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