A Sun photometer (or sunphotometer) is a kind of light meter that measures the light from the Sun. Most Sun photometers measure sunlight at discrete colors or wavelengths. All Sun photometers measure only the sunlight arriving directly from the Sun and not the sunlight scattered from MOLECULES and AEROSOLS (or particles) in the sky. Therefore a Sun photometer must be pointed directly at the Sun.
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A SUN PHOTOMETER measures the sunlight in a narrow spectral band that passes through the Earth's atmosphere. If the EXTRATERRESTRIAL CONSTANT for a particular Sun photometer is known, a Sun photometer can measures the transmission (T) of sunlight through the atmosphere. The clearer the sky, the higher the transmission of sunlight. AEROSOLS such as smoke and dust absorb or scatter sunlight. Therefore, aerosols reduce the sunlight transmitted through the atmosphere.
Sun photometers are ideal for monitoring natural haze and air pollution. The transmission of sunlight through the atmosphere (T) or the optical thickness (or optical depth) of the atmosphere. T is slightly reduced (and optical thickness slightly increased) when sunlight is scattered from molecules of air. T is also reduced by natural haze and air pollution. Natural haze can be caused by water vapor, natural forest and brush fires, dust, pollen, gases emitted by plants and trees, sea salt and volcanic eruptions. Air pollution is a byproduct of automobile emissions, coal-burning power plants, intentional burning of forests and rangelands, certain industrial and mining operations and dust from unpaved roads and agricultural fields.
Some Sun photometers measure the amount of certain gases in the atmosphere. This is done by measuring the T (or optical thickness) at two different wavelengths, one of which is strongly absorbed by the gas being measured and the other which is not. The ratio of the two measurements is then used to determine how much of the absorbing gas is in a column through the atmosphere. This method is often used to measure ozone and water vapor.
The extraterrestrial constant is the total amount of sunlight arriving at the top of the atmosphere. The extraterrestrial constant is often represented as Io in Sun photometer formulas. A Sun photometer at the top of the atmosphere would not need to observe the Sun with a narrow FIELD OF VIEW since outside the atmosphere there are no air molecules or aerosols to scatter sunlight. Since Sun photometers usually measure discrete colors or wavelengths of sunlight, the extraterrestrial constant they measure is that of the specific color or wavelength and not all the light emitted by the Sun. You can use the LANGLEY METHOD to measure the extraterrestrial constant for a Sun photometer such as the VHS-1.
The field of view is the solid angle of the sky viewed by a Sun photometer. A field of view (or FOV) of 2 degrees means that the instrument is observing a circular segment of the sky which subtends an angle of 2 degrees. The Sun subtends an angle of approximately half a degree.
Samuel Langley pioneered the measurement of sunlight for the Smithsonian Institution. He used BEER'S LAW to measure the EXTRATERRESTRIAL CONSTANT of sunlight at the top of the atmosphere without leaving the surface of Earth. Though the method is very simple, it gives surprisingly accurate results. Therefore, it has been used by atmospheric scientists for more than a century. Briefly, the Langley method requires that you make measurements of direct sunlight with a Sun photometer for half of a very clear day. You then make a graph which plots the natural logarithm (ln) of your measurements (on the y or upright axis) against the AIR MASS (on the x or horizontal axis) during each measurement. If the day was clear and you pointed the instrument directly at the Sun, the data points plotted on the graph will fall along or very near a perfectly straight line. Extending this line to where it crosses the y axis at an AIR MASS of 0 will give the ln of the extraterrestrial constant for the instrument. See the VHS-1 manual for details about using your computer to compute the linear regression of your data to arrive at the extraterrestrial constant.
August Beer was a German physicist who lived from 1825 to 1863. Beer's specialty was optics. He developed the principle known as Beer's Law, which explains how the intensity of a beam of light is reduced as it passes through different thicknesses of a medium. A simplified version of Beer's Law that gives the OPTICAL THICKNESS of the atmosphere measured by a Sun photometer is the natural logarithm (ln) of the instrument's EXTRATERRESTRIAL CONSTANT (Io) minus the ln of the intensity of sunlight measured by the instrument divided by the AIR MASS. (See the VHS-1 INSTRUCTIONS for the complete formula.)
Optical thickness or optical depth describes how much light passes through a material. Since the amount of light that passes through a material can be quite small (less than a fraction of 1%) or very large (nearly 100%), optical thickness is based on a logarithmic scale. There are various definitions. When the optical thickness of the atmosphere is measured, the usual definition is that the optical thickness is -ln T, where T is the transmission of the atmosphere. If T is 0.95, then the optical thickness is 0.05. The optical thickness of the atmosphere is higher than in space because of Rayleigh scattering and MIE SCATTERING.
Air mass (m) is the thickness of atmosphere through which a beam of sunlight travels. At any sea level location, the air mass is 1 when the Sun is straight overhead at solar noon. A simplified formula for air mass is m = 1/sine of the Sun's angle over the horizon. Therefore, when the Sun is 30 degrees over the horizon, m = 2 (1/0.5). A more refined version of this basic equation which accounts for the curvature of the Earth is used in the VHS-1 spreadsheet.
Molecules of air scatter sunlight. Air molecules scatter ultraviolet and blue wavelengths much more efficiently than red and infrared wavelengths. You can easily observe Rayleigh scattering. Shine a bright flashlight or, even better, a red laser beam, through a dark room. The uniform glow you see that outlines the beam of light is caused by Rayleigh scattering. Bright speckles in the beam are caused by AEROSOLS (usually particles of dust) in the air. Light scattered from aerosols is caused by MIE SCATTERING. Caution: If you use a laser for this demonstration be sure the beam is well away from eye level. The beam should not be pointed at people or at shiny or reflective surfaces.
Aerosols are particles suspended in air. They range in size from a fraction of a micrometer to a few hundred micrometers. They include smoke, bacteria, salt, pollen, dust, various pollutants and tiny droplets of water. The scattering of light from aerosols is called MIE SCATTERING.
Aerosols are very efficient at scattering sunlight. Some aerosols, especially black carbon particles created by fires, efficiently absorb sunlight. You can easily observe Mie scattering. Shine a bright flashlight or, even better, a red laser beam across a dark room. The uniform glow you see that outlines the beam of light is caused by Rayleigh scattering (light scattered by molecules of air). Bright speckles in the beam are caused by AEROSOLS (usually particles of dust) in the air. This is Mie scattering. This demonstration can be made much more dramatic by introducing chalk dust or smoke into the light beam. Caution: If you use a laser for this demonstration be sure the beam is well away from eye level. The beam should not be pointed at people or at shiny or reflective surfaces.
If the EXTRATERRESTRIAL CONSTANT measurement for your VHS-1 is carefully made on a very clear day and if the LED, sunlight hole and sunlight alignment target are all carefully aligned, then the instrument should be able to make measurements which are repeatable to within about 1% for identical sky conditions. For best results, make haze measurements at mid-morning, noon and mid-afternoon. You will probably find that the noon measurements are not quite as repeatable as measurements made earlier and later.
Yes. Comparatively few Sun photometers are in use around the world. Since recent studies have shown that aerosols can block considerable sunlight, thus causing a cooling effect on Earth's climate, there is renewed interest in Sun photometer measurements. Since the optical filters used to select specific colors of sunlight degrade over time, scientists have been mistrustful of measurements made by an earlier generation of Sun photometers. The VHS-1 does not use a filter. Instead, it uses a long-lived, highly stable LIGHT-EMITTING DIODE (LED) to detect a narrow band of light wavelengths. This is the key advantage of the VHS-1. Also, scientists are much more interested in results using instruments that have been described in peer-reviewed scientific journals. The VHS-1 meets this qualification. See REFERENCES. Of course it is essential that you properly CALIBRATE your instrument using the LANGLEY METHOD for its data be useful to scientists.