Spectrum vs wavelength from image (Matlab)

Question

With reference to this example FFT, but with a linear wavelength scale I am dealing with the extraction of the spectrum versus wavelength of pictures with a certain resolution. However, at the beginning, when the script must create a frequency domain, there is something that is unclear to me. Why the calibration is made in temporal domain (see bold text). I might be wrong but there should be a spatial/frequency conversion. And if this must be time, where do I get the information about the temporal grid resolution and grid size? Can someone explain this point?

lam = 800e-9; % Wavelength (m)  (I think this is the excitation or emission wavelength)
c = 3e8; % Light speed (m/s)

**nt = 8192; % Temporal grid resolution    
T = 400*1e-15; % Temporal grid size (s)
dt = T/nt; % Temporal pixel spacing**

df = 1/(nt*dt); % Frequency pixel spacing
ff = [(0:nt/2-1) (-nt/2:-1)]*df; % Frequency grid 
ff = fftshift(ff);