The Speed of Sight: Measuring Differences Between Conscious Visual Response Times of Human Eyesight using LEDs and Simple Electronics

Mark Valentine, Electrical Engineer

Introduction

The human eye responds more quickly to moderate light levels than to dim light. One of the simplest ways to demonstrate this effect uses LEDs. Figure 1 shows a circuit in which two matched green LEDs are activated together with the same switch. One LED produces a light that is of moderate intensity. That is, it does not prevent the eye from seeing dimmer light from an adjacent source, nor does exposure cause persistence in the eye. The other LED produces a light nearly 10 times fainter, since it is in series with a resistor nearly ten times larger that reduces the current, thus, the light intensity of the LED, by a factor of ten.

Before using the circuit in Figure 1, darken the surroundings and allow 30 minutes for your eyes to adjust to the low light level. While viewing the two LEDs with one eye covered, press the push button switch. Note that the moderately illuminated LED appears first, and then a split-second later the dim LED appears. Because both LEDs are controlled by the same switch, and because there is no “warm up” time associated with LED illumination, the two LEDs begin to emit light at the same instant. However, human eyesight requires more time to consciously register the sudden illumination of a dim light source than a moderate light source. Therefore, light from the moderate LED appears before light from the dim LED each time the button is pressed. Furthermore, this observable delay directly represents the difference between the respective conscious visual response times of the eye for each of the two LEDs.

One method of measuring this delay electronically might be to connect the push button switch of the circuit in Figure 1 to the “start” signal of an electronic stopwatch. The observer could then press the “stop” button when the dim light is observed. However, this would be unreliable, not to mention taxing on the observer, since it introduces the additional mental chore of a visual-tactile reflex to supply the “stop” signal to the stopwatch.

Retinal Response Timer Circuit

A more reliable technique for measuring the delay can be realized using relatively simple electronics. It is based on the more precise ability of the observer to discern whether or not two physically adjacent visual events—specifically, the individual illuminations of two identical LEDs driven to different luminous intensities—have occurred simultaneously. The circuit for implementing this technique, the Retinal Response Timer Circuit, is shown in Figure 2.

This circuit, like the circuit in Figure 1, drives two identical green LEDs with different currents. However, the LEDs blink automatically at an adjustable rate within a range of about once every two seconds to seven times per second, and the illumination of the moderate LED occurs after each illumination of the dim LED by a user-adjusted delay. Both LEDs then remain illuminated for a short time before switching off together, which begins another timing cycle. The circuit in Figure 2 also includes a dim red LED that is constantly illuminated and physically situated between the two blinking LEDs to serve as a visual reference for the observer. This ensures that the light from each blinking LED will only stimulate a single corresponding region on the retina. To operate the circuit, the user gazes at the red LED (making the same viewing preparations as for the circuit in Figure 1 and also viewing the LEDs from an unchanging position) and adjusts the delay until the two blinking LEDs appear to turn on at the same time. Once this condition has been established, the delay between the two LED drive signals then represents the difference between the respective conscious visual response times for each of the two blinking LEDs.

The circuit in Figure 2 uses the 556 dual timer IC to perform the timing functions. There are two independent timer units within the 556, which shall be referred to hereafter as “Timer 1” and “Timer 2." Each timer performs a different role in the visual response timing circuit. Timer 1 is configured as a low frequency oscillator that directly drives LED 3, the dimly illuminated LED. The output of Timer 1, pin 5, generates a square wave by charging and discharging capacitor C2 between two voltage levels defined inside the chip. The voltage of C2 is monitored by pin 2 and pin 6, which are tied together. When the Timer 1 output voltage is high, C2 charges through resistors R9 and R10 until its voltage reaches about 2/3 of the supply voltage. This leads the Timer 1 output to change to a low voltage, which causes C2 to discharge until its voltage reaches about 1/3 of the supply voltage. When that occurs, the output for Timer 1 switches back to a high voltage, and the cycle repeats.

Timer 2 performs the delayed drive function for LED 1, the moderately illuminated LED. It is controlled by two signals from Timer 1 that operate in unison. One signal is the output of Timer 1 (pin 5), which feeds pin 8 of Timer 2. When pin 8 is driven low, it forces the output of Timer 2 (pin 9) to a high voltage. When Pin 8 is driven high, the output of Timer 2 can be switched to a low voltage under the control of pin 12. The other control signal supplied by Timer 1 is pin 1, which is essentially a switch tied to ground. When the output of Timer 1 (pin 5) goes high, this switch is opened, blocking the current path to resistor R1 and allowing capacitor C1 to charge. The voltage on C1 is monitored by Timer 2 via pin 12. When this voltage exceeds an internally defined threshold (which is adjusted by the combined resistance of R4 and R5 between pin 11 and ground) in Timer 2, the output of Timer 2 (pin 9) becomes a low voltage and drives LED 1. When the output of Timer 1 becomes a low voltage, the switch on pin 1 closes again, quickly discharging C1 to a low voltage through resistor R1. The low output of Timer 1 also resets Timer 2 (by driving pin 8 high), immediately forcing its output to the high state, which turns LED 1 off to prepare for a new timing cycle. Of course, LED 2, the red diffuse lens LED situated between the two green LEDs, is constantly illuminated.

One thing to consider is that the power supply used in the prototype circuit was a bank of 4 series connected AA cells. The adjustable delay of the circuit should not be affected by changes in battery voltage, but the intensity of the LEDs will be affected. Therefore, if the circuit is operated for extended periods of time, it might be desirable to supply power with a regulated power supply. A 9-volt battery powered supply with a regulated 5-volt output is shown in Fig. 3. This power supply should allow the circuit to operate for long periods of time without changes in LED intensity, but values for resistors R6, R7 and R8 should be slightly reduced if it is desired to generate the same luminous intensities produced by the LEDs in the original circuit with a 6-volt supply.

The circuit in Figure 2 can be constructed on a modular breadboard available from Radio Shack (Catalog Numbers 276-174 or 276-175) with the components listed below in Table 1. This allows the circuit to be tested and debugged or for the evaluation of different LEDs from the 20-piece LED assortment pack listed in the parts list. However, a more reliable and permanent form of construction would be to build the circuit on a printed circuit board enclosed within a large project box. A large hole could be drilled in the box for viewing the LEDs, and electrical tap points for the LED drive signals could be routed to the outside of the box for convenient electrical access so the delay between them could be easily measured.

Table 1: Parts List for Retinal Response Timer Circuit

Manual or automatic methods can be used for measuring the delay, once it has been properly adjusted, between the two drive signals for the blinking LEDs. On one occasion, I correctly adjusted the delay and then simply changed the series resistors of the two blinking LEDs to make them brighter. Then, carefully listening to a ticking quartz watch, the delay was estimated to be 0.25 second. Though I am not musically inclined, I recommend this method for people who have a good sense of rhythm.

Another method involves performing electronic surgery on a digital wristwatch. Many department stores sell inexpensive models that feature a stopwatch function. A bipolar transistor can act as a switch that, when connected with pieces of magnet wire to the push button contacts in a watch, allows the stopwatch function to be controlled by an external electronic circuit. The thing to remember is that a transition from low-to-high on pin 5 of the 556 timer IC represents the “start” signal, and a high-to-low transition on pin 9 represents the “stop” signal. Another item to consider is that some means must be provided to ensure that only one “start” and one “stop” signal are generated per measurement. With some ingenuity and craftsmanship, it should be possible to make an attractive instrument that can automatically perform highly accurate delay measurements at the press of a button.

Digging Deeper: Resources for Further Experimentation

As with everything else, a great deal of information regarding human eyesight, color theory, and light are available online. Below are a few links relating to those topics. Also included are sources for LED history and electronic components vendors.

• http://www.uctc.net/papers/207.pdf (Describes mathematical models for vision.)
• http://home.att.net/~RTRUSCIO/COLORSYS.htm (Explains color mixing.)
• http://www.ozoneservices.com/glossary/u/uv-a.htm (Explains UV-A and UV-B.)
• http://ledmuseum.home.att.net/ (LED Museum.)
• http://www.lsdiodes.com/ (LED vendor.)
• http://www.radioshack.com/ (Website for Radio Shack.)

Going Further: Other LED-Based Optical Experiments

I hope that experimentation with the circuits presented here will lead to a new appreciation for the human sense of sight and for the remarkable capabilities of commercially available LEDs. To encourage further exploration in these areas, a few experiments relating to the information found in the previously mentioned web sites are included below. When developing these or other new experiments or devices, please be safe and have fun!

•  Reverse this experiment: Modify the circuit in Figure 2 to make the LEDs turn on at the same time, but turn off at different times with an adjustable delay, noting that the dim LED might need to turn off after the moderate LED to create the illusion that both LEDs go dark at the same time. Measure the delay between the two drive signals, if any, and compare it to the delay in the original experiment.

•  Compare the “white” light from a white LED with the light from an incandescent lamp using a prism to separate the colors from each source.

•  Make a “Gatling” strobe light from seven LEDs in a ring, all pointed up and angled slightly inward toward a common central point above the ring. Each LED should be a different color from the set of seven primary colors, and all the LEDs should be driven individually in sequence at an adjustable rate. Shine the mixed light on a white sheet of paper. At what rate must the sequence occur for the mixed colors to appear white?

•  Use a 380 nm UV LED and some fluorescent paper or a CdS photoresistor to determine the effectiveness of sunglasses and eyeglasses at blocking UV-A light.