A photodiode-array spectrometer for the detection of atmospheric trace gases has been developed to take diffuse solar zenith and, using a n artificial light source, horizontal long path measurements. Of the numerous factors involved in determining the minimum amount of a detectable gas, including its spectral characteristics, atmospheric phenomena and the algorithm used, the present study examines only the features of the spectrometer and sensor. The spectrometer's spectral dispersion is accurately calculated and its internal stray light kept to a minimum. The linear image sensor's dynamic range and aperture-response function are analyzed along with those phenomena that can alter the actual signal such as veining glare, blooming, etaloning and dark current; the latter two are treated in depth. Etaloning is linked to optical interference in the sensor's passivating layer. A simple model of it, as well as a check of the relative stability over time of the transmission peaks, are included. The analysis of the dark current indicates the existence of a photo-induced component that is inversely proportional to the incident radiation flux. It yields a simple analytical equation describing the phenomenon for the sensor's various elements. This makes it possible to derive the correct dark current value in relation to both the incident radiant energy and the spectral range investigated.