fig1 shows an exemplary embodiment of a receptacle 1 of a contact lens package . such contact lens package usually comprises the receptacle 1 and a closure , which usually is attached , for example laminated ( in case of a foil ), to a top surface 2 of the receptacle 1 . as the closure is of no specific importance to the invention , it is not shown in the drawings , as detection of the presence or absence of the contact lens can be performed with or without closure . the receptacle 1 is provided with a concavely shaped cavity 3 , in which a contact lens is to be accommodated . corresponding to the concavely shaped cavity 3 , the receptacle 1 may have a convexly curved bottom 4 . supporting flanges 5 and 6 , which extend at the sides of the receptacle 1 towards the bottom 4 thereof , facilitate a stable placement of the receptacle 1 on a supporting surface . the receptacle 1 may be made of polypropylene , for example . in a packaging station of an automated contact lens manufacturing line , a contact lens is placed into the concavely shaped cavity 3 of the receptacle 1 which is subsequently filled with a liquid , such as water or saline . in order to be able to detect whether or not a contact lens is present within the cavity 3 of the receptacle 1 a detector which is sensitive to infrared radiation may be arranged in a lens detection station in order to observe the receptacle 1 as a whole or at least a portion thereof which comprises the cavity 3 . the detector may be embodied as or comprise an infrared sensor or an infrared camera . in fig1 the directions from which the detector may observe the receptacle 1 are indicated with arrows b and h , respectively . arrow b indicates that the detector observes the receptacle 1 from beneath the receptacle 1 ( i . e . the detector is arranged beneath the bottom 4 of the receptacle ). arrow h indicates that the detector observes the receptacle 1 from a lateral side of the receptacle 1 ( in the embodiment shown from about horizontally ; i . e . the detector is arranged at a lateral side thereof ). fig2 shows schematically an image obtained with an infrared camera which is arranged beneath the bottom 4 of the receptacle 1 , such that the receptacle 1 is observed from underneath . the outlines of the receptacle 1 and its cavity 3 are clearly visible . a contact lens 10 within the cavity 3 of the receptacle 1 is shown in black contrast , corresponding to the absorption of infrared radiation by the material the contact lens is made of , as detected by the infrared camera . fig3 shows an image from an infrared camera which is arranged at a lateral side of the receptacle ( about horizontally relative to the receptacle ) such that the receptacle 1 is observed from laterally . again , the outlines of the receptacle 1 are clearly visible . the contact lens 10 is shown in shades ranging from black to grey . this is a result of the different amounts of absorption of infrared radiation , which is dependent from the length of travel of the infrared radiation through the material of the contact lens 10 within the cavity 3 of the receptacle 1 . the observation from a lateral side of the receptacle 1 also may provide clear information as to whether more than one contact lens is present within the receptacle 1 . the diagram in fig4 shows two graphs representing the absorbance a r of a receptacle made of polypropylene and the absorbance a l of a contact lens made of a silicone hydrogel material such as from a material obtained from a mixture of the following substances , with “% ( w / w )” indicating the weight percentage per total weight : on the abscissa wavenumbers are shown ( unit : cm − 1 ) while the ordinate shows the absorbance at the respective wavelength ( unit : %). no absolute numbers and dimensions are specified on the ordinate , since they depend on the specific detector used , from the amplification factor in the electronics in the respective wavelength range , etc . in any event , the difference in absorptions is sufficient to reliably detect the presence or absence of a contact lens in the cavity 3 of the receptacle 1 . the respective wavenumber on the abscissa corresponds to the reciprocal of the wavelength (= 1 / λ ). the two graphs in fig4 show that the receptacle made of polypropylene has peaks of the absorbance in a different wavelength range than the material the contact lens is made of . thus , in the analysis of the detected infrared radiation the two materials can be reliably distinguished . for example , in the embodiment shown a first wavenumber range of 1300 cm − 1 to 1210 cm − 1 ( corresponding to a wavelength range of 7 . 7 μm to 8 . 3 μm ) and a second wavenumber range of 1150 cm − 1 to 1000 cm − 1 ( corresponding to a wavelength range of 8 . 7 μm to 10 μm ) are particularly advantageous , since in these ranges the absorbance a l of the contact lens and the absorbance a r of the polypropylene receptacle are significantly different so that a reliable detection of the contact lens can be performed . the graphs shown in the diagram of fig5 shows the absorbance a w of water and the absorbance a s of a saline , respectively . again , on the abscissa wavenumbers are shown while the ordinate shows the absorption at the respective wavelength in %. the two graphs representing the respective absorbance a w and a s are very similar to each other and have peaks in the absorbance practically in the same wavelength range . a comparison of the absorbance a r , a l , a w and a s represented by the graphs shown in fig4 and fig5 shows that the absorbance a l of the contact lens has maxima in a wavenumber range ( wavelength range ) which is clearly distinct from the maxima in absorbance a r , a w and a s of the polypropylene receptacle and of water and saline , respectively . thus , the passive detection method employing a detector for detecting infrared radiation and for the analysis of the detected infrared radiation may lead to clear and unambiguous results as to whether or not a contact lens is accommodated within the receptacle , and as to whether or not more than one contact lens is present within the receptacle . the detection method may be performed even with contact lens packages in which the receptacle has been loaded with a contact lens and has been filled with water or saline , and after the receptacle has been provided with a removable closure ( for example a foil ), which may have been attached , for example laminated , to a top surface of the receptacle . however , detection may also be performed when the receptacle has been loaded with a contact lens but before water or saline have been added , or after water or saline have been added but before the foil has been attached to the receptacle . although the invention has been described with the aid of a specific embodiment , it is evident to the person skilled in the art that this embodiment has been described by way of example only while it represents a more general teaching , and that various changes and modifications are conceivable without departing from this general teaching underlying the invention . therefore , the scope of protection is not intended to be limited by the embodiment described , but rather is defined by the appended claims .