KWAI Chung Hospital's decision to open its doors to the public was no doubt a well-meaning attempt to show mental care in a good light and dispel some of the prejudices and stigma attached to mental illness. But it has backfired badly. Showing off the hospital's facilities is one thing. Showing off the patients is another. Mental patients crave and deserve privacy. They should not be denied it. In a society which still does stigmatise the mentally ill, it is particularly insensitive to expose them to public attention, whatever the motives. Nor is it fair to argue the patients had been informed and given the choice to be moved on to other wards if they objected to public scrutiny. That kind of decision would be hard enough for a mentally healthy person, knowing that it would mean being on display to an expected 2,000 to 3,000 visitors. How can it be asked of a patient in a mental ward? Mental hospitals are not zoos. The days are long gone when the ''mad'' could be considered legitimate subjects for family entertainment and could be teased and tormented. The fact that visitors were to be forbidden from taking photographs or talking to the patients is in itself a recognition of the inmates' need to preserve their fragile dignity. With a little extra thought the hospital management should have made the leap of imagination required to understand the discomfort and feelings of inadequacy their patients will suffer as a result of this open day. It cannot be beyond the skills and imagination of qualified hospital doctors to find more constructive and positive ways to attack the necessary task of educating the public - and to ease those patients who are ready for it back into contact with the outside world.