From the PHB, p. 152:
Lantern, Hooded. A hooded lantern casts bright light in a 30-foot radius and dim light for an additional 30 feet. Once lit, it burns for 6 hours on a flask (1 pint) of oil. As an action, you can lower the hood, reducing the light to dim light in a 5-foot radius.
PHB errata says that you are not supposed to be “blinded by darkness”, but can’t see anything that is concealed by the darkness:
Vision and Light (p. 183). A heavily obscured area doesn’t blind you, but you are effectively blinded when you try to see something obscured by it.
But when you are in the darkness yourself, you can see things that are not. That means that a hooded lamp will be visible, since it still creates a lit area around.
Assuming that your foes will see the light at any range regardless, what is the point of reducing the lit area?