto the water and back onto the photodiode. Ensure that the laser light doesn't hit metal surfaces which may cause it to scatter in random directions.

Schematic of the experimental setup.

LabView

LabVIEW is a graphical programming for measurement and automation. In this investigation you will use LabVIEW to collect and analyse your data.

Useful shortcuts:

To toggle between the front panel and block diagram press [Ctrl] + [e]
To view the help menu, press [Ctrl] + [h]
Objects on the block diagram are connected with wires along which, data flows
To create a wire, left click at the output terminal of the starting block then again at the input of the final block
To view the function palette right click in an empty space on the block diagram

Tasks:

Getting started: To launch the LabVIEW program:

Open the `LabVIEW Outreach' folder on the desktop.
Open the file `Specularity' and wait for LabVIEW to load.
Open the file `Front Panel.vi' in the Project Explorer.

Modifying the code: Although most of the LabVIEW structure you will need is in place, before you can use LabVIEW to take data you will need to carry out the following tasks:

Find where the signals from the DAQ card enter the LabVIEW code and wire them into the labelled nodes.
The data read in from the thermistor and photodiode needs to be passed and converted into a format that you can interpret. You will need to make connections to the output graphs (shown on the front panel) in the block diagram and ensure that all necessary conversions are included! To do this, insert the right objects (mathematical operators) to convert the temperature signal into ^OC and to subtract the background light from the photodiode signal. Hint: See Figure below for an example of addition in LabVIEW.
Wire the thermistor and photodiode signals into the correct output graphs.
Insert the necessary componenets to calculate specularity from your photodiode signal and the mean temperature from the thermistor.

Hardware: You will need to connect your photodiode and thermistor to the LabVIEW DAQ. Use the table below for key to wire colour coding.

(a)Table of wire connections for DAQ card. b) Example of addition operation in LabVIEW.

Connect the photodiode to AI 0 and ground
Connect the thermistor to AI 1+ and AI 1-
Connect the thermistor power leads to AO 0 and ground (the DAQ provides the power for the thermistor)

Correcting the photodiode offset: The photodiode will give an output voltage even if the laser is not illuminating it. You will need to subtract this offset level from the signal you record. In order to do this, you will need to first measure the offset (think about what conditions you want to measure this under) and then incorporate this into labview.

Collecting the Data

In order for the peltier cooler to work effectively the aluminium block it is mounted to conduct heat away on must be covered by ice chips. Heat generated by the peltier can then be transfered away from the water/ice sample. However, the peltier itself must not become submerged as the ice melts — be careful not to use too much ice.
To freeze the water, set the peltier current to 2~A (it should take no longer than 10 minutes to freeze.) Ensure the water is fully frozen before starting to collect data.
Once the water has frozen, switch off the peltier cooler to allow the ice to melt as you take your data set.
Before taking the first measurement press the white 'run' arrow in the top left hand corner of the front panel in LabVIEW. You only need to do this once.
To take a measurement, press the green 'Run' button on the front panel and rotate the polariser continuously (through the full 360^O). LabVIEW records for 10 seconds so try and complete multiple rotations within this time.
LabVIEW determines the specularity by measuring the maximum and minimum values recorded by the photodiode during the 10 second window. It then calculates:

$I = I_0 \cos^2(\theta)\,.$

Once the measurement has been taken LabVIEW will ask if you want to save and plot or delete the data.
Repeat the measurement for different temperatures as the ice melts to build up a profile of specularity vs. temperature.

Analysis and Conclusion

Once you have collected your data you need to look if there are any trends or conclusions you can draw from it.

What is your result?
Is it what you expected? If not, why not?
Can you explain why you obtained the result you did?
Are there any systematic errors/ limitations of the equipment that might have affected your result?
How could you improve your investigation?