Emotion is an important mental and physiological state, influencing cognition, perception, learning, communication, decision making, etc. It is considered as a definitive important aspect of user experience (UX), although at least well developed and most of all lacking experimental evidence. This paper deals with an application for emotion detection in usability testing of software. It describes the approach to utilize the valence arousal space for emotion modeling in a formal experiment. Our study revealed correlations between low performance and negative emotional states. Reliable emotion detection in usability tests will help to prevent negative emotions and attitudes in the final products. This can be a great advantage to enhance Universal Access.