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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies [quote][i]Originally posted by kemeyer[/i] [br]Origin Ver. and Service Release (Select Help-->About Origin):8.1 SR1 Operating System:Vista I have a question regarding the capabilities of the nlbegin/nlfit/nlend x-functions. Basically, I have a dataset of index(n) vs wavelength(lambda) that I initially fit to the sellmeier equation: n = sqrt(A + B*Lambda^2/(Lambda^2-C) + D*Lambda^2/(Lambda^2-E)) Where n is the dependent variable and Lambda is the independent. I've created a user-defined function for it and was able to fit the data using the analysis>>Non-linear fit>>new dialog option from Origin's menus. After this fit, I get parameters A, B, C, D, and E. What I want to do is use these parameters for the next step, which is finding the parameter alpha in the equation: n = sqrt(A + B*Lambda^2/(Lambda^2-C)(1+alpha*(T-23)) + D*Lambda^2/(Lambda^2-E)(1+alpha*(T-23))) Where I have the data of Temperature (T) vs. index(n) at a given wavelength(lambda).I know they need to be fixed, but I am unsure of how to go about doing this. Should I assign nltree:= to the output of the first fitting? One problem I see is that in the second equation, Lambda is a constant (but a different one for each data set I am looking at). Here, the independent variable is T, the dependent is still n, and the parameter is alpha (A, B, C, D, E, and Lambda will all be fixed). Do you have any suggestions or pertinent examples I could look at? I searched the wiki page for Origin but I didn't see anything that clarified or answered my questions. I'd really appreciate any help you could give me, Katie [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.