LX1972

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 5

Ambient Light Detector

Copyright

© 2004

Rev. 1.1b, 2005-10-31

TM

®

SIMPLIFIED BLOCK DIAGRAM

Current

Amp

VDD

VSS

Figure 3 –

Simplified Block Diagram

APPLICATION NOTE

define the light source. Lux is a unit for the measurement

of illuminance, which is the photometric flux density or

visible light flux density.

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

µ

W/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, which

corresponds to yellowish-green light. At 555nm, the

If the photo sensor had a truly photopic response, it would

produce the same output current for the same number of 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 LX1972 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.

For ease of automatic testing of the LX1972 the ATE

(automatic test equipment) light source is configured with

white LED’s whose current is adjusted to output a calibrated

flux density at 550nm. This allows consistent and repeatable

testing of the sensor but corresponds to a light source unlike

that typically found in a office, home or sunlit environment.

In practice, the user needs to place the sensor in the target

environment and calibrate the sensors output current range to

match the application objective. This is easily accomplished

by adjusting the output resistor, which sets the sensor’s gain.