Application Notes—Photoconductive Cells
Factoring in the power outputs of the LEDs, in this case at a forward
drive current of 10 ma, coupling factors (matching factor multiplied by
power output) for the various LED/material type combinations can be
generated.
Storage in the dark will change both the sensitivity and decay time of
the cell.
APPLICATION NOTE #6
A Low Cost Light Source for Measuring
Photocells
Normalized LED/Photocell Coupling Factors @ 10 mA
LED Type
λ (nm)
Type Ø
Type 3
P
GaP
GaAsP/GaP
GaAsP/GaP
GaAsP/GaAs
AIGaAs
569
58
3%
5%
3%
5%
The Light Source used in the measurement of photocell resistance
635
655
66
17%
11%
47%
47%
—
13%
9%
must be characterized for intensity and spectral composition.
PerkinElmer uses a tungsten filament lamp having a spectral output
approximating a black body @ 2850 K with a known candlepower
output at a specified voltage and current.
35%
31%
—
GaP/GaP
GaAIAs
697
880
940
While calibrated lamps of this type are available from the National
Institute of Standards and Technology (formerly NBS) and private
testing labs, a low cost alternative is to use a 100 W, inside frosted,
tungsten filament lamp available from any home or hardware store.
Such a lamp operated at 120 VAC will produce approximately 90
candlepower (cp) of illumination and a color temperature of 2700 K to
2800 K.
GaAs
—
—
Once gain, this data is intended as a general guide. LED power
outputs can vary 5:1 between manufacturer lots.
APPLICATION NOTE #5
Assembly Precautions
The relationship between candlepower and footcandle is:
When soldering the cell leads take all measures possible to limit the
amount of heating to the photocell. The maximum recommended
soldering temperature is 250°C with a solder duration of 5 seconds.
Heat sink the LEDs if possible. Keep soldering iron 1/16 inch (1.6 mm)
minimum from base of package when soldering.
candle power
(distance in feet)2
footcandle = ---------------------------------------
Since this equation assumes a point source of light, the distance
between lamp and detector should be at least five times the lamp
diameter.
Avoid chemicals which can cause metal corrosion. Do not clean the
plastic coated cells with organic solvents (ketone types). Check with
factory for specific cleaning recommendations.
There are some characteristics of incandescent lamps which should
be noted:
Finally refrain from storing the cells under high temperature and/or
humidity conditions. If cells are stored in the dark for any length of time
please “light adept” before testing (see section on Light History Effect).
1. Color temperature increases with increasing wattage.
2. When operated at a constant current, light output rises with time.
25
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
