Core Project: Activity Six

Hands On Physics

Haze
Core Project
Calibrating the Sun Photometer

Overview:

Now it is time to calibrate the instrument. When your Sun Photometer is successfully calibrated, you ready to compare the information about HAZE which you collect with the information collected by others. You can now add your scientific observations to the HAZE data bank.

QUICK START! You can begin using your Sun Photometer immediately without calibrating it. Just remember to record your measurements and the date and time you made them in your notebook. But eventually you must calibrate the instrument. You can then insert all the data you previously collected into your calibration formula.

Calibrating the Sun Photometer:

All sun photometers determine the clarity of the atmosphere, and thus the blockage of direct sunlight caused by haze, smoke, dust and smog, by measuring the intensity of sunlight. When properly calibrated, different sun photometers will give nearly the same result.

One way to calibrate a sun photometer is to illuminate its detector with light from a calibrated light source known as a standard lamp. The problem with this approach is that calibrated lamps cost hundreds of dollars. And they must be powered by very stable power supplies that cost a few thousand dollars.

Fortunately there's a free, highly stable standard lamp that anyone can use. It's the Sun! Although sunlight varies slightly in intensity during the solar cycle, for practical purposes, the Sun can be considered a standard lamp. In fact, for more than a century scientists have used the Sun to calibrate various kinds of Sun photometers.

During the early 1900's, Samuel Langley and Charles Abbot of the Smithsonian Institution perfected the method used to calibrate Sun photometers that is still used today. The method is named for Langley in honor of his pioneering work in this important field.

You can use the Langley method to calibrate a Sun Photometer so that its accuracy is as good as a professional instrument costing hundreds of dollars.

Calibration Supplies

Be sure to collect all the necessary supplies BEFORE your calibration session. Otherwise you may get off to a late start. Before beginning, decide how you will measure the Sun's angle at each measurement. You can use the TERC VHS-1 spreadsheet that accompanies this manual. If you don't have access to a computer, you can use one of the methods described in the Concepts Section.

Here's a challenge: Measure the Sun's angle using the TERC VHS-1 spreadsheet and one or more of the methods described in Concepts. Then compare your results.

In addition to what you will need to measure the angle of the Sun (see TERC VHS-1 spreadsheet or the Concepts Section), here's what you will need:

Your Sun Photometer (completely assembled and working)
Digital voltmeter
Digital watch or clock
Method for measuring the Sun's angle (see separate section)
Notebook and a pen or pencil
Sunglasses, a hat and possibly sun screen
Linear graph paper
Digital calculator with natural logarithm (ln) and sine (sin) keys ..or.. a computer with spreadsheet software
Comfortable, safe place to sit while making measurements
Cold drinks and snacks

An Important Note About Sun Safety... You'll need to spend up to half a day in the Sun to calibrate the TERC VHS-1. Half a day in the Sun can cause fair-skinned people to get a sun burn, so be sure to wear long sleeves and rub sun screen on exposed skin if you are susceptible. No matter what shade or color your skin is, be sure to wear a hat and sunglasses to protect your eyes from the ultraviolet radiation (UV-B) scattered from the sky. Skin color doesn't matter when it comes to protecting your eyes from dangerous UV-B!

WARNING: AVOID DAMAGE TO YOUR EYES! NEVER LOOK AT THE SUN, EVEN WHILE WEARING SUNGLASSES! ALWAYS WEAR SUNGLASSES WHEN LOOKING AT THE SKY!

Calibration Procedure

You will need to make observations over a wide range of Sun angles to calibrate the TERC VHS-1. This is the same method pioneered by Samuel Langley nearly a century ago. For best results, begin early in the morning and make measurements until the Sun reaches the highest point in the sky. Or begin when the Sun is at its highest point and measure until evening. If you have a full day, making measurements all day will give you two calibrations.

Select a day when the sky is blue with few or no clouds.
Calibrate your watch against the time signals broadcast by radio station WWV at the frequencies of 5, 10, 15 and 20 MHz.
Before each calibration measurement it's important to check the sky to make sure no clouds or cloud haze cover the Sun. Block the Sun with a book or other opaque object held at arm's length and look for nearby clouds or cloud haze.

WARNING: ALWAYS WEAR SUNGLASSES WHEN LOOKING AT THE SKY! NEVER LOOK DIRECTLY AT THE SUN EVEN IF YOU ARE WEARING SUNGLASSES!

Switch the Sun Photometer switch to the ON position, press the Sun port on the instrument against a dark object (black paper or your clothing) and write the number displayed by the voltmeter in the third of four columns in your notebook. This number is known as your instrument's dark signal. You must measure it each time you measure the Sun.
Carefully point the VHS-1 at the Sun by centering the bright circle of light that passes through the upper angle bracket directly over the Sun target. While holding the VHS-1 as steady as possible, read the voltage indicated on the voltmeter. The numbers on the voltmeter display will flicker up and down. For best results, pick the highest number that appears when the unit is pointed directly at the Sun. This number is called the Sun signal. Write the Sun signal in the third column in your notebook and the exact time in the first column. Here is a sample notebook entry for the prototype TERC VHS-1 from 17 March 1996. Time Dark Sun Sky
012:47:40 0.235 3.12 Clear
If you plan to determine the Sun's angle using one of the manual methods described in Concepts , try to make your Sun angle and VHS-1 measurements as close together as possible. Ideally, one student should make the VHS-1 measurements and another the Sun angle measurements while a third student records the data in the notebook.
Space your measurements 15-20 minutes apart when the Sun is high in the sky. As the Sun rises (or descends), make the observations more frequently.
Figure C5
The Sun's Angle
The Sun's angle over the horizon changes much faster in the early morning and late afternoon than at midday.

PLAN AHEAD! Before its time to increase the number of measurements, be sure you have something to drink and maybe some snacks. Use the rest room before beginning your rapid series; there won't be time during the actual measurements!

Before ending your calibration session, review your notes to make sure everything is legible and complete. Do not guess any times you may have forgotten to record! Instead, don't use any data when you don't know the time it was measured. Write a brief summary about the sky conditions during the measurement time. When you are satisfied that your notes are complete, you and any others who helped should sign and date the notebook.