Ganong's light screen is an instrument used to stop the supply of light to a part of a leaf so that we can study the importance of light factor for the preparation of food by the leaf through photosynthesis. 

Instead of Ganong's light screen we can make use of a black paper to test the importance of light factor in photosynthesis. A piece of black paper is taken and a star shaped design is cut at the center of the black paper and is attached over the surface of the leaf with help of clips. The place where star shaped cutting is the place where light is available for the leaf and at the other place because of black paper light is not available. Where light available on the leaf, when iodine test is conducted it gives positive result. 
For this experiment to conduct, first the plant must be kept in a dark room for 48 hours to destarch the plant