Application Notes—Photoconductive Cells
APPLICATION NOTE #2
APPLICATION NOTE #3
Light Resistance Measurement Techniques
Spectral Output of Common Light Sources
The light resistance or “on” resistance (RON) of a photoconductor cell
is defined as the resistance of the cell as measured at a special light
level using a light source with a known output spectrum. Furthermore,
the cell must be “light adapted” for a specific period of time at an
established level of illumination in order to achieve repeatable results.
Incandescent lamps can be considered as black body radiators whose
spectral output is dependent on their color temperature. The sun has
approximately the same spectral radiation distribution as that of a black
body @ 5900 K. However, as viewed from the surface of the earth, the
sun's spectrum contains H O and CO absorption bands.
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The industry standard light source used for light resistance
measurements is a tungsten filament lamp operating at a color
temperature of 2850 K. Specifying the 2850 K color temperature for the
light source fixes the spectral output (i.e. the tungsten filament light has
fixed amounts of blue, green, red, and infrared light).
For consistency and ease of comparing different cells, PerkinElmer
lists light resistance values for its photocells at two standard light
levels: 2 fc (footcandles) and at 10 lux. The footcandle is the old,
historical unit for measuring light intensity and is defined as the
illumination produced when the light from one standard candle falls
normally on a surface at a distance of one foot. The lux (the metric unit
of light measurement) is the illumination produced when the light from
one candle falls normally on a surface of one meter. The conversion
between footcandle and lux. is as follows:
Black Body Sources Output vs. Wavelength
Fluorescent lamps exhibit a broad band spectral output with narrow
peaks in certain parts of the spectrum. Shown below is a plot of the
light output of a typical daylight type fluorescent tube.
1.0 fc = 10.76 lux
1.0 lux = 0.093 fc
As explained in the section on “Selecting a Photocell”, the “light
history” effect necessitates the pre-conditioning of the cell before a
light resistance measurement is made. PerkinElmer stores all cells at
room temperature for 16 hours minimum at 30 – 50 fc (about 320 - 540
lux) prior to making the test measurement.
Sometimes the design engineer or user does not have access to the
precision measurement equipment necessary to determine the light
levels or light intensities of the application. Should this prove to be a
problem, calibrated photocell samples with individual data can be
provided by PerkinElmer.
Fluorescent Lamp Output vs. Wavelength
Due to their long operating lifetimes, small size, low power
consumption, and the fact they generate little heat, LEDs are the light
sources of choice in many applications. When biased in the forward
direction LEDs emit light that is very narrow in spectral bandwidth (light
of one color). The “color” of the light emitted depends on which
semiconductor material was used for the LED.
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