the position illustrated in fig6 oscillator 70 drives light 46 on and off at 10 cycles per second at a 50 percent duty cycle i e the light is on half the time and off half the time the output of light 46 shines on photodiode 52 and generates a current proportional to the amount of light emitted such current travels through resistor 68 and a voltage is thus applied to the input of display 32 the number generated by the display corresponds to the intensity of light 46 the present display value is compared with numbers so generated in the past to assure that there has been no drop off of light intensity next switch 28 is closed thus causing oscillator 70 to drive light 46 continuously a second higher reading is generated on digital display 32 and this reading is also compared with readings taken in the past with switch 28 closed to assure no drop off in steady state operation of light 46 once it is determined that the output of light 46 is consistent with past uses of the apparatus the test to determine either a subject 39 s cff or icff threshold can proceed to set apparatus 10 for testing a subject 39 s icff frequency switches 26 28 are positioned as shown in fig6 knob 34 is turned to adjust potentiometer 62 so that light emitting diode 44 is flashing at a frequency of between 5 and 15 cycles per second as will be recalled digital display 32 displays.