For best accuracy, comparison should be made on clear, non-polluted, summer days within one hour of solar noon.

Enter input parameters in the blue cells at right. Definitions are shown below.  

Sensor must be level and perfectly clean. Enter your measured solar radiation in the blue "Measured PPF" cell at far right.

Difference between the model and your sensor is shown in the yellow "DIFFERENCE FROM MODEL" cell at right.

Run the model on replicate days. Contact Apogee for recalibration if the measured value is more than 5 % different than the estimated value. You will be contacted within two business days.

For a discussion on model accuracy and
sensitivity of input parameters, CLICK HERE.

  Latitude = ˚    Model Estimated PPF = µmol m-2 s-1  
    Longitude = ˚    Measured PPF = µmol m-2 s-1  
    Longitudetz =  ˚    Difference from Model = %  
    Elevation =       m      
  Day of Year =     Name:  
    Time of Day =   E-mail:  
    (6 min = 0.1 hr)      
    Daylight Savings =  + hr   Phone:  
    Air Temperature = C   Serial #:  
    Relative Humidity = %   Comments:  
        Please include all requested information.

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  Latitude = latitude of the measurement site [degrees]; for southern hemisphere, insert as a negative number; info may be obtained from http://itouchmap.com/latlong.html    
  Longitude = longitude of the measurement site [degrees]; expressed as positive degrees west of the standard meridian in Greenwich, England (e.g. 74° for New York, 260° for Bangkok, Thailand, and 358° for Paris, France).

  Longitudetz = longitude of the center of your local time zone [degrees]; expressed as positive degrees west of the standard meridian in Greenwich, England          
      In the US, the values are 75˚, 90˚, 105˚, and 120˚ for Eastern, Central, Rocky Mountain, and Pacific time zones; 0˚ for Greenwich, England; 345˚ for Paris, France; 255˚ for Bangkok, Thailand; DETERMINE THE CENTER OF YOUR TIME ZONE.  

ASCE Standardized Reference Evapotranspiration Equation. American Society of Civil Engineers.  Reston, Virginia, USA. 2005.

On the above Web site, select:
 -- Draft Main Report Body
        without Appendices (PDF 1.2 MB)
 -- Appendices * (PDF, 7.4 MB)

              * Appendix D describes clear sky  procedure

  Elevation = elevation of the measurement site [meters]; 1 m = 3.28 ft; CONVERSION CHART    
  Day of Year = numeric day of the year (0-365); DAY OF YEAR CHART    
  Time of Day = numeric time of the day in tenths of hours (0-24); 6 min = 0.1 hr    
  Daylight Savings = correction for daylight savings time (enter 1 if on daylight savings time, 0 if not)    
  Air Temperature = air temperature at the time of measurement [C]    
  Relative Humidity = relative humidity at the time of measurement [%]    
  Measured PPF = measured value of Photosynthetic Photon Flux [µmol m-2 s-1] from the sensor being tested    
  Model Estimated PPF = estimated Photosynthetic Photon Flux [µmol m-2 s-1] incident on horizontal plane for clear sky conditions    
  Difference from Model = difference in percent between the measured and estimated values of radiation    
  Solar Constant = solar constant for the mean distance between the Earth and sun 1361 W m-2    
  Energy in PAR = average energy content of photosynthetically active radiation (PAR) 222000 J mol-1    
  PAR / Solar = ratio of photosynthetically active radiation (PAR) to incoming shortwave radiation (SWi) 0.45 J J-1      
  Kt = atmospheric turbidity coefficient (must be set to 1 for clean air) 1.0        
  dr = inverse relative distance factor for the distance between Earth and sun        
  d = solar declination ˚      
  eqt = equation of time (More info on NOAA Web site) min      
  Solar N = time of solar noon        
  Solar Z = solar zenith angle ˚      
  PB = barometric pressure of the measurement site kPa    
  eA = air vapor pressure kPa    
  w =  precipitable water in the atmosphere mm    
  Kb = clearness index for direct beam radiation      
  Kd = transmissivity index for diffuse radiation      
  SWa = extraterrestrial radiation W m-2