A display apparatus includes a display having a plurality of pixels, and a controller which selects a pattern corresponding to a gradation of gradation data. The optical head has a low profile, e. The key is then generated and, lastly, the key is represented in either an external form or an internal form. The delay line includes a plurality of delay cells connected in series. The system includes cryoprobes and a control system which operates the cryoprobes to accomplish freezing in two stages, including a high power freeze and a low power freeze.