ISL29124

FN8454 Rev 2.00

Page 13 of 16

January 13, 2015

Applications Information

Figure 13 shows a plot of the 1931 standard normalized spectral

response of various types of light sources for reference.

System Compensation and RGB to XYZ

Transform (Chroma Meter)

The accuracy of the RGB sensor is extremely sensitive to the

opto-mechanical design of the system in which it resides. The

compensation setting and calculation of RGB to XYZ transform

should be characterized within that environment with as many

standard illuminants as possible. A minimal recommended set

would include A, F2 and D65 illuminants (see Notes 11, 12, 13

on page 11 and “References” on page 15 about IEC 1931,

Planckian locus and standard illuminants). The two most

important opto-mechnical features are FOV (field of view FWHM)

and optical filters as example of tinted cell phone glass through

which the sensor will detect the ambient lighting. With the

combination of the FOV and a large sample for the filter

(30x30mm) it is possible to determine the best compensation

and XYZ transform coefficients. It is also possible to project the

accuracy of the measurement system.

RGB → XYZ TRANSFORM

Once the proper compensation setting is determined, measure

the RGB values of the various illuminants at this value. Calculate

the RGB to XYZ transform coefficients based on the measured

result against appropriate Chroma Meter Standard (using x and y

values).

X, Y, and Z are in the IEC system which specifies the color and

brightness of a particular homogeneous visual stimulus.

R, G, and B are digital output from the sensor.

Cs are coefficients. These coefficients will be changed

respectively depending on the system setup.

COMPENSATION

The compensation adjustment is used to balance the various

illuminants of interest (A, F2 and D65 recommended) such that

the value measured at the same power level (measured with a

Lux meter) is the closed value. Since the compensation

adjustment is piecewise linear, the proper setting can be

determined by extrapolating from a pair of measurements and

calculating the closest intersection of the sources of interest.

The Configuration register (Reg 0x02[7:0]) allows coarse tuning

B7 and fine tuning (B[5:0] of the residual infrared component

from the ALS output.

The recommended procedure for determining ALS IR

compensation is as follows:

• Illuminate the ISL29124 based design configuration with a no

IR F2 light source. Record the ALS measurement and the Lux

level.

• Illuminate the device with A, and D65 with heavy IR and the F2

light sources. Take an ALS measurement and Lux level

measurement.

• It really depends on the system setup in order to adjust the

Configuration register (Reg 0x02, B7 and B[5:0]) to

compensate for the IR contribution.

• Repeat previous steps until the IR light source contribution to

the ALS measurement is under 10% assuming there is no

change in Lux level due to IR light source.

Figure 14 example shows how to calculate the compensation for

varying levels of infrared components such as A, F2 and D65 (see

Notes 11, 12 and 13). With compensation adjustment from 0%

to 100%, the crossing point is the IR compensation value, which

makes tighter variation of varying level of infrared components.

This setup system is a sensor without IR tinted glass and

illuminates with 3 different light sources. Since it is not under IR

tinted glass, then reg0x2 setups like b7 = ‘0’ and B[5:0] is at

about 25% compensation adjust (%/range), which means about

40 codes.

FIGURE 13. 1931 STANDARD NORMALIZED SPECTRAL RESPONSE

OF LIGHT SOURCES

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

350 380 410 440 470 500 530 560 590 620 650 680 710 740 770 800 830

WAVELENGTH

RED

GREEN

BLUE

1931 STD RED

1931 STD GREEN

1931 STD BLUE

NORMALIZED TO GREEN

(EQ. 1)

X

Y

Z

CXR CXG CXB

CYR CYG CYB

CZR CZG CZB

x

R

G

B

=

FIGURE 14. IR COMPENSATION VALUE

0

2000

4000

6000

8000

10000

12000

0

20

406080

100

COMPENSATION ADJUSTMENT (% RANGE)

D65

A

F2