[ TERC VHS-1 Sun Photometer ] [ Manual TOC ]

The TERC VHS-1 Sun Photometer

The TERC VHS-1 Sun photometer which forms the principle subject of this manual detects a narrow range of wavelengths without using a filter [2, 3]. The result is an instrument which costs only a fraction of the price of the most inexpensive filter instruments [3]. The key to the TERC VHS-1 is its use of a common light-emitting diode (LED) as a detector of light [2]. LEDs are designed to make, not detect, light. But it happens that they detect light quite well. Even better, unlike detectors designed specifically to detect light, LEDs detect a relatively narrow band of wavelengths. For example, an LED that emits greenish-yellow light at a peak wavelength of about 555 nanometers (nm) detects green light at a peak wavelength of about 525 nm over a spectral width of about 50 nm.

The main function of the TERC VHS-1 Sun Photometer is measuring haze. But it also has other uses. Here's what you will be able to do with this versatile instrument:

• Measure the Sun's extraterrestrial constant, the signal the instrument would indicate if you were able to fly the Space Shuttle and point it at the Sun during a space walk. You will do this, of course, without leaving the ground. You will use the same method which scientists with the Smithsonian Institution first used in the 1880's, even though they were unable to prove that their measurements were correct until rockets first carried Sun photometers into space.

• Measure changes in the clarity of a column through the atmosphere (the aerosol optical depth) caused by smog, haze, smoke, dust, wispy clouds, fog and volcanic haze. You can do this over the course of a day. Or you can do it at the same time every day for a year or more and help scientists better understand the annual cycle of haze where you live.

• Measure the light transmission of water, leaves and other materials at about 525 nm.