VT33CT datasheet(30/76 Pages) PERKINELMER

30 / 76 page

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.

Normalized LED/Photocell Coupling Factors @ 10 mA

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

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.

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.

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).

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

The Light Source used in the measurement of photocell resistance

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.

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.

The relationship between candlepower and footcandle is:

Since this equation assumes a point source of light, the distance

between lamp and detector should be at least five times the lamp

diameter.

There are some characteristics of incandescent lamps which should

be noted:

Color temperature increases with increasing wattage.

When operated at a constant current, light output rises with time.

LED Type

P (nm)

Type Ø

Type 3

GaP

569

GaAsP/GaP

635

17%

13%

GaAsP/GaAs

655

11%

AIGaAs

47%

35%

GaP/GaP

697

47%

31%

GaAIAs

880

—

GaAs

940

—

footcandle

candle power

distance in feet

()

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