They say the Sun shines on the righteous—and if you want to know precisely how righteous you are, you'd better invest in a
pyranometer! Widely used by weather and climate scientists, these
strange looking instruments measure the amount of sunlight
hitting Earth's surface at a particular place and time. How do they
work? Let's take a closer look!
Photo: Sunlight varies all the time, from its theoretical minimum at sunrise (as here) and sunset to its theoretical maximum at midday. But how can you measure it more precisely?
Unless you're lucky enough to live in a tropical paradise, you probably see
quite a bit less of the Sun than you might like. Earth orbits the Sun and, at the
same time, spins around on a tilted axis, so we get variations in
sunlight across our planet each hour of the day and each day of the
year. That's essentially what gives us our seasons and climate.
Now suppose you had the job of comparing how much sunlight
different places receive. How would you go about it? You could lounge
about in the Sun all day in different places and see how brown you
get, but unless you wear copious amounts of sunscreen, that will
be very dangerous—and it's not exactly scientific! What about
rigging up a small solar panel to an electricity meter? You could
carry the panel round the world with you, measure how much
electricity it generates in each place, and use that to compare the
amount of sunlight.
Very roughly speaking, this is what pyranometers
do—although they work in a much more precise and scientific way.
What they measure is the solar radiation falling on a horizontal
surface in watts (the amount of energy received each second) per
square meter. Technically that's known as insolation.
Chart: How insolation varies at different times of day during the year. There is obviously most sunlight
in summer (red) and least in winter (gray), with a moderate amount in fall/autumn and spring. The total solar energy (the area under the curves) is much greater in summer both because the days are longer and because the sun is "higher" in the sky (the insolation is greater).
Sunlight may look yellow, but it actually consists of a very broad spectrum of
electromagnetic radiation, ranging in wavelength from about 280
nanometers (nm, which are billionths of a meter) up to about 4000 nanometers. This includes both visible
"white light" (the familiar rainbow spectrum ranging from red and orange
through to indigo and violet) and invisible electromagnetic
radiation, including ultraviolet (UV) and infrared (IR). Although
our eyes can't see much of this light, pyranometers do their best to
detect as much of it as possible, because it all counts as sunlight.
Photo: It's important to remember that sunlight contains a spectrum of different light wavelengths, including some we can't see. Different pyranometers can give very different results because they measure different parts of the spectrum. While good thermopile pyranometers measure pretty much the whole spectrum, chip-based pyranometers measure a much narrower range of wavelengths.
If you're good with your Greek, you'll know that pyr means
fire, ano indicates something up above, and meter
suggests measurement, so a pyr-ano-meter measures "fire from up
above"—sunlight, in other words. Broadly speaking, there are two
different kinds of pyranometers. Although they do the same job, they work in
very different ways. Thermopile-type pyranometers measure sunlight from the heat it generates; chip-type pyranometers
measure sunlight from the electricity it generates.
The very best pyranometers are described as laboratory-grade, research-grade, reference-grade, secondary
standard (ISO), or high quality (WMO). Slightly lesser instruments are described as first class (ISO) or
good quality (WMO), while second class (ISO) are the next best grade.
A typical, laboratory-grade pyranometer is essentially just a thermopile (a collection of
thermocouples, perhaps 50–100 in the best instruments) mounted on a
black carbon disc, which generates electricity according to how hot
it gets (how much solar radiation falls on it).
That's not quite all there is to it, however! Scientists are
serious people who like to be sure that when they measure something,
other things aren't getting in the way and spoiling their data. So
pyranometers have some extra features. Most noticeably, there's a
dome made from one or two layers of ground and polished optical glass
or acrylic plastic covering the thermopile, which eliminates air
movements and dirt that might affect the measurements (the curved
outer surface also ensures any raindrops fall away quickly). A small,
replaceable cartridge of silica gel (or other dessicant) inside the
pyranometer absorbs any dew. Since a pyranometer typically sits outside in an
exposed position, its case has to be made from something like
toughened, rustproof, anodized aluminum. Usually, there's a
built-in spirit level so you can be sure your pyranometer is flat
(though some are designed to be used on inclined surfaces as well).
Photo: A pyranometer quietly going about its job, measuring solar radiation. Note the double glass in the dome and the weatherproof cable coming out of the side that carries electrical signals, corresponding to the strength of the solar radiation, out to a computer. Photo by Steve Wilcox courtesy of US Department of Energy/National Renewable Energy Laboratory (US DOE/NREL).
When sunlight falls on a pyranometer, the thermopile sensor
produces a proportional response typically in 30 seconds or
less: the more sunlight, the hotter the sensor gets and the greater
the electric current it generates. The thermopile is designed to be
precisely linear (so a doubling of solar radiation produces twice as
much current) and also has a directional response: it produces
maximum output when the Sun is directly overhead (at midday) and zero
output when the Sun is on the horizon (at dawn or dusk). This is
called a cosine response, because the electrical signal from the pyranometer varies with the
of the angle between the Sun's rays and the vertical.
Here's what you'll find inside a really high-quality pyranometer that uses a thermopile to measure solar radiation:
Outer dome made from a hemisphere of optical-quality glass.
Inner dome made from a smaller hemisphere of optical glass.
Black carbon disk (illuminated by the Sun) absorbs a broad range of wavelengths of solar radiation and acts as the sensing element.
Second, control disk (not illuminated by the Sun) acts as a comparison and compensating element. Any sources
of temperature rise other than solar heat (perhaps an air-conditioning unit positioned nearby) will warm both disks equally, so we can be reasonably confident that the difference between the two disks (and the temperature rise we're measuring) is caused only by the Sun.
Thermopile temperature sensor compares the temperature rise of the two disks.
Output lead (usually about 10m or 30ft long).
Replaceable silica gel cartridge (dessicant) absorbs moisture to prevent dew forming inside on cold nights.
Adjustable screw legs let you level the pyranometer using its built-in, high-precision spirit level, which is sensitive to
a fraction of a degree (not shown on this diagram).
Photo: You can use small photovoltaic solar cells like these to measure solar radiation.
Not all pyranometers use thermopiles. You can also get less sophisticated (and considerably cheaper) solar-cell pyranometers, based on light-sensitive semiconductor chips, that give more approximate measurements. The best thermopile pyranometers are designed to respond more or less equally to a substantial band of incoming light wavelengths (this is sometimes described as a flat wavelength response). Lesser, chip-based pyranometers don't do this. Their main drawback is that
they don't respond linearly to a broad band of solar radiation but
only to a limited range of wavelengths; so while a high-quality
pyranometer might measure wavelengths from 280–2800 nanometers, a
solar-cell version might respond to wavelengths in a much
narrower band from about 300–1100 nanometers (with a peak in the infrared region from around 800-1100nm). But unless you're making
really detailed measurements in a laboratory, that may be perfectly fine for your needs.
How does a chip-type pyranometer work?
Here's the inside of a typical, inexpensive pyranometer that uses a light-sensitive chip instead of a thermopile.
This is a drawing of an actual pyanometer designed in the 1990s by inventors David and Arthur Beaubien as a low-cost instrument for measuring, specifically, the ultraviolet part of the spectrum. It uses a simple photodiode (a light-sensing chip) mounted in a sealed housing, with filters and a phosphor above the photodiode restricting the light passing through to the precise part of the spectrum we want to measure. I've colored and numbered the original diagram and simplified the explanation to make it easier to follow.
Dome made from a hemisphere of glass or quartz designed to transmit solar radiation to the sensor equally well from
any angle of the Sun.
Outer housing, typically made from a stiff thermoplastic, provides insulation from heat and electrical interference.
Central part of the pyranometer where sunlight enters and is measured.
Filter restricts the range of solar radiation entering and being measured. This particular pyranometer is designed for measuring
UV light, so this filter and the one beneath are designed to restrict the wavelength of the light passing through to the detector.
Phosphor absorbs incoming radiation of a particular wavelength (in this case, ultraviolet-B) and re-emits radiation of a different wavelength that the photodiode is sensitive to (in this case, green light).
Second filter allows only light produced by the phosphor to pass through.
Aluminum cylinder provides central support structure for other components. To improve the accuracy of measurements, it can be maintained at a precisely controlled temperature using a miniature heater (not shown).
Photodiode measures the solar radiation (or, more precisely, the solar radiation that has passed through the filters and been converted by the phosphor). The photodiode is very carefully chosen so it's responsive only to the range of sunlight we want to measure.
Amplifier increases output from the photodiode so it's easier to detect.
Solar and Infrared Radiation Measurements by Frank Vignola, Joseph Michalsky, and Thomas Stoffel. CRC Press, 2019. A broad introduction that includes an interesting history of solar measurement (from ancient times to the present day); a comprehensive overview of pyranometers, radiometers, and similar instruments; and a guide to setting up a complete solar monitoring station. Suitable both as a general introduction and a technical reference.
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