Spectroscopic diagnostics of solar coronal plasmas critically depends on the uncertainty in the measured line intensities .
One of the main sources of uncertainty is instrumental scattered light , which is potentially most important in low-brightness areas .
In the solar corona , such areas include polar and equatorial coronal holes , which are the source regions of the solar wind ; instrument-scattered light must thus pose a significant obstacle to studies of the source regions of the solar wind .
In this paper we investigate the importance of instrument-scattered light on observations of equatorial coronal holes made by the Hinode/EIS spectrometer in two different phases of the solar cycle .
We find that the instrument-scattered light is significant at all temperatures , and in both regions it amounts to approximately 10 % of the average intensity of the neighboring quiet Sun regions .
Such contribution dominates the measured intensity for spectral lines formed at temperatures larger than Log T = 6.15 K , and has deep implications for spectroscopic diagnostics of equatorial coronal hole plasmas and studies of the source regions of a large portion of the solar wind which reaches Earth .
Our results suggest that the high temperature tail in the coronal hole plasma distribution with temperature , however small , is an artifact due to the presence of scattered light .