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Frequently Asked Questions Because All Sun Photometers Will See 1 Frequently Asked Questions because all sun photometers will see 1. What is a sun photometer and what does it some scattered light and direct light from the sky measure? around the sun. A sun photometer is a type of light meter that The cone of light a sun photometer’s detector sees measures the amount of sunlight . Most sun is called its field of view, and it is desirable to have photometers measure the amount of sunlight for a this cone as narrow as possible. The GLOBE sun narrow range of colors or wavelengths. All sun photometer’s field of view is about 2.5 degrees, photometers should measure only the sunlight which GLOBE scientists have concluded is a arriving directly from the sun and not the sunlight reasonable compromise between desires for scattered from molecules and aerosols. Therefore accuracy and practical considerations in building a sun photometer is pointed directly at the sun a handheld instrument. The basic tradeoff is that and the light is collected through a small aperture the smaller the field of view, the harder the (hole or opening) that prohibits scattered sunlight instrument is to point accurately at the sun. Very from reaching the instrument’s detector. expensive sun photometers, with motors and 2. The GLOBE sun photometer uses a light- electronics to align the detector with the sun, emitting diode (LED) as a sunlight detector. typically have fields of view of 1 degree or less. What is an LED? Studies have shown that the error introduced by A light-emitting diode is a semiconductor device somewhat larger fields of view is negligible for that emits light when an electrical current flows the conditions under which a GLOBE sun through it. The actual device is a tiny chip only a photometer should be used. fraction of a millimeter in diameter. In the GLOBE 4. How important is it to keep the sun sun photometer, this chip is housed in a plastic photometer from getting hot or cold while housing about 5 mm in diameter. You can find I’m taking measurements? these devices in a wide range of electronic The LED detector in your sun photometer is instruments and consumer products. The physical temperature-sensitive, so its output is slightly process that causes LEDs to emit light also works influenced by its temperature. Therefore, it is very the other way around: if light shines on an LED, it important to protect your instrument from getting produces a very small current. The electronics in too hot in the summer or too cold in the winter. your sun photometer amplifies this current and In the summer, it is essential to keep the converts it to a voltage. instrument case out of direct sunlight when you Generally, the wavelength of light detected by an are not actually taking a measurement. In the LED is shorter than the wavelength of light emitted winter, it is essential to keep the instrument warm by the same LED. For example, certain red LEDs — you can tuck it under your coat between are relatively good detectors of orange light. The measurements. Never leave your sun photometer LED in the GLOBE sun photometer emits green outside for extended periods of time. light at about 565 nm. It detects light with a peak The sun photometer case itself provides some at about 525 nm, which is a little further into the protection from temperature changes that can blue part of the light spectrum than the emission affect the electronics inside. If you follow these color. precautions and take your measurements as 3. What is the field of view of a sun quickly as possible, then your measurements will photometer, and why is it important? be acceptable. In extreme conditions (winter or The equation that describes theoretically how to summer), you should consider making an interpret sun photometer measurements requires insulating housing for your sun photometer. You that the instrument should see only direct light can use styrofoam or other foam plastic. Cut holes from the sun — that is, light that follows a straight for the on/off switch and the sunlight aperture, line path from the sun to the light detector. This and a channel for sunlight to get from the front requirement can be met only approximately alignment bracket to the target on the back GLOBE® 2002 Aerosol Protocol - 18 Atmosphere bracket. The hole for the sunlight aperture should voltage is less than 7.5 V. You should check the Welcome Introduction be no smaller in diameter than the thickness of battery no less frequently than once every three the insulating material itself, and in no case should months. If you still believe you have a problem, it be smaller than about 1 cm. contact GLOBE for help. 5. I dropped my sun photometer. What 7. What does it mean to calibrate a sun should I do now? photometer? Fortunately, the components inside your sun A sun photometer is considered to be calibrated photometer are virtually indestructible, so they if its extraterrestrial constant is known. This is should have survived being dropped. Check the the voltage you would measure with your sun case for cracks. Even if the case is cracked, you photometer if there were no atmosphere between should still be OK. Just tape over the cracks — you and the sun. As an exercise, you could think use something that is opaque, such as duct tape. about pointing your sun photometer at the sun Open the case and make sure that everything looks from the open cargo bay of the space shuttle as it OK. In particular, make sure that the battery is orbits Earth above the atmosphere. The voltage still firmly attached to the terminals on the battery you measure would be your instrument’s holder. If the alignment brackets have moved or extraterrestrial constant. This value depends are loose as a result of the fall, straighten them primarily on the wavelength at which your sun and retighten the screws. Apply a little photometer detects light and also on the distance cyanoacrylate glue (“super glue”) at the base of between Earth and the sun. (This distance varies each alignment bracket. Then you should realign slightly because Earth follows a slightly elliptical, your sun photometer by following the alignment rather than a circular, path around the sun.) Protocols protocols. Note that, for this exercise, you do not have to When you have done these things, try making a worry about limiting the field of view. Why? measurement. If measurements look OK, they Because outside the atmosphere there are no probably are, as the actual calibration of your molecules or aerosols to scatter sunlight. Hence, instrument will not be affected by this kind of your sun photometer will see only direct sunlight. accident. As a practical matter, sun photometers can be 6. How do I know if my sun photometer is calibrated by inferring the extraterrestrial constant working properly? from measurements made at Earth’s surface. This ActivitiesLearning A When you turn your sun photometer on without is called the “Langley plot” method. These pointing it at the sun, you should measure a measurements are difficult to make at low voltage in the range of about (0.000 to 0.020 V) elevation sites with variable weather. GLOBE sun (10 to 20 mV ). When you point your instrument photometers are calibrated against reference directly at the sun, the voltage should increase to instruments that have been calibrated using a value in the range of about 1 to 2 V. Only in very measurements taken at Mauna Loa Observatory, hazy conditions, late in the afternoon, or early in which is widely accepted as one of the best the morning, should you see a sunlight voltage locations for such work. less than .5 V. If you do not see the expected It is an interesting project to make your own voltages, then your sun photometer is not working. Langley plot calibrations and compare the results The most likely reasons for a sun photometer not with the calibration assigned to your sun to work are that the battery has come loose from photometer. If you would like to do this, contact ppendix the connectors on the battery holder or that the GLOBE. battery is too weak to power the electronics. If the 8. Can I make my own sun photometer? battery seems to be seated properly in the battery You can purchase a sun photometer kit. holder, then test the voltage. As indicated in the Constructing a sun photometer involves soldering procedure for changing your sun photometer’s some electronic components, which is a skill battery, you should replace the battery if the students need to learn from someone who has GLOBE® 2002 Aerosol Protocol - 19 Atmosphere done it before. You can start taking measurements the light and the size of the aerosols. This kind of as soon as you have assembled your instrument, particle-light interaction is called Mie scattering, however, at some point, you must send your sun named after the German physicist Gustav Mie, photometer to the GLOBE Atmospheric Haze who published the first detailed mathematical Science Team for calibration before your data can description of this phenomenon in the early part be accepted into the GLOBE Data Archive. of the twentieth century. 9. How often must I take sun photometer 12. What is optical thickness? measurements? Optical thickness (or optical depth) describes how The protocols ask that you take measurements much light passes through a material. The amount every day, weather permitting. In some parts of of light transmitted can be quite small (less than the world, it is possible to go many days without a fraction of 1%) or very large (nearly 100%).
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