The New Tungsten-Filament Lamp Standards of Total Irradiance

The National Bureau of Standards standard of total irradiance as presently issued in the form of a 50-W carbon filament lamp was originally calibrated more than fifty years ago. Recently, needs for higher accuracy and wider ranges of total irradiance have necessitated the setting up of three sizes (100 W, 500 W, and 1000 W) of tungsten-filament lamp standards of total irradiance. These standards operate at a higher temperature than was possible with the carbon-filament lamps, and are shielded, except for a narrow area of the bulb in front of the filament, so the reception of long wavelength flux from the lamps is reduced to a minimum. The new lamps were calibrated by the use of a blackbody at a known temperature together with a quartz plate whose spectral transmittance was accurately determined. The quartz plate limits the flux received from the blackbody to the spectral region below about 4.5 micro and thus reduces errors resulting from water vapor absorption at 6 micro and longer wavelengths. Comparisons sow the new standards to be in close agreement with the carbon-filament lamp standard.