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LX1970 Datasheet(PDF) 4 Page - Microsemi Corporation |
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LX1970 Datasheet(HTML) 4 Page - Microsemi Corporation |
4 / 10 page Microsemi Integrated Products 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 4 Copyright © 2002 Rev. 1.4b, 2005-08-10 LX1970 Visible Light Sensor PRODUCTION DATA SHEET I N T E GRA T ED PR ODUC T S MEASUREMENT CIRCUIT CONF IGURATIONS A A VDD VS ISNK ISRC White LED Current Source LX1970 A VDD SRC IR LED 910nm LX1970 White LED Current Sources % Figure 1 – Light Current Measurement Circuit Figure 2 – IR sensitivity Measurement Circuit APPLICATION NOTES LIGHT UNITS In converting from µW/cm2 to Nits it is necessary to define the light source. Nits are units for a measurement of luminance, which is the apparent brightness of an illuminated flat surface. µW/cm2 is a measurement of irradiance or the measurement of electromagnetic radiation flux both visible and invisible. The first step in the conversion process is to convert irradiance to illuminance, which essentially involves running the irradiant flux through a photopic filter. In normal ambient a photopic curve is used and in dark ambient a scotopic curve (dark adapted eye) is used. If the light is composed of only one wavelength, a conversion chart will tell the conversion factor to convert µW/m2 to lux (lumens/m2). If more than one wavelength is used, the light spectrum of the irradiance must be applied to the photopic filter to determine the resultant illuminance. The most sensitive wavelength for the normal light adapted human eye is 555nm. At 555nm, the conversion factor is 683 Lux = 1W/m2 = 100µW/cm2. Therefore 14.6µW/cm² = 100 lux at 555nm. The next step in the conversion process is to convert illuminance to luminance. The units for illuminance are lux or lumens/m2. The units for luminance are Nits or lumens/m2/steradian. Assuming the illuminance falls on a Lambertian surface which has perfect dispersion and total reflection, the conversion from lux to nits is 3.14 lux falling on a Lambertian reflector produces 1 Nit. Therefore 100 Lux will produce 31.4 Nits. If the photo sensor had a truly photopic response, it would produce the same output current for the same number of nits or lux, regardless of the color of the light. However, because the match is not perfect, there is still wavelength dependency particularly at the ends of the visible spectrum. In the case of the LX1970 the peak photo response is at 520nm, however depending on the light source, what the human eye perceives as ‘white’ light may actually be composed of peak wavelengths of light other than 520nm. For instance a typical fluorescent lamp includes dominant light not only near 550nm but also at 404 and 435nm. Incandescent light sources such as standard tungsten lights generate substantial IR radiation out beyond 2000nm. |
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