Method of detecting fraud

A method of detecting fraud during use of a device for capturing an imprint of a body part using a principle of total internal reflection with dark field, the said device comprising a first transparent plate comprising an upper face on which is placed a body part to be verified, a light source comprising a plurality of LEDs illuminating exclusively in the direction of the upper face, a light-opaque screen situated below the first transparent plate, a second transparent plate situated below the opaque screen and an image sensor situated below the second transparent plate, the two transparent plates, the opaque screen and the image sensor being parallel and the opaque screen comprising a network of holes intended to let light rays arising from the light source reach the said image sensor, the said light rays allowing the image sensor to generate an image of the said imprint, wherein the method comprises:

• using the said device to acquire a first imprint image with a lighting of the body part to be verified by the light source, termed the first type of lighting, such that the whole surface of the body part to be verified in contact with the transparent plate returns light;
• using the said device to acquire at least one second imprint image with a lighting of the body part to be verified by the light source, termed the second type of lighting, such that at least one LED is lit up;
• when a plurality of LEDs is lit up, the said LEDs are at least a predefined distance apart such that the sub-part of the finger D returning the light emitted by an LED is disjoint from any other sub-part of the finger D returning light illuminated by another LED;
• obtaining for each second imprint image an item of information representative of a light level re-emitted by the body part to be verified by using the said second imprint image and the first imprint image;
• and
• comparing the said item of information with a reference item of information representative of a light level re-emitted by a true body part placed on the transparent plate so as to validate that the body part to be verified is a true body part.