referring to fig1 a preferred embodiment of the present invention is illustrated as a tabletop apparatus for detecting and measuring the total indicated runout in a cutting tool , particularly a conventional dual - fluted twist drill 10 . the twist drill 10 is mounted horizontally in an electrically powered , variable - speed rotary v - block 12 . a light source 14 directs a collimated beam of light transversely across the rotating twist drill 10 toward a photosensor module 16 . the light source 14 and the photosensor module 16 are mounted on a platform 18 supported by an optical bench 20 extending parallel to the drill 10 . the position of the platform 18 along the optical bench 20 may be varied by means of a line screw 22 to accommodate twist drills 10 of different lengths or to measure runout at different points along a particular drill . ordinarily , runout is measured at or near the tip of a cutting tool such as the drill 10 illustrated in fig1 . other elements illustrated in fig1 are a circuit box 24 , including a main power on / off switch 26 for the rotary v - block 12 , an associated power indicator light 28 , power circuit fuse 30 , and motor start switch 32 . a motor control unit 34 includes coarse and fine motor speed controls 36 and 38 , respectively , and a digital motor speed readout 40 . not shown in fig1 is the digital voltmeter and associated circuitry that provides a readout of the runout in the drill 10 . referring to fig2 the light source 14 in the preferred embodiment includes a ten - watt halogen lamp 42 and associated spherical reflector 44 . light from the lamp 42 is focused by a set of condensing lenses 46 onto a frosted glass diffusion plate 48 positioned adjacent an opaque plate 50 having a pinhole aperture 52 . the frosted plate 48 and the pinhole aperture 52 act as a point source of light . light from the pinhole 52 is refracted by a pair of collimating lenses 54 to provide a collimated beam of light . the collimated beam is passed through a vertical slot 55 in an opaque exit plate 56 and directed orthogonally at the rotating twist drill 10 so as to pass the drill on both sides . referring to fig2 and 3 , a portion of the light beam passes on opposite sides of the drill 10 to impinge on two matched pn silicon photocells 58 and 60 mounted on a single insulating wafer 62 . the photocells 58 and 60 , each approximately 5 mm wide by 10 mm long , are separated by a gap approximately 0 . 25 mm wide that is oriented parallel to and centered on the drill 10 opposite the light source , such that the photocells 58 and 60 are equally shadowed by the drill 10 . the photocells 58 and 60 are matched with one another to have substantially identical output characteristics . additionally , the photocells are each selected to have a variation in output of not greater than five percent when scanned linearly over the active surface area with a light beam passed through a 0 . 007 - inch wide slot . suitable photocells are commercially available , for example , from silicon sensor , inc ., of dodgeville , wisconsin . the energy level of the collimated beam that illuminates the photocells 58 and 60 is , in the preferred embodiment , about 600 microwatts / cm 2 . with this level of illumination , the output of each photocell is about 0 . 06 microamp per thousandth of an inch of illumination along the lengthwise axis of the photocell . in fig3 the crosshatched areas of the photocells 58 and 60 illustrate schematically the portions of the photocells 58 and 60 shadowed by the drill 10 in position for runout measurement . a slotted opaque disc 64 is located at the front of the photosensor module 16 to minimize entry of ambient light and define the illuminated areas of the photocells 58 and 60 . the operation of the apparatus is illustrated functionally in fig3 . briefly , the two photocells 58 and 60 each produce an output signal that is of relatively constant voltage , on the order of 0 . 45 volt dc , and which varies in current in proportion to he area of the photocell that is illuminated . the photocell output signals are applied to a dual channel current - to - voltage converter 66 . the current - to - voltage converter 66 is preferably an operational amplifier - type converter to minimize the load drawn from the photocells and thereby obtain substantially linear photocell responses . the converted photocell signals are applied to each of a summing amplifier 68 and a differential amplifier 70 , which are the first elements in a drill speed indicator circuit and a runout indicator circuit , respectively . briefly , in the drill speed indicator circuit the summed signal from amplifier 68 is passed in sequence through a bandpass filter 72 , a variable - gain amplifier 74 , and a sine - to - square wave converter 76 . the squared signal from the square wave converter 76 is applied to a frequency counter 78 . the frequency counter 78 is adapted to provide an output corresponding to to the actual drill speed , in rpm , to the drill speed indicator 40 . in the runout indicator circuit , the parallel outputs of the current - to - voltage converter 66 are applied to the differential amplifier 70 and the output thereof is applied to a variable - gain amplifier 80 that is employed to calibrate the apparatus . the output from the variable - gain amplifier 80 is passed through a low - pass filter 82 to an ac - to - dc converter 84 . the dc output of the converter 84 is measured with a digital voltmeter 86 and provided to a digital runout indicator 88 . the embodiment thus far described and illustrated in fig3 is particularly adapted to the measurement of runout in , for example , a two - fluted twist drill . in operation , the drill is initially rotated in the rotary v - block 12 at approximately 300 rpm . the actual speed varies somewhat with the diameter of the drill 10 . variations in the output signals from the photocells 58 and 60 due to design profile variations in the drill , that is , due to the flutes in the drill , are in phase with one another and are therefore additively combined in the summing amplifier 68 . runout components of the signals , which are out of phase , are approximately cancelled in the summing amplifier 68 . the summing amplifier thus serves to remove the runout components from the flute comonents of the photocell signals . the filter 72 is preferably a state variable bandpass filter having a q value of approximately 10 . the bandpass filter 72 operates in addition to the summing amplifier 68 to further remove runout components from the photocell signals . for example , when runout is to be measured with the drill turning at 300 rpm , the bandpass filter 72 is selected to have a center frequency of approximately 10 hz , or twice the rotational frequency of the drill 10 . the flute components of the drill turning at 300 rpm have a frequency of approximately 10 hz and are therefore passed through the bandpass filter 72 , whereas lower frequency runout components are removed . the filtered flute components are applied through the variable - gain amplifier 74 and the sine - to - sqaure wave converter 76 to the frequency counter 78 . the frequency counter 78 counts the output of an internal time base oscillator during each cycle of the approximately 10 - hz flute signal . with appropriate selection of the time base oscillator , the readout of the frequency counter 78 provides a direct indication of the actual drill rpm . in practice , the operator adjusts the speed of the rotary v - block 12 with the coarse and fine motor speed controls 36 and 38 ( fig1 ) until an rpm readout of 300 is obtained at the drill speed indicator 40 . once the drill speed has been set at 300 rpm , the runout portion of the circuit is enabled to provide a direct reading of runout in the drill 10 . specifically , the differential amplifier 70 and the low - pass filter 82 each operate to pass runout components of the photocell signals and remove flute components . flute components , being in phase with one another , are largely cancelled in the differential amplifier 70 . remaining flute components are removed by the low - pass filter 82 , which is preferably based on a tenth - order , low - pass butterworth active filter circuit having a unity gain and an operating frequency of approximately 5 hz . the filter 82 is preferably adapted to provide approximately 36 db attenuation one octave above the operating frequency and approximately 60 db attenuation at three or more times the operating frequency . with the drill turning at 300 rpm , runout components have a frequency of 5 hz and are therefore passed through the low - pass filter 82 . the output of the low - pass filter 82 is a 5 - hz sine wave signal having an amplitude proportional to the runout of the drill 10 . this signal is applied to the ac - to - dc converter 84 , which preferably consists of a digital peak detector that acquires and holds the maximum peak - to - peak voltage of the 5 - hz signal received from the filter 82 . the advantage of using a peak detector is that , with a 5 - hz signal , a more rapid determination of the drill runout is obtained than can be obtained with other types of ac - to - dc converters , for example a rectifier . a suitable peak detector is available commercially from daytronics corporation of dayton , ohio , and is identified as a model 9279 max / min module . the output of the ac - to - dc converter 84 is a dc signal having a voltage proportional to the runout of the drill 10 . the digital voltmeter 86 measures this voltage and provides a direct indication of the drill runout , in thousandths of an inch , at the digital runout indicator 88 . in practice , the runout determination takes approximately two seconds after the drill 10 is brought up to speed . fig4 illustrates schematically in block diagram an alternative preferred embodiment of the invention that does not require the drill 10 to be rotated at a constant , predetermined speed . elements of the embodiment of fig4 that are substantially the same as elements of the embodiment of fig3 are like numbered . the primary advantage of the embodiment of fig4 is that it may be employed in the field to measure runout in cutting tools directly in their working environment . for example , the apparatus may be employed to measure runout in the cutting tool of a milling machine without having to remove the cutting tool from the machine . the embodiment makes use of a frequency - to - voltage converter 90 that receives as input the summed and filtered flute components of the photocell signals . as in the previous embodiment , the summing amplifier 68 and the bandpass filter 72 effectively remove runout components . the frequency - to - voltage converter 90 provides a variable voltage control signal on line 92 to a voltage - controlled , low - pass filter 94 . the cutoff frequency of the low - pass filter 94 is thereby varied in response to the rotational speed of the drill 10 . the remainder of the embodiment of fig4 is essentially the same as the embodiment of fig3 . however , it will be appreciated that the method and apparatus represented by fig4 can be more advantageously employed to measure runout in rotating tools or other objects where their removal for mounting in a rotary v - block or other device would be difficult or otherwise undesirable . although the present invention is described and illustrated with reference to preferred embodiments of the invention , it will be understood that modifications , substitutions , and alterations that may be apparent to one of ordinary skill in the art may be made without departing from the spirit of the invention . accordingly , the scope of the invention is defined by the following claims .