Obscura v Lucida

The short answer: A camera lucida, like the LUCY or LUCID-Art, is a device used to reflect a large image over your drawing surface, so you can sketch out the subject.

A camera obscura is a device that projects an image onto a screen to help you with perspective, tonal rendering, composition, handling of light, and getting a classic ‘Vermeer look’ to your paintings. The camera obscura can also be very useful for photographers.

Below is a detailed video answer with images and diagrams: