TSL2581, TSL2583

LIGHT-TO-DIGITAL CONVERTER

TAOS134A − JULY 2012

20

r

r

Copyright

E 2012, TAOS Inc.

The LUMENOLOGY

r Company

www.taosinc.com

APPLICATION INFORMATION: SOFTWARE

Calculating Lux

The TSL258x 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 TSL258x 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 CH1/CH0 = 0.00 to 0.25

Lux = 0.105

 CH0 − 0.208  CH1

For CH1/CH0 = 0.25 to 0.38

Lux = 0.1088

 CH0 − 0.2231  CH1

For CH1/CH0 = 0.38 to 0.45

Lux = 0.0729

 CH0 − 0.1286  CH1

For CH1/CH0 = 0.45 to 0.60

Lux = 0.060

 CH0 − 0.10  CH1

For CH1/CH0 > 0.60

Lux/CH0 = 0

FN Package

For CH1/CH0 = 0.00 to 0.30

Lux = 0.130

 CH0 − 0.240  CH1

For CH1/CH0 = 0.30 to 0.38

Lux = 0.1649

 CH0 − 0.3562  CH1

For CH1/CH0 = 0.38 to 0.45

Lux = 0.0974

 CH0 − 0.1786  CH1

For CH1/CH0 = 0.45 to 0.54

Lux = 0.062

 CH0 − 0.100  CH1

For CH1/CH0 > 0.54

Lux/CH0 = 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 with

the TSL2583FN. The source code is intended for embedded and/or microcontroller applications. All floating

point arithmetic operations have been eliminated since embedded controllers and microcontrollers generally

do not support these types of operations. Because floating point has been removed, scaling must be performed

prior to calculating illuminance if the integration time is not 400 msec and/or if the gain is not 1

× as denoted in

the source code on the following pages.