Haze-SPAN: Haze Sun Photometer Atmospheric Network
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How do I calibrate the VHS-1?

When you use the LANGLEY METHOD to determine the EXTRATERRESTRIAL CONSTANT for your instrument, you are calibrating your instrument.

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How often should I calibrate the VHS-1?

It's wise to use the LANGLEY METHOD to CALIBRATE the VHS-1 at least once each year. Ideally, you should try to use the Langley method on 3 very clear days.

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How do I save my data?

When using the LANGLEY METHOD, it's wise to record your data in a notebook. You should also record your daily observations in your notebook. Be sure to note the date, exact time and the sky color and condition (clouds, haze, etc.). It's much easier to process your data if you have access to a computer. You can use the VHS-1 spreadsheet for data processing. Be sure to always make a backup copy of your data on a separate, removable disk! Store this disk in a safe place away from the computer.

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What is a light-emitting diode (LED)?

A light-emitting diode is a semiconductor device which emits light when an electrical current flows through it. Semiconductor diodes can also be used as light detectors. Light-emitting diodes are good detectors of light of specific colors. Generally, the wavelength of light detected by an LED is shorter than the wavelength of light emitted by the same LED. For example, certain red LEDs are relatively good detectors of orange light.

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Can I use different LEDs to detect wavelengths other than the green wavelengths detected by the VHS-1?

Yes! Usually an LED detects light having a wavelength somewhat less than the wavelength it emits. This means a blue LED will detect UV-A light. A near-infrared LED will usually detect red light. We'll have much more information about other kinds of LEDs later. See REFERENCES for additional information.

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Why is it necessary to flatten the end of the LED with sandpaper?

There are two reasons. First, the epoxy dome of an LED acts like a lens. This lens might focus enough sunlight on the LED chip to cause it to be damaged. Second, the instrument will be much harder to point accurately if you don't remove the LED lens. By flattening the end of the LED with sandpaper, the instrument can be pointed slightly away from the center of the Sun and still provide usable data.

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How flat should I make the end of the LED?

You should remove ALL of the curved portions of the lens. Any curved portion that remains will act like part of a lens. 10 to 20 strokes with sand paper is all it should take.

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Can I use a more recent op amp which can be powered by only one battery?

Definitely. We used the 741 in the VHS-1 only because of its wide availability. It's perfectly acceptable to use a more recent, single-supply op amp.

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Can I replace the solderless breadboard with a circuit board?

Sure. The original VHS-1 was designed to be assembled without the need to solder. That's why it is built with a solderless breadboard. Whether you use a breadboard or circuit board, be sure the distance from the inside of the case to the LED is the same as that given in the MANUAL.

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What do I do if the VHS-1 doesn't work properly?

Check to see if your instrument give a large signal when pointed directly at the Sun and a small signal when dark. If not, then recheck the wiring. Be sure the LED and op amp are installed in the proper direction! Be sure the volt meter is set to read voltage and not resistance or current.

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What do I do if the voltage from the VHS-1 is too high when the instrument is pointed at the Sun?

Some LEDs are more sensitive than others. If you use a very sensitive LED, the VHS-1 may "saturate" when pointed directly at the Sun. In other words, it may produce a steady voltage of around 4-5 volts without changing much. You will need to reduce the resistance of the resistor to lower the gain (amplification) of the op amp. Try cutting the resistance in half and see what happens.

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What do I do if the voltage from the VHS-1 is very small when the instrument is pointed at the Sun?

Some LEDs are less sensitive than others. They may provide a rather small signal (less than half a volt) when the instrument is pointed directly at the Sun. You can increase the signal by increasing the resistance of the resistor to increase the gain (amplification) of the op amp. Try doubling the resistance and see what happens.

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I assembled a modified version of the VHS-1 described in
SCIENTIFIC AMERICAN. Can I still submit data to Haze-SPAN?

Yes! But we need to know how your instrument differs from the VHS-1. Here are the most important things you need to tell us:

  1. What is the distance from the front surface of the LED detector to the sunlight port?
  2. What is the diameter of the sunlight port?
  3. What is the specific color, part number and source (distributor, electronics parts store, etc.) of the LED? (If possible, please provide the manufacturer's name.)

This information will allow us to determine how the field of view of your instrument differs from the VHS-1. The LED information will tell us the wavelength your instrument detects.

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My Sun photometer is a professional instrument. Can I submit data to Haze-SPAN?

Yes! You will need to tell us how your instrument differs from the VHS-1. The operating instructions supplied with your instrument should tell us what we need to know. Here are the most important things we need to know:

  1. What is the manufacturer's name and model number of the instrument?
  2. What is the instrument's field of view?
  3. What wavelength or wavelengths of sunlight does the instrument measure?
  4. If known, what is the spectral bandpass of the filters used in your instrument?
  5. The field of view and wavelength information will tell us how your instrument differs from the VHS-1.
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Has the VHS-1 been beta tested?

The VHS-1 was beta tested by two classes of humanities majors from 9 countries at the University of the Nations in Hawaii and Switzerland during 1996. The first students who built the VHS-1, Brian Maxwell from Alabama and Cindy Sigamoney of South Africa, did so with only a few notes to guide them. Dr. John Kuhne, Dean of Science at the University of the Nations, was also among the first to beta test the instrument.

The VHS-1 was also beta tested by 82 science teachers from across the United States during a workshop in Houston, Texas, sponsored by the SS&C project of the National Science Teachers Association. This group was headed by Bill Aldridge, formerly Executive Director of the National Science Teachers Association, and Dr. Linda Crow of the Baylor College of Medicine. In addition to these beta tests, several students have made VHS-1 instruments for science fairs. The following questions are based on these beta tests.

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Are there any print references to the VHS-1?

The VHS-1 Sun photometer is based on work published in a peer-reviewed scientific journal.

The use of LEDS as detectors in Sun photometers was first described Forrest M. Mims III in Sun Photometer with Spectrally Selective Light-Emitting Diodes as Spectrally Selective Detectors, Applied Optics 31, 6965-6967, 1992).

A team of scientists from India has independently tested the use of light-emitting diodes in a Sun photometer. Their results were published in a paper by Y. B. Acharya, A. Jayaraman, S. Ramachandran and B. H. Subbaraya in Compact Light-Emitting Diode Sun Photometer for Atmospheric Optical Depth Measurements, Applied Optics 34, 1209-1214, 1995).

LED Sun photometers are also described in these non-peer reviewed publications:

  • Science Fair: Ticket to Your Future by Forrest M. Mims III (Radio Shack, 30-31, 1992).
  • Engineer's Mini-Notebook: Environmental Science Projects by Forrest M. Mims III (Radio Shack, 37-41, 1995).
  • How to Measure the Solar Constant by Forrest M. Mims III (Science Probe: The Amateur Scientist's Journal, 1, 93-100, 123, 1991).
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