BH1751FVI

Technical Note

10/16

www.rohm.com

2010.02 - Rev.A

© 2010 ROHM Co., Ltd. All rights reserved.

●Adjust measurement result for influence of optical window. (sensor sensitivity adjusting )

BH1751FVI is possible to change sensor sensitivity. And it is possible to cancel the optical window influence (difference with

/ without optical window) by using this function. Adjust is done by changing measurement time. For example, when

transmission rate of optical window is 50% (measurement result becomes 0.5 times if optical window is set ), influence of

optical window is ignored by changing sensor sensitivity from default to 2 times

Sensor sensitivity is shift by changing the value of MTreg (measurement time regisiter). MTreg value has to set 2 times if

target of sensor sensitivity is 2 times. Measurement time is also set 2 times when MTreg value is changed from default to 2

times.

ex) Procedure for changing target sensor sensitivity to 2 times.

Please change Mtreg from”0100_0101” (default) to”1000_1010” (default * 2).

1) Changing High bit of MTreg

ST

Slave Address

R/W

0

Ack

01000_100

Ack

SP

2) Changing Low bit of MTreg

ST

Slave Address

R/W

0

Ack

011_01010

Ack

SP

3) Input Measurement Command

ST

Slave Address

R/W

0

Ack

0001_0000

Ack

SP

* This example is High Resolution mode, but it accepts the other measurement.

4) After about 240ms, measurement result is registered to Data Register.

( High Resolution mode is typically 120ms, but measurement time is set twice. )

The below table is seeing the changable range of MTreg.

Min.

Typ.

Max.

changable range of

MTreg

binary

0001_1111

( sensitivity : default * 0.45 )

0100_0101

default

1111_1110

( sensitivity : default * 3.68 )

decimal

31

( sensitivity : default * 0.45 )

69

default

254

( sensitivity : default * 3.68 )

It is possilbe to detect 0.23lx by using this function at H-resolution mode. And it is possilbe to detect 0.11lx by using this

function at H-resolution mode2.

The below formula is to calculate illuminance per 1 count.

H-reslution mode: Illuminance per 1 count ( lx / count ) = 1 / 1.2 *( 69 / X )

H-reslution mode2: Illuminance per 1 count ( lx / count ) = 1 / 1.2 *( 69 / X ) / 2

1.2 : Measurement accuracy

69 : Default value of MTreg ( dec )

X : MTreg value

The below table is seeing the detail of resolution.

MTreg value

lx / count

at H-resolution mode

lx / count

at H-resolution mode2

0001_1111

1.85

0.93

0100_0101

0.83

0.42

1111_1110

0.23

0.11