The "Hold-to-Light" binder in the David P. Campbell Postcard Collection deals primarily with cards that contain some type of visual effect when viewed. The majority of the cards’ visual effects can be viewed when held up a light. For this, they are usually called HTL (hold-to-light) cards, or transparency cards. There were three specific processes used in printing these HTL Cards: Die-cutting, Transparency, and Slide-Transparency.
"Hold-to-Light and Other Specialty Postcards,"
Student Projects from the Archives: Vol. 1
, Article 7.
Available at: https://ideaexchange.uakron.edu/spa/vol1/iss1/7