the present invention will be described below with reference to the accompanying drawings . in the following description , well - known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail . fig4 is a flow chart illustrating a method of discriminating a latent fingerprint from that of a biomass according to the present invention , and fig1 is a block diagram illustrating a function of an apparatus for discriminating a latent fingerprint from that of a biomass according to the present invention . for the sake of convenience , the description of a method will be made along with an apparatus according to the present invention . if a person to be authenticated for access touches an imaging surface with one finger , the backlight control means 503 switches off the backlight ( s 101 ). the image acquisition means 505 then acquires an image of the fingerprint ( s 103 ), and the fingerprint detection means 507 detects the existence of a fingerprint ( s 105 ). as described above , if a fingerprint has been detected during the “ off ” state of the backlight , the latent fingerprint discrimination means 509 determines that a latent fingerprint has been detected due to an external light ( s 109 ). if no fingerprint has been detected during the backlight &# 39 ; s “ off ” state , the latent fingerprint discrimination means 509 determines the inputted fingerprint to be a fingerprint of a biomass ( s 109 ). in step ( s 105 ) of detecting a fingerprint , the fingerprint can be detected by adding gray levels of the image as described above . to be specific , differences in gray levels between adjacent pixels of the acquired image on the x - axis or the y - axis of a coordinate are added , as shown in fig5 ( s 106 ). as the next step , the latent fingerprint discrimination means determines whether the added value is greater or less than a pre - set reference value ( s 107 ). if the added value is greater , this means that a fingerprint has been detected while the backlight was in its “ off ” state , and it is determined that the acquired fingerprint image is a latent fingerprint ( s 108 ). if the added value is less than or equal to the pre - set reference value , this means that a fingerprint has not been detected ( s 113 ), and it is determined that the acquired fingerprint image is a fingerprint of a biomass . [ 0053 ] fig6 is a flow chart illustrating a process of determining whether or not a fingerprint exists according a second embodiment of the present invention . a fingerprint image is acquired by switching off the backlight , and all the gray level values of the pixels of the image are added ( s 206 ). if the added value is less than the pre - set reference value , it means that a fingerprint has been detected ( s 208 ), and it is determined that the acquired image is a latent fingerprint image . [ 0054 ] fig7 is a flow chart illustrating a process of determining whether or not a fingerprint exists according to a third embodiment of the present invention . a fingerprint image is acquired by switching off the backlight , and an average value m as well as a dispersion value d of the gray levels of the pixels of the image is calculated ( s 306 ). if it is determined that m is less than a first reference value and that d is greater than a second reference value ( s 307 ), it means that a fingerprint has been detected ( s 308 ). therefore , it is determined that the acquired image is a latent fingerprint . the above embodiment represents a case of distinguishing a latent fingerprint from that of a biomass by acquiring an entire image of a fingerprint . as mentioned above , time is wasted while receiving a frame of the image in this case and time can be saved by acquiring only a part of the image , rather than the entire image , to detect a latent fingerprint while the backlight is in an “ off ” state . [ 0056 ] fig8 is an exemplary pattern of a half of the entire image acquired from a latent fingerprint . a latent fingerprint can sufficiently be discriminated from that of a biomass with only about half of the entire fingerprint image . the same method as described above is applied for discriminating a latent fingerprint from that of a biomass . [ 0057 ] fig9 is an exemplary pattern of a mid - part image acquired from a latent fingerprint by using a windowing function of an image sensor . when using such a windowing function of the image sensor for receiving only a part of an image , the time consumed for receiving the fingerprint image will be reduced as the width w of the window is narrow , and consequently , the time consumed for determining the existence of a latent fingerprint will also be reduced . [ 0058 ] fig1 is a flow chart illustrating a method of discriminating a latent fingerprint from that of a biomass according to a fourth embodiment of the present invention . image acquisition means 505 switches on the backlight under a control by backlight control means 503 ( s 401 ) to acquire an image ( s 402 ). as the next step , the fingerprint detection means 507 determines whether or not a fingerprint has been detected ( s 403 ). in the affirmative , the image acquisition means 505 switches off the backlight under a control by the backlight control means 503 ( s 404 ) to acquire an image ( s 405 ). the fingerprint detection means 507 then determines whether or not a fingerprint has been detected while the backlight was in its “ off ” state ( s 406 ). in the affirmative , discrimination means 509 determines the detected fingerprint to be a latent fingerprint ( s 407 ). in the negative , the discrimination means 509 determines the detected fingerprint to be a fingerprint of a biomass ( s 408 ). the same method as shown in fig5 to 7 is used for detecting a fingerprint according to the fourth embodiment , as in case of the third embodiment . in the fourth embodiment shown in fig1 , an image is acquired while the backlight is in an “ on ” state to detect the existence of a fingerprint , and the existence of a fingerprint is detected by acquiring an image while the backlight is in an “ off ” state when a fingerprint has been detected . an image acquired using a similar method of receiving an image while switching on and off the backlight in the course of acquiring the image contains a part acquired under an “ on ” state of the backlight and a part acquired under an “ off ” state of the backlight that appear in turn . [ 0060 ] fig1 is an exemplary pattern of a fingerprint image acquired by switching on and off a backlight . part “ a ” represents an image acquired under an “ off ” state of the backlight , while part “ b ” represents an image acquired under an “ on ” state of the backlight . as described above , it is determined that an image was acquired from a fingerprint of a biomass because a fingerprint has not been detected in the part “ a ”, which was acquired while the backlight was in the “ off ” state , but has been detected in the part “ b ”, which was acquired while the backlight was in the “ on ” state . it is well known to those skilled in the pertinent art that the switching on and off of the backlight is controlled by software , and that the image part acquired while the backlight was in the “ on ” state is distinguished from the image part acquired while the backlight was in the “ off ” state by calculation . while the invention has been shown and described with reference to certain embodiments to carry out the invention , it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention as defined by the appended claims . the technical concept of the present invention lies in acquiring an image while the backlight was in the “ off ” state and discriminating a latent fingerprint from a fingerprint of a biomass based on the fingerprint image detected from the acquired image .