VTL5C2SLASH2 datasheet(28/76 Pages) PERKINELMER

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Application Notes—Photoconductive Cells

APPLICATION NOTE #2

Light Resistance Measurement Techniques

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.

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:

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.

APPLICATION NOTE #3

Spectral Output of Common Light Sources

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 H2O and CO2 absorption bands.

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.

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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