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Sunlight plays huge roles in life as we know it. The red and blue wavelengths of sunlight trigger the photosynthesis that is essential for the growth of plants. The tilt of Earth’s axis causes the seasons: It’s summer in the hemisphere tilted toward the sun and winter in the hemisphere tilted away from the sun.

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Figure A

I’ve been tracking the daily intensity of sunlight at noon since September 1989 using a variety of homemade instruments. While my instruments work well, they require batteries and power switches, both of which eventually need to be replaced. Here we will build the ultra-simple radiometer shown in Figure A that needs no power switch or battery, for it’s powered by the sunlight it measures. This radiometer will teach you much about the daily cycle of sunlight and how it’s modulated by the seasons, clouds, and air pollution.

How It Works

For many years photographers used light meters that employed a selenium photocell connected to an analog meter. The spectral response of selenium closely resembles that of the color response of the human eye. The sunlight sensor for our radiometer is a silicon photodiode, which is actually a miniature solar cell. While its spectral response peaks in the near infrared, it is far more sensitive than selenium photocells. As shown in Figure B, the cathode pin of the photodiode is connected directly to the negative terminal of a 0–1 milliampere (mA) analog panel meter. The anode pin of the photodiode is connected to the positive terminal of the meter through a 550Ω or similar resistor that reduces the peak photodiode’s current to less than 1mA in full sunlight.

Figure B

Figure B

While analog panel meters are considered old-fashioned, the one used here is key to the simplicity of the radiometer. Analog meters lack the resolution of digital readouts, and they’re bulkier. Yet analog panel meters have a very long lifetime and other significant advantages over their digital counterparts. In this project the meter requires no battery, because it’s powered by the light sensor. And the swinging needle of a panel meter shows changes and trends that are immediately recognizable, unlike the flickering numbers displayed by a digital readout.

As for reliability, decades ago I built an analog radiometer much like the one described here. Its purpose was to measure the power of the beam emitted by near-infrared LEDs. That instrument works as well today as it did when I built it in 1970, and the version described here should have an equally long life.

Build Your Radiometer

The radiometer shown in Figure A was assembled on a mini-clipboard to provide space for a log sheet and a bubble level. You can also use a standard-size clipboard or a piece of thin plywood or rigid plastic. To follow the design shown in Figure A, bore a ¼” hole in the board for the audio jack, above where the meter will be mounted. Place your meter over the board with its top edge centered under and at least 1″ below the audio jack hole. Mark the position of the meter’s 2 screw terminals. Measure the diameter of the screw terminals (the ones on mine are ⅛”) and bore holes for them.

Remove the nuts from the meter’s screw terminals, and insert the terminals into the holes you drilled. On the back of the board, place a solder lug over each terminal, and secure the meter and lugs in place with the nuts as shown in Figure B.

Solder a short length of connection wire between the negative meter lug and the audio jack lug. Solder a 550Ω resistor between the positive meter lug and the audio jack’s center terminal. (You may need to experiment with the best resistor value if you use a photodiode other than the one specified.)

Figure C

Figure C

Finally, install the photodiode. You can install it directly on the clipboard, but I’ve installed it in an audio plug to form an interchangeable sunlight probe as shown in Figure C. For the latter method, remove the audio plug’s plastic cover. Next, spread the photodiode pins outward and carefully inspect the upper side of the photodiode. A tiny wire bonded to the top surface of the silicon wafer goes to the anode (+) pin (Figure D). Thread the photodiode pins through the connection holes in the plug’s 2 terminals.

Figure D

Figure D

IMPORTANT: The anode pin should be inserted through the audio plug’s center terminal.

Carefully solder the photodiode pins to their respective terminals. After the connections cool, clip off the ends of the pins extending beyond the terminals.

Using the Radiometer

Test the radiometer by pointing a flashlight at the sunlight probe. The meter needle should move slightly. Take the radiometer outside during daylight, and the needle will move much more. If it exceeds 1mA, you’ll need to increase the resistance of the resistor. For serious studies, use a bubble level to keep the instrument level during measurements. Be sure to place the radiometer opposite the shadow formed by your head.

The opening shot, here again below as Figure E, shows fisheye photos of the sky for 3 different summer cloud conditions. Note that the highest current occurred when the sun was closely surrounded by clouds. Also note the surprisingly high current when the sun was blocked by a large cloud.

Figure E

Figure E

You can produce serious data by measuring sunlight at solar noon for a full year. I’ve done this at my Texas site for more than 26 years using a variety of DIY radiometers, and the graphed data below shows an obvious seasonal cycle. Clouds, dust, and smoke can greatly reduce sunlight. The thickest haze in Texas is caused by Saharan dust from Africa and power plant haze from the Ohio Valley during summer and fall. Smoke from distant forest fires and agricultural burning in Mexico also causes thick haze.


Going Further

» To study daily trends in the data and the effects of clouds, you can make a video or elapsed-time photos of the meter.
» Use the radiometer to monitor the red wavelengths of sunlight that stimulate photosynthesis by placing a red filter over the photodiode.