Oh most Blessed Light Divine
Shine within these hearts of thine
And our inmost being fill
O lux beatissima
Reple cordis intima
To be filled with light connotes to me a complete lack of any need for fear, for suspicion, for lack of trust. Nothing evil can hide; everything is plain.
The irony is of course that we do fear the light. The whole contest between light and darkness in Scripture (especially from St. John) demonstrates clearly that sin is comfortable with darkness and fears light. So light can actually be a very threatening thing to our sinful status quo. Praying for an influx of the Holy Spirit is not for the faint of heart. As C. S. Lewis tells us about Aslan, he is wild, but he is good.
Even though the light of the Holy Spirit is pure goodness, we are likely to experience it as pain because of our impurities, St. John of the Cross reminds us: "The soul, because of its impurity, suffers immensely at the time this divine light truly assails it." (The Dark Night, Bk. 2 Ch. 5.5)
So this prayer is really one of deep surrender. When we know that God's ways and plans and desires for us are higher than our own, we can trust Him to do within us whatever He wants. But fasten your seatbelt. God answers all sincere invitations.