Solar Brightness

Bright light


A photometer made by making a grease spot on white paper can be used to compare the brightness of the sun to the brightness of a lamp. By finding a position at which the sun is as bright as the lamp the power output of the sun can be estimated.

 Piece of white typing paper 10 cm square or larger

 Drop of cooking oil


 A 200 Watt lightbulb and lamp to hold it. (It must be a spherical bulb not a flood light. a 150 Watt bulb can be used in a pinch.)

 Optional photometer
two pieces of translucent white plastic at least 2.5 cm (1 inch) square a piece of aluminum foil the same size as the plastic transparent tape or a rubber band

To make a grease spot photometer, place a small drop of food oil in the center of the paper and spread it around into a circle about 2 cm in diameter with your finger. optional assembly
To make a plastic photometer, place the aluminum foil between the two pieces of plastic and hold them all together with the rubber band or tape.

Place the lamp outside in direct sunlight. Hold the photometer paper between the lamp and the sun. Move the photometer toward and away from the lamp until the brightness of the lamp is the same as the brightness of the sun.(The grease spot will be as bright as the surrounding paper or when viewing the photometer from its edge, one side will be as bright as the other.) Measure the distance from the center of the lamp to the photometer. Notice the difference in color between sunlight and incandescent lamp light.

The lamp filament is cooler than the sun. Cooler objects emit proportionately more red light than blue light and so appear redder in color. (The filament is about 2900 K while the sun is about 5800 K.) The light intensity decreases inversely proportional to the square of the distance. This inverse square proportionality can be used to quantitatively compare the brightness of the bulb and the brightness of the sun.

Math Root

The brightness of a light is how bright it appears to the human eye. The intensity of a light is the power per meter squared carried by the light. If the lamp were to put out 200 Watts of power in visible light, then its intensity at a distance of 1 meter would be 200 Watts spread over the area of a sphere 1 meter in diameter, (200/4ºr2). Since the area of a sphere increases as the square of its radius, the intensity of a light source which spreads uniformly decreases as the inverse square of the distance from the center of the source (the radius). When the photometer indicates that the brightness of the lamp is equal to the brightness of the sun then the intensity of the lamp is approximately equal to the intensity of light from the sun.

Intensity of lamp = intensity of sun
(power of lamp/distance to lamp2 )= (power of sun/distance to sun2)
(Plamp/D2lamp)= (Psun/D2sun)
Psun = (PlampD2sun/D2lamp)

The distance to the sun is 1.5 x 1011 m.

Example calculation.

If the brightness of a 200 W light bulb equals the brightness of the sun at a distance of 10 cm then:
Psun = Plamp (1.5 x 1011 m)2 / (0.1m)2
= Plamp x 2.25 x 1024 So the sun is over 1024 times as bright as the 200 Watt bulb. The power of the sun is actually 4 x 1026 Watts. Not all of the sun's power is in the visible, however more of the sun's power is in the visible than the lightbulb's power, since hotter objects have a larger fraction of their light in the visible than in the infra-red.

Here are three complications to this experiment: The atmosphere of the earth scatters and absorbs sunlight. This will make our estimate of the solar power too small. This experiment only compares the power of the sun and lightbulb in the visible portion of the spectrum. An incandescent lamp emits less than 5% of the power it consumes as visible light. The lamp thus emits 5% of 200 Watts as light or about 10 Watts. The sun is about twice the temperature as the lightbulb filament. As a result, the sun emits a larger fraction of its total power as visible light. See the activity titled Solar Thermal for a continuation of this activity. In which the total energy output of the sun at all wavelengths is compared to the energy output of the lamp. The human eye is more sensitive to the spectrum of light emitted by the sun than to the reddish light emitted by the filament. This means that when the brightnesses of the two sources look equal the intensities are not exactly equal.(The intensity of the sun is slightly less than that of the lamp when they look to be the same brightness.)

Paul Doherty



©1997 The Exploratorium 3601 Lyon Street San Francisco, CA 94123