In a food processor blend raspberries to a purée with the lemon and orange juice and sugar. Strain to discard raspberry seeds.
Pour the preparation into an ice-cream machine and when the sorbet is ready, transfer it to a pre-chilled mould and place in the freezer.
If you don't have an ice-cream maker, place preparation in a stainless-steel bowl in the freezer. When the purée starts to set, whisk if for 10 to 15 seconds and return to freezer.
Repeat this procedure until the purée is too hard to whisk. The whisking lightens the sorbet and prevents large ice crystals from forming.