A lightbox pop-up is a pop-up that appears over the visited page while the latter is darkened and inactivated - links don’t work. The lightbox effect ensures the visitor attention focuses on the overlay though the visitor can always use a closing cross.
Lightbox pop-up are used on websites to promote content and offers. They can be used:
for subscribtion form (email or membership)
for content offer (webinar, white paper)
for special offer
for live chat
as an ad space - like old pop-up window ads.
To reduce intrusiveness and according to the purpose of the lightbox, the appearance of the pop-up may be managed through cookies and visitor behaviors.
For more details and info about lightbox impact, see also lightbox subscribe form.
An example of lighbox pop-up usage: