User Manual

BH1750FVI
Technical Note
11/17
www.rohm.com
2009.04- Rev.B
© 2009 ROHM Co., Ltd. All rights reserved.
●Adjust measurement result for influence of optical window. ( sensor sensitivity adjusting )
BH1750FVI 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 の値
lx / count
at H-resolutin 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