  The Extreme ultraviolet SpectroPhotometer (ESP) is one of five channels of
the Extreme ultraviolet Variability Experiment (EVE) onboard the NASA Solar
Dynamics Observatory (SDO). The ESP channel design is based on a highly stable
diffraction transmission grating and is an advanced version of the Solar
Extreme ultraviolet Monitor (SEM), which has been successfully observing solar
irradiance onboard the Solar and Heliospheric Observatory (SOHO) since December
1995. ESP is designed to measure solar Extreme UltraViolet (EUV) irradiance in
four first order bands of the diffraction grating centered around 19 nm, 25 nm,
30 nm, and 36 nm, and in a soft X-ray band from 0.1 to 7.0 nm in the zeroth
order of the grating. Each band's detector system converts the photo-current
into a count rate (frequency). The count rates are integrated over 0.25 sec
increments and transmitted to the EVE Science and Operations Center for data
processing. An algorithm for converting the measured count rates into solar
irradiance and the ESP calibration parameters are described. The ESP pre-flight
calibration was performed at the Synchrotron Ultraviolet Radiation Facility of
the National Institute of Standards and Technology. Calibration parameters were
used to calculate absolute solar irradiance from the Sounding Rocket flight
measurements on 14 April 2008. These irradiances for the ESP bands closely
match the irradiance determined for two other EUV channels flown
simultaneously, EVE's Multiple Euv Grating Spectrograph (MEGS) and SOHO's
Charge, Element and Isotope Analysis System / Solar EUV Monitor (CELIAS/SEM).
