Vishokaa vaa jyotishmati
Vishokaa: Free of grief, blissful
Jyotishmati: Resplendent with light (of gyan), enlightened
Patanjali lists other ways
By stilling oneself in a mind, which is free from grief and illuminant with gyan also (the chitta gets purified).
It is the impurities which lead us to attach ourselves to the kleshas which are the cause of suffering or pain, and also which cloud our vision, when one stills his/her focus on a being/mind who is content/in a state of bliss and resplendent with light, the chitta gets purified.