The button labeled Parameters in the main window of SPRAY opens the following dialog which gives access to important simulation parameters:

In the 'Spectral range' section you can set the spectral range for the ray tracing simulation. Specify the minimun, the maximum, the number of spectral points and the unit. For the latter you can choose between 1/cm (wavenumbers), nm (wavelength), eV (energy), micron (wavelength) and THz (frequency).

The 'Angle resolution' is important for interface objects. These compute (before the simulation is started) the angular dependence of the reflectance and transmittance using the specified number of points for the range 0 ... 180 degrees. Usually this does not take too long compared to the subsequent ray-tracing with possibly many thousands of rays. Hence there is no reason to go to a low angle resolution unless you have a large number of interfaces in your model.

The 'Number of photons per spectral point' determines how many rays are processed for each spectral point. How many rays you need depends very strongly on the questions that you have to answer.

Finally you can set the 'Max. number of interactions' for each ray. After a ray has been emitted by the light source SPRAY counts how many interactions this ray has with the objects of the setup. If the specified maximum value of interactions is reached before the ray reaches infinity or is absorbed the tracing of this ray is stopped. This is to avoid situations where a ray is reflected back and forth forever between two ideal mirrors, for example. Usually you do not have to change this value unless you study very special setups.