TSL2568, TSL2569

LIGHT-TO-DIGITAL CONVERTER

TAOS091D − DECEMBER 2008

21

The LUMENOLOGY

r Company

r

r

Copyright

E 2008, TAOS Inc.

www.taosinc.com

APPLICATION INFORMATION: SOFTWARE

Calculating Lux

The TSL256x is intended for use in ambient light detection applications such as display backlight control, where

adjustments are made to display brightness or contrast based on the brightness of the ambient light, as

perceived by the human eye. Conventional silicon detectors respond strongly to infrared light, which the human

eye does not see. This can lead to significant error when the infrared content of the ambient light is high, such

as with incandescent lighting, due to the difference between the silicon detector response and the brightness

perceived by the human eye.

This problem is overcome in the TSL256x through the use of two photodiodes. One of the photodiodes

(channel 0) is sensitive to both visible and infrared light, while the second photodiode (channel 1) is sensitive

primarily to infrared light. An integrating ADC converts the photodiode currents to digital outputs. Channel 1

digital output is used to compensate for the effect of the infrared component of light on the channel 0 digital

output. The ADC digital outputs from the two channels are used in a formula to obtain a value that approximates

the human eye response in the commonly used Illuminance unit of Lux:

Chipscale Package

For 0 < CH1/CH0

 0.35

Lux = 0.00713

 CH0 − 0.00975  CH1

For 0.35 < CH1/CH0

 0.45

Lux = 0.00813

 CH0 − 0.01250  CH1

For 0.45 < CH1/CH0

 0.52

Lux = 0.00935

 CH0 − 0.01521  CH1

For 0.52 < CH1/CH0

 0.67

Lux = 0.00394

 CH0 − 0.00482  CH1

For 0.67 < CH1/CH0

 0.85

Lux = 0.00337

 CH0 − 0.00396  CH1

For CH1/CH0 > 0.85

Lux = 0

TMB Package

For 0 < CH1/CH0

 0.35

Lux = 0.00763

 CH0 − 0.01031  CH1

For 0.35 < CH1/CH0

 0.50

Lux = 0.00817

 CH0 − 0.01188  CH1

For 0.50 < CH1/CH0

 0.60

Lux = 0.00723

 CH0 − 0.01000  CH1

For 0.60 < CH1/CH0

 0.72

Lux = 0.00573

 CH0 − 0.00750  CH1

For 0.72 < CH1/CH0

 0.85

Lux = 0.00216

 CH0 − 0.00254  CH1

For CH1/CH0 > 0.85

Lux = 0

The formulas shown above were obtained by optical testing with fluorescent and incandescent light sources,

and apply only to open-air applications. Optical apertures (e.g. light pipes) will affect the incident light on the

device.

Simplified Lux Calculation

Below is the argument and return value including source code (shown on following page) for calculating lux.

The source code is intended for embedded and/or microcontroller applications. Two individual code sets are

provided, one for the chipscale package and one for the TMB package. All floating point arithmetic operations

have been eliminated since embedded controllers and microcontrollers generally do not support these types

of operations. Since floating point has been removed, scaling must be performed prior to calculating illuminance

if the integration time is not 402 ms and/or if the gain is not 16

 as denoted in the source code on the following

pages. This sequence scales first to mitigate rounding errors induced by decimal math.

extern unsigned int CalculateLux(unsigned int iGain, unsigned int tInt, unsigned int

ch0, unsigned int ch1, int iType)