Fiber-optic trans-illumination (FM) method of caries
diagnosis is based on the principle of:
When Laser beam falls on carious enamel, it gives a bluish hue. A fluorescent dye is taken up by carious dentine which can be viewed easily There	is	a	different index	of	light	transmission	for decayed or sound enamel. The conductance of normal and decayed tooth tissue is different.