GaP photodiode
(Hamamatsu G1962)
Collimator
(Note 1)
Filter
NOTES
1. Collimation in the TOPS
instrument is provided by 5mm-
diameter apertures in the end
of the housing approximately
90mm from the photodiodes.
2. Exact resistances depend on
transmission of the filter. The
range will be from 10M to 100M.
3. Use one 1N914 for a 6V supply
or three 1N914s for a 7V supply.
100pF
Rf (a)
Rf (b) (Note 2)
Gain
select
TLC271
2
3
6
7815
4
100K
zero
adjust
.01μF
1M
1M
6V –7V Power
Liquid crystal readout
(Acculex DP-650)
D.P. 1
In (+)
Hold
+5V
1N914
(Note 3)
In (–)
Ground
Fig. A: Circuit diagram for
one of the two UVB sun
photometers in the original
homemade TOPS- 1 from
1990. All the components
are still available. Fig. B: The
TOPS project earned a 1993
Rolex Award that provided
funds for the development
of a first-generation micro-processor-controlled TOPS
(Microtops) by Scott
Hagerup. Fig. C: This global
ozone image was acquired
while NASA’s Nimbus- 7 satellite was providing accurate
data during 1991. On this
day TOPS- 1 measured 284.4
Dobson units (DU) of ozone,
and the satellite measured
281.5 DU.
A
Photography and images by Forrest M. Mims III (A, B); Goddard Space Flight Center, NASA (C)
BC
inexpensive op-amp integrated circuit to amplify the
current produced by a UV-sensitive photodiode. An
interference filter passed only the UVB wavelengths
from about 300nm–310nm, while blocking the
visible wavelengths.
I described how to make two versions of the UVB
radiometer in “The Amateur Scientist” column in
the August 1990 Scientific American. This article
also described how the radiometer detected significant reductions in solar UVB when thick smoke
from forest fires at Yellowstone National Park drifted
over my place in South Texas in September 1988.
This meant that my simple UVB radiometer
formed half of an ozone monitor. So I built two
radiometers inside a case about half the size of a
paperback book. One radiometer’s photodiode was
fitted with a filter that measured UVB at 300nm, and
the second was fitted with a 305nm filter. I named
the instrument “TOPS” for Total Ozone Portable
Spectrometer. (Full details are at
bit.ly/9JOth9.)
Ozone strongly absorbs UV, and the amount of
ozone in a column through the entire atmosphere
layer can be determined by comparing the amount
of UV at two closely spaced UV wavelengths. This is
possible because shorter wavelengths are absorbed
more than longer wavelengths.
TOPS was calibrated against the ozone levels
monitored by NASA’s Nimbus- 7 satellite. This
provided an empirical algorithm that allowed TOPS
to measure the ozone layer to within about 1% of
the amount measured by the satellite. During 1990,
ozone readings by TOPS and Nimbus- 7 agreed
closely. But in 1992, the two sets of data began to
diverge so that TOPS was showing several percent
more ozone than the satellite.
When I notified the ozone scientists at NASA’s
27 Make:
