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VTL5C1 Datasheet(PDF) 28 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 28 Page - PerkinElmer Optoelectronics |
28 / 76 page ![]() 23 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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