APPROACHING THE FINAL FRONTIER
■ BY L. PAUL VERHAGE
LED BASED PHOTOMETER
THERE’S A NEW SENSOR that I’d like to share with you this month that only
requires a few LEDs and a resistor. Therefore, you probably already have
everything in your junk box needed to build it.
As you read in this article, you’ll see that I still
have a few concerns about the performance
of this sensor. Even flying three of them into
near space didn’t answer all the questions. Oh
well, that’s just a reason to launch another near
space mission. ■ FIGURE 1. LED photometer.
Years ago, while reading magazines in the Physics library at
Kansas State University, I came across
a report by Forrest Mims about using
LEDs as a color specific photometer.
Now a photometer is an instrument
that measures the intensity of light
falling on it. That sounds just like a
photocell, or light sensitive resistor,
NOTE: All PCB patterns and files are
available on the Nuts & Volts website
doesn’t it? However, photocells are
sensitive to a wide band of the
spectrum, from at least infrared to the
visible, while photometers are
sensitive only to a narrow band of
color; say, a particular shade of red.
To make a photocell sensitive to a
specific color, it needs a filter covering
it that lets only a particular color
through (these are usually expensive
interference filters and not cheap
colored gels). Instead of an expensive
narrow band photometer or a wide
band filtered photocell, I’m using an
LED to make an inexpensive medium
wide band photometer.
LEDs by their nature only emit a
narrow band of colors. That color is
determined by the way the silicon (or
in the case of blue LEDs, the gallium
nitride) is cooked up. This means that
unlike a traditional incandescent light
bulb, the color of an LED’s plastic case
doesn’t control its color. The color of
an LED’s housing merely indicates
what color it will emit (and that’s
useful information when the LED is
sitting in your junk box).
Then there’s the white LED to
make it a bit more complicated. White
LEDs don’t emit white light (which is a
combination of all the colors, from red
to blue, in the visible spectrum).
Instead, white LEDs are blue LEDs
combined with a yellow colored
phosphor. So, white LEDs emit
blue light and excite their phosphors
to emit yellow light (which when
combined, looks white to our eyes).
■ FIGURE 2. The emission spectrum
of an LED. This chart came from the
LED Museum ( http://home.att.net/~led
museum/) and is from a Nichia blue-green LED. This LED — like all the
others — emits a narrow band of color
in the electromagnetic spectrum.