DEW1.2. LEDs As Light Sensors

Materials

• Digital multimeter with alligator clips
• 2 LEDs: Infrared (IR) and Red
• Lamp
• Red laser
• Protractor

Part I: Parts of LEDs

Make sketches of each LED from the top and two views from the side (perpendicular to each other). What are the differences and similarities between the two LEDs?

IR LED
Top	Side 1	Side 2
Red LED
Top	Side 1	Side 2

The wavelength of light generated by the LED is controlled by the width of the slot in the metal plate (visible from the side views). Light is emitted from the cup at the top of the LED.

Part II: Testing the LEDs

How can we see if the IR LED is working if our eyes do not detect IR light? Fear not: a smart phone camera (or digital camera) comes to the rescue. The sensors in digital cameras can detect IR. Use your smartphone or digital camera with an LCD screen to detect IR. Turn the camera on, point it toward the top of the IR LED after the battery is connected. What do you see?

If you have a TV remote control handy, try pointing it toward your camera and press a button on the remote control. TV remotes work by sending out pulses of IR.

Part III: Measuring Light with LEDs

Part IV: Angle of Sensitivity

Using either a light bulb or a red laser, point the top of the LED toward the light source. Slowly rotate the LED to see at what point the voltage on the multimeter drops significantly. Measure this angle for the red LED. If using a red laser, does the IR LED generate any electrical current? Use an incandescent light to test the IR LED.

LED	Angle of Sensitivity
Red
IR

If you have straws that fit over the LEDs, create a sleeve to let only light hit the top of the LED. Test with the LEDs you just used. Does this affect the angle of sensitivity?

Part V: Measure Light Sources In the Classroom

Locate several different types of light sources in the room, for example, regular light bulbs (incandescent), fluorescent light bulbs, halogen bulbs, the source of light for an optical scanner, LEDs on various electronic equipment. Using either tape measures or the tile on the floor (typically 1 foot squares), systematically record the current generated by several of these light sources from various distances away.

Light Source	Distance	Voltage from Red LED	Voltage from IR LED

Although these instruments cannot measure the magnitude of red and IR light being emitted from these light sources, are you surprised at how much IR is given off by light bulbs? IR is heat energy and does not contribute the visible light of a light source. In that sense, IR is wasted energy for a visible light source.

Part VI: Measure Warm and Cool Things

How much IR and red does your hand emit? How about a hot pad? Ice? 

Using a box, place a warm or cold source in the box and place the LED into the box with the LED still connected to the multimeter outside the box. Make sure the top of the LED is pointing toward the object being measured. You may need to use tape, string, or wire to hold the LED in place. You could also use a dark closet and have a student hold the LED and another record the observation while standing outside the closet.

Warm and Cold Things

Object	Voltage from Red LED	Voltage from IR LED

Although our bodies do emit IR, the wavelengths are in the far or thermal IR region. The IR being detected with these LEDs is in the near IR region, which is generated from very hot light sources. All objects emit light, but cooler objects emit light of longer wavelengths. Hotter bodies emit light of shorter wavelengths. For example, blue stars are hotter than red stars (blue is a shorter wavelength than red). 

Part VII: Measuring Reflection, Transmission, Absorption

Although these instruments cannot measure actual light magnitudes, they can be used to measure the percentage of reflectance, transmittance, and absorption of light from an object. 

1. Determine that the object does not emit light (test in dark room).
2. Measure the voltage generated by incoming light (use sunlight).
3. Measure the voltage being reflected off the object. The percent of light reflected is the voltage from reflected light / voltage from incoming light.
4. If you can get behind the object, say it is a leaf or a piece of paper or fabric, measure the voltage of light traveling through the object. The percent of light being transmitted through the object is voltage from transmitted light / voltage from incoming light.
5. Finally, the light being absorbed from the object is:

Light absorbed = 100% – %reflected – %transmitted.

Part VIII: Measuring Reflection of
Land Covers

Go outside with your instruments, preferably on a sunny to partly cloudy day.

First, measure the intensity of the sunlight by pointing the LED toward the Sun. 

Do not look directly at the Sun while doing this. 

Estimate your pointing by either:

• (a) watching the voltage and moving the LED to find the maximum value, or 
• (b) looking at the shadow of the LED on the ground—the smallest shadow of LED will mean it’s pointing directly at Sun.

Second, point your LED at the ground and record the voltage and the surface type. If you have a partly cloudy sky, continue to measure the intensity of sun light before each measurement. 

Measure a variety of surfaces. Record your observations and measurements.

Reflection of Land Cover 

These images below show coal mining in eastern Ohio. In these images vegetation is red, water is black to dark blue, and the mines are bright gray. Federal Law requires the restoration of mined lands to their approximate original contours and that reclaimed land support either the same or better land uses than it supported before mining. To meet this requirement the Muskingum mines, as well as other mines, are replanted to grassland, for agricultural use. The mining company replaces the topsoil, grades the soil, and applies grass seed and mulch.