Eppley Precision Spectral Pyranometer (PSP)
This radiometer measures sun and sky irradiance in the range of wavelengths 0.285 to 2.8
microns, including most of the solar spectrum. The PSP is intended to weight the energy
flux in all wavelengths equally. It is a “hemispheric receiver” intended to approximate the
cosine response for oblique rays. Specifications:
1. Sensitivity approximately 9 microvolts per W/m2. Thus an irradiance of
1000W/m2 will produce a DC potential of 9 millivolts.
2. Impedance is approximately 650 Ohms. Thus, if current is drawn from the PSP,
the output voltage will drop.
3. Temperature dependence is 1% over -20 to +40C
4. Linearity is plus or minus 0.5% from 0 to 2800 W/m2
5. Response time is 1 second (for a 1/e relaxation)
6. Cosine response: 1% for 0 to70 degrees from zenith; 3% from 70 to 80 degrees
7. Size 5.75 inches diameter; 3.75 inched high
8. Weight 7 pounds
The YCEO PSP instrument is an old model; at least 30 years old. New instruments can be
purchased from the Eppley Laboratory Company in Philadelphia for about $2300. When
handling the PSP take care to avoid hitting or dropping it.
The output potential from the PSP can be measured with a good digital voltmeter. For
most applications a precision of at least 0.1 millivolt is required. A voltmeter precision of
0.01millivolt would give improved results (e.g. Extech 530, Fluke 87, Agilent U12521a).
For normal irradiance measurements, the PSP should be mounted horizontally with an
unobstructed view of the full sky in all directions. On clear days, the irradiance will rise
and fall smoothly with the sun, largely controlled by the cosine of the solar zenith angle.
On cloudy days, the irradiance will be much lower and will change quickly with time.
The PSP can be used to separate the direct and indirect solar irradiance by blocking either
the direct beam or the indirect skylight. To block the direct beam, shadow the sensor
dome with a small opaque object held about a meter away. To block the