turning now to the drawing , and in particular to fig1 and 2 thereof , there is shown a hand - held optical reader 10 . the reader has a housing 12 , which includes a hand - grip 14 , and a reader head 15 . a depressible switch 18 is provided on the front of the hand - grip 14 , which actuates the device when depressed by an operator &# 39 ; s finger . a cable 16 attaches to the base of the hand - grip 14 for supplying power and for transmission of signals between the reader and a host device ( not shown ), which can be a point - of - sale terminal , or a computer . an assembly 37 is disposed in the rear of the reader head 15 . mounted thereon is a light source 38 which can be a microlaser or the like , and one or more photodetectors 34 , which must be optically shielded from the light source 38 . shielding can be accomplished by suitable opaque thin film barriers ( not shown ) appropriately disposed in the photodetectors , if they are realized as an integrated circuit , or by an appropriate configuration of the laser &# 39 ; s containment or lensing system . alternatively , the light source 38 can be mounted behind the photodetectors 34 to prevent direct transmission of light therebetween . power and signals are communicated to and from the assembly 37 via a cable 39 leading to suitable signal processing electronics 48 , which can include a decoder and a microcomputer . a transceiver , well known in the art , can also be incorporated into the electronics 48 , for communication to and from a remote host . in the case where a transceiver is included , then cable 16 will generally be limited to carrying power only , or it could be omitted entirely , and a battery ( not shown ) placed in the handle portion 14 for supplying power to the unit . light emitted by the light source 38 strikes a negative lens 35 , which is mounted on a wheel or disk 36 . light exiting the lens 35 continues outwardly through the housing 12 at an angle of divergence . a plurality of lenses can be mounted on the wheel 36 , and can be selected in order to achieve a predetermined angle of divergence . the support member 30 is configured as a frustum of a cone , having a small rear end disposed proximate the lens 35 , and a wide front end disposed near the front of the reader head 15 . an optical element 31 can optionally be placed in the rear end of the support member 30 to further adjust the light beam produced by the light source 38 and the lens 35 , as may be required . the support member 30 is stabilized by an outer retainer 40 , which is mounted in the housing 12 . a transparent guide 20 is disposed on the front portion of the reader head 15 . the guide 20 is preferably configured as a frustum of a cone . it removably attaches to the retainer 40 by bayonet pins 24 and is inserted into a bayonet adapter 41 located at the front end of the reader head 15 , so that it aligns with the support member 30 . the bayonet pins 24 are carried in bayonet tracks 45 . a spring 44 disposed in a recess of the retainer 40 bears on a shoulder 42 of the support member 30 , urging the support member into forward movement . the support member 30 abuts the bayonet adapter 41 , and forces the pins 24 into terminal indents 46 where they are locked in place . the spring 44 also functions to displace the assembled support member 30 and transparent guide 20 in a forward direction , so that the transparent guide 20 is in an extended position with respect to the reader head 15 . the guide 20 could also be secured to the reader head 15 by other conventional methods , such as threads , or snap fittings . as best seen in fig6 and 7 , the assembled device can also be actuated by pressing it against a data carrier . in fig6 the guide 20 is disposed near a data carrier 56 , and , as a result of the unopposed action of spring 44 , is in a fully extended position . in fig7 the guide 20 is applied to the data carrier 56 and is subjected to pressure exerted by an operator ( not shown ). the force exerted by the data carrier 56 on the cone 20 counteracts the spring 44 , and moves the transparent guide and the support member 30 rearward a short distance into the housing 12 . switch contacts 61 , 62 disposed on the support member 30 and on a non - movable surface such as disk 36 are thereby brought together to electrically actuate the unit in the same manner as depressible switch 18 . the light source 38 is preferably disabled until one of the switches is closed for safety and to conserve power . as is known in the art , other components of the reader can also be enabled and disabled according to the state of the switches , such as the electronics 48 , and the photodetectors 34 . while actuation of the device is responsive to a mechanical contact switch , it will be understood by those skilled in the art that other types of switches can be substituted for contacts 61 , 62 . for example optical beam interruption switches could be employed , as could various pressure sensors and proximity sensors using principles such as , for example , change in capacitance . the result in each case is that the device is actuated in response to increased mechanical pressure exerted by the operator pushing the reader against the data carrier . referring now to fig3 the transparent guide 20 extends outwardly of the housing 12 , and is dimensioned to encompass a symbol 52 of predetermined type when the transparent guide 20 is placed on a data carrier 56 . the symbol can be , for example , a two - dimensional bar code symbol . turning now to fig4 there is shown a diagram illustrating the function of a transparent guide 420 which is configured as the frustum of a cone , and placed on a data carrier 452 . light radiation emitted by a laser source 438 emerges from a negative lens 435 at an angle of divergence α that is equal to the conical angle β of the frustum . this laser can be of any suitable type ; a 660 nm laser is suitable . the guide 420 is constructed of a transparent material such as glass or plastic . the source 438 is aligned with the axis of the guide 420 , so that the negative lens 435 aligned therewith projects a beam of light that diverges parallel to the walls of the frustum , as shown by marginal rays 405 , 406 , and it will be evident that the incident beam is fully encompassed by the wall of the guide 420 , which is proximate the marginal rays 405 , 406 . light incident on the data carrier 452 is scattered in accordance with well known substrate interactions with the data carrier 452 , and most of it returns in the general direction of the lens 435 , eventually reaching a photodetector 434 . in fig5 it will be seen that the incident light rays are scattered in various directions . some of the scattered rays strike the internal wall of the guide 420 , for example at points 441 , 442 at less than a critical angle , so that the light is internally reflected at points 443 , 444 to reenter the interior of the guide at points 445 , 446 . only a small amount of light is transmitted through the outer surface of the transparent guide 420 . an observer viewing the guide thus sees a dim red glow when the laser is actuated . turning again to fig1 it is desirable that the diameter at the front end of the guide 20 , indicated by dimension d , exceed the vertical dimension of the reader head 15 . also the diameter preferably exceeds the width dimension of the reader head . the operator , positioned generally behind the reader 10 , can thus observe substantially the entire circumference of the front end of the guide 20 . except for the relatively narrow profile of the hand grip 14 , the housing 12 does not block the operator &# 39 ; s view of the intersection of the front end of the guide 20 with the data carrier . alternate guides 20 can be provided for reading symbols of different sizes and at different working distances . to use the device , the operator attaches a suitable guide 20 to the reader head 15 , and positions the front end of the guide 20 on a data carrier having a symbol thereon , and holds his / her eyes behind the reader head . while maintaining an essentially stationary position , the operator then views the front end of the guide 20 , and confirms that the symbol to be read is fully encompassed within the perimeter of the guide . the reader 10 is then actuated either by depressing the switch 18 , or by pressing the device against the data carrier and causing the guide 20 and the support member 30 to retract into the housing and close switch contacts 61 , 62 to initiate a reading operation . while this invention has been explained with reference to the structure disclosed herein , it is not confined to the details set forth and this application is intended to cover any modifications and changes as may come within the scope of the following claims :