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VT43N4 Datasheet(PDF) 10 Page - PerkinElmer Optoelectronics |
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VT43N4 Datasheet(HTML) 10 Page - PerkinElmer Optoelectronics |
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10 / 76 page  5 Selecting a Photocell Specifying the best photoconductive cell for your application requires an understanding of its principles of operation. This section reviews some fundamentals of photocell technology to help you get the best blend of parameters for your application. When selecting a photocell the design engineer must ask two basic questions: 1. What kind of performance is required from the cell? 2. What kind of environment must the cell work in? Performance Criteria Sensitivity The sensitivity of a photodetector is the relationship between the light falling on the device and the resulting output signal. In the case of a photocell, one is dealing with the relationship between the incident light and the corresponding resistance of the cell. Defining the sensitivity required for a specific application can prove to be one of the more difficult aspects in specifying a photoconductor. In order to specify the sensitivity one must, to some degree, characterize the light source in terms of its intensity and its spectral content. Within this handbook you will find curves of resistance versus light intensity or illumination for many of PerkinElmer’s stock photocells. The illumination is expressed in units of fc (foot candles) and lux. The light source is an incandescent lamp. This lamp is special only in that the spectral composition of the light it generates matches that of a black body at a color temperature of 2850 K. This type of light source is an industry agreed to standard. Over the years PerkinElmer has developed different “types” of photoconductive materials through modifications made to the chemical composition of the detector. For a given type of photoconductor material, at a given level of illumination, the photoconductive film will; have a certain sheet resistivity. The resistance of the photocell at this light level is determined by the electrode geometry. RH = ρH (w / l ) where: RH = resistance of cell at light level H ρ H = sheet resistivity of photoconductive film at light level H w = width of electrode gap l = length of electrode gap Sheet sensitivity ( ρ H) for photoconductive films at 2 fc are in the range of 20 M Ω per square. The ratio w / l can be varied over a wide range in order to achieve design goals. Typical values for w / l run from 0.002 to 0.5, providing flexibility for terminal resistance and maximum cell voltage. Spectral Response Like the human eye, the relative sensitivity of a photoconductive cell is dependent on the wavelength (color) of the incident light. Each photoconductor material type has its own unique spectral response curve or plot of the relative response of the photocell versus wavelength of light. The spectral response curves for PerkinElmer’s material types are given in the handbook and should be considered in selecting a photocell for a particular application. |
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