C :: Calculate Black Body Spectra Over User-entered Range Of Wavelength And Temperatures

I tried to write a code to calculate black body spectra over an user-entered range of wavelength and temperatures. The equation I'm trying to code is the second one this image (stolen from Wikipedia)The syntax to run it is bbgrid lambda_inic lambda_final temp_inic temp_final inc_T inc_lambda

where bbgrid is the name of the program, lambda_inic and lambda_final are the limits of the wavelenght range (in units of angstroms, 1A=10⁻⁰m), temp_inic and temp_final are the limits of the temperature range (in Kelvins) and inc_T and inc_lambda are the increments. What I want to do is, given the ranges of temperatures and wavelengths, to run the code over the lambdas and the temperatures.

The problem is that the behaviour of the intensities (what I'm calculating) is erratic. Sometimes it is highly positive, sometimes immensely negative and turning between those two. As an example of an output file, I'm getting things like this:

3100 1915076038
3110 -1233496775
3120 1741010116
3130 1229780625
3140 421722788
3150 -1874760945
3160 1654746252
3170 1062468321
3180 -795217626
3190 -1141129750
3200 -1570716956
3210 539385985

While I was trying to debug the code, I found the problem may reside in the exponential factor in the denominator. I wrote some lines to calculate and print on the screen only the exponential, and it was oscillating like crazy. The output file should produce curves like this:

Code:

#include<stdlib.h>
#include<stdio.h>
#include<math.h>
#include<string.h>

[code]....