I'm putting the Optris calculations into a spreadsheet -- the following is 
documentation of the formulae used in readable form

        From the Optris "IR Basics" documentation  (Page 7)                     
                                
                                                                
                                                                
                From the actual object temperature (To) and ambient (Ta)        
                                        
                                                                
                        To      Actual temperature                      e       
emmisivity
                        Ta      Actual ambient                  C       a 
constant in the calorimeter
                        Tp      Temperature of pyrometer                        
n       exponent -- depends on wavelenght
                                                                
                        U =  C *( e*To^n  + (1- e)*Ta^n - Tp^n)                 
                
                                                                
                Measured temperature reported by the calorimeter                
                                
                                                                
                        Tm      Measured temperature                            
                                Note : the optris equation uses the same symbol 
for Tm and To --- 
                                 so it seems to be self-referential             
                
                                                                
                        Tm = root(n,(U  - C*Ta^n + C*e*Ta^n + C*Tp^n)/ C*e)     
                                
                                Note that a lot of the "e" cancel out, leaving 
1/e terms                                
                                root(n,val) can be computed as power(val,1/n) 
-- some languages have problems with this                         
                                                                
                        Pm      Total power calculated from Tm                  
        
                                                                
                        Pm =    a * e * ( Tm^4 - Ta^4)  

I'll put these equations into the spreadsheet and see what happens for various 
"n" (wavelength) and "e" emmisivity                      

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