LED's as Light Detectors
Tanya M. Sansosti
Stony Brook University
Laser Teaching Center
Optics Rotation Project 2, Spring 2002
Advisor: Dr. John Noé
Why This Project?
Given my strong interest in optics and atmospheric sciences, for my
second Optics Rotation project Dr. Noé and I were originally
considering doing a project which focused on how a laser beam
might behave if you shine it through various fluid media that scatter
light. However, we came across a website that involved using an LED as
a light detector rather than a light source to estimate the amount of
haze present in the atmosphere. Naturally, I was immediately intrigued
to have the opportunity to work on an atmospheric optics project.
The Haze-Span website, intended for amateur scientists and students,
is run by Dr. Forrest M. Mims III. It offers detailed instructions on
how to build a home-made sun photometer and use it for understanding
haze in the atmosphere. For my purposes, understanding what an LED is,
how it can be used to detect light rather than emit it, and which
wavelengths are best detected, were more important than anything else.
What is an LED?
"A light-emitting diode is a semiconductor device which emits light
when an electrical current flows through it. Semiconductor
diodes can also be used as light detectors. Light-emitting
diodes are good detectors of light of specific
colors. Generally, the wavelength of light detected by an LED is
shorter than the wavelength of light emitted by the same
LED. For example, certain red LEDs are relatively good detectors
of orange light." 
Using an LED to Detect Sunlight
LEDs are designed to make, not detect, light. But it happens that
they detect light quite well. Even better, unlike detectors designed
specifically to detect light, LEDs detect a relatively narrow band of
wavelengths. For example, an LED that emits greenish-yellow light at a
peak wavelength of about 555 nanometers (nm) detects green light at a
peak wavelength of about 525 nm over a spectral width of about 50
For the haze detector: light from the Sun causes the LED
detector to generate a tiny electrical current much like a solar cell
does. This current goes to the operational amplifier, which is
connected so that the LED current is transformed to a voltage and
boosted by the amount of the feedback resistance in ohms.
At the Haze-Span website, Dr. Mims uses the LED to build a sun
photometer. This is a device which is designed to measure the
intensity direct sunlight penetrating the atmosphere. It can be used
to measure haze since this blocks some direct sunlight and the
photometer is not designed to measure diffuse or scattered light in
the atmosphere. So, basically, this device designed by Forrest Mims
is an inexpensive, efficient tool, practical for the amateur
scientist, that can detect how much haze exists in the atmosphere.
Using an LED to Detect Laser Light
Rather than building a copy of the sun photometer, I
decided to start by studying the wavelength response of colored
LED's using a monochromator as a source of colored light.
The first step was to understand more clearly
how the instrument worked and to calibrate it. Here is some of what I saw:
The above images show the monochromator and it's interior
workings. There is a rotating diffraction grating inside whose motion
is controlled by the dial and the angle is determined by the reading
on the dial. According to a plate on the front of the instrument, the
grating has 675 lines/mm. There is also a mirror, which can be seen
to the right in the third picture. The mirror is facing downward to
reflect the light source inside which can be better viewed in the
following three images:
Once I had a better understanding of what I'd be working with, my next
task was to calibrate the instrument. Here is a list of the steps I
1) I used a protractor to determine the angle of the diffraction
grating for several different dial settings. For one end of the dial
(R=0.7) the angle was estimated to be 10 degrees, and for the other
end of the dial (R=1.6), the angle was about 30 degrees. The mechanism
inside is set up in such a way that the angle is a linear
function of the dial reading.
2) Next, I set up a spreadsheet that calculated the wavelength of
light passing through the instrument as a function of the dial
reading, using the grating equation: n * lambda = d * sin (theta),
where n is the order of diffraction, lambda is the wavelength, and
d is the grating interval. This gave a prediction for lambda
versus dial reading R that was almost a straight line.
3) The two calibration constants that fix the predicted curve were
adjusted slightly to make yellow light in first and second order agree
with observations. Yellow was picked because yellow is a specific
color to match, and wavelengths shorter than yellow were off the dial
in first order.
4) Next I measured the dial reading for four specific colors in
red, we compared the color with a
helium/neon laser (633 nm). For yellow, we compared it with a sodium
lamp (589.5 nm), and for green, we
green laser (532 nm) in the lab. For blue, unfortunately, we had no other
tool to use but our unaided eye, and so visually determined the
wavelength to be 474 nm, which is the average wavelength for the blue
range (455 nm - 492 nm).
5) The four data points were found to be very close to the predicted calibration
curve. A further very small adjustment in the calibration constants brought
data and curve into exact agreement as shown in the plot. The plot shows
first, second and third order curves. The red line is an exact calculation
of the grating equation while the blue lines are a linear approximation,
which can be seen to be quite accurate.
The LED and My Results
Under the recommendation of Dr. Mims, The LED that I used to make
measurements of laser and sunlight was shaved, or flattened, to a
point where there was no longer a rounded edge. He suggested doing
this with a piece of coarse sand paper.
Next, I had to create a set up for the LED and monochromator where a
movable lens could be placed in between the light source and the
LED. For me, proved to be a bit of a challenge itself. I have
displayed: a movable monochromator facing the LED edge on with an
adjustable lens in the middle. Off to the side is an amplifier that is
reading the current off of the LED.
Using a Radio Shack 276-022A green LED, I was
able to produce the following curve:
I was also able to take some reading with the LED outside, under
sunlight. For these measurements, I used different color LED's
to compare what different readings I might get. Many of the LED's were
not flattened on the end which made it difficult to get a steady
Outside, using different color LED's with unflattened edges, we got the
following readings on a voltmeter with *magnified* sunlight. The sun was
quite bright even though it was late afternoon (about 5 PM) because
the sky was very clear that day.
red: ~.1 microamps
yellow: ~ 330 microamps
green: ~ 365 microamps
And for using different color LED's with just indirect sunlight (no magnifier):
red: ~.1 microamps
yellow: ~ 4 microamps
green: ~ 6.6 microamps
I also took some measurements with the flattened, polished green LED
outside under sunlight without a magnifying glass and got readings
that ranged from 1.4 microamps to 1.9 microamps, depending on the
angle the sunlight was hitting the LED. These readings, of course,
were much more steady since the LED had a flattened tip.
I would sincerely like to thank Dr. John Noé for his extreme
patience and kindness in working with me on this project. He has been
of great support. My deepest gratitude to him.
I would also like to thank Professor Bob DeZafra for his input and
support on this project.
And finally, I would like to thank Professor Harold Metcalf for giving
me the motivation and courage to sign on for a class that I had such
little background in. Truly, thank you.
 Britney's Lasers
is an LED?
 When Hazy
Skies Are Rising
 Mims III, Forrest M. "Sun Photometer with Light-emitting diodes as
spectrally selective detectors" Applied Optics. Vol. 31,
No. 33. November 20, 1992.
 Mims III, Forrest M. "The Amateur Scientist: How to Monitor
Ultra-Violet Radiation form the sun" Scientific American. August 1990.
 Morys, Marian; Mims III, Forrest M.; Anderson, Stanley E. "Design,
Calibration and Performance of Microtops II Hand-Held Ozonometer"
Presented at the 12th International Congress on Photobiology, Vienna,
Tanya M. Sansosti