This windows helps translating between different physical units often used in different areas of optics. 
Additionally the approximate color corresponding to the entered spectral position is indicated.

On the left side you simply enter a value in any of the four controls and the other three fields will be changed accordingly as soon as you press ENTER or leave the input field. The selector behind each field allows to quickly change between different orders of magnitude (e.g. between micrometer, nanometer, Angstrom etc.). 

The following relations apply: 

• wavelength = c/frequency 
• wavenumber = frequency/c 
• photon_energy = h*frequency

On the right side you can translate spectral shifts, linewidths or coherence lenghts between the different notations. 
The relations here are: 

• d_wavelength = -wavelength^2*d_frequency/c 
• d_wavenumber = d_frequency/c 
• coherence_length = 1/d_wavenumber 
• d_photon_energy = h*d_frequency

 

From the menu "index of refraction" it is possible to change the index of refraction of the surrounding medium. ToptiCalc can automatically calculate the index of refraction of dry air (at 15°C, according to the SCHOTT glass catalogue) in order to help translating wavelengths measured with a spectrometer in air atmosphere into the frequencies etc. 
The currently used index and the corresponding speed of light is always displayed in the status bar at the bottom of the window.