Application Note

VISHAY SEMICONDUCTORS
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Optical Sensors
Application Note
Designing the VEML6075 into an Application
By Reinhard Schaar
UVA / UVB LIGHT SENSOR WITH I2C INTERFACE
The VEML6075 is an advanced ultraviolet (UVA / UVB) light sensor with an I2C protocol interface and designed in the
CMOS process.
An accurate VEML6075 UVI sensing system requires visible and infrared noise compensation and a teflon diffusor for cosine
angular response correction. The UVI formulas and related UVI formula coefficients are discussed here in detail. The coefficient
extraction method and a calculated example are also presented in this application note.
VEML6075
GND 1
4 VDD
VDD
Timing
controller
State machine
I2C interface
UV-PD
SDA 2
3 SCL
Oscillator
Fig. 1 - Block Diagram of the VEML6075
The VEML6075 is easily operated via a simple I2C command. It incorporates a photodiode, amplifiers, and analog / digital
circuits into a single chip. The VEML6075’s adoption of FiltronTM UV technology provides the best spectral sensitivity to cover
UVB and UVA / UVB spectrum sensing. It has excellent temperature compensation and a robust refresh rate setting without the
need for an external RC low-pass filter.
The device can be used as a solar UV indicator for cosmetic / outdoor sport handheld products or any kind of consumer
products.
The VEML6075 comes within a very small surface-mount package with dimensions of just 2.0 mm x 1.25 mm x 1.0 mm
(L x W x H).
The VEML6075 operates within a supply voltage range from 1.7 V to 3.6 V. The necessary pull-up resistors at the I2C line can
be connected to the same supply as the micro controller is connected to, between 1.7 V and 3.6 V.
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The VEML6075 shows linear sensitivity to solar UVB as well as UVA light and its sensitivity can easily be adjusted with selecting
the proper integration times.
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1.7 V to 3.6 V
R1 R2
GND (1)
1.7 V to 3.6 V
C1
100 nF
Host
Micro Controller
VDD (4)
VEML6075
SDA (2)
I2C bus data SDA
SCL (3)
I2C bus clock SCL
Fig. 2 - Application Circuit
The value for the pull-up resistors should be 2.2 kΩ.
The supply current of this device in activated measuring mode is 500 μA typical, whereas in shut-down mode (SD = 1) it is
typically just 800 nA. The operating temperature range is specified for -40 °C to +85 °C.
BASIC CHARACTERISTICS (Tamb = 25 °C, unless otherwise specified)
PARAMETER
SYMBOL
MIN.
TYP.
MAX.
Supply operation voltage
VDD
1.7
-
3.6
V
Supply current
IDD
-
480
-
μA
VIH
1.5
-
-
VIL
-
-
0.8
VIH
1.4
-
-
VIL
-
-
0.6
Tamb
-40
-
+85
°C
IDD (SD)
-
800
-
nA
I2C signal input
I2C signal input
TEST CONDITION
Logic high
Logic low
Logic high
Logic low
VDD = 3.3 V
VDD = 2.6 V
Operating temperature
Shutdown current
Light condition = dark;
VDD = 1.8 V, Tamb = 25 °C
UNIT
V
V
UVA sensitivity
IT = 50 ms (1)
-
0.93
-
counts/μW/cm2
UVB sensitivity
IT = 50 ms (2)
-
2.1
-
counts/μW/cm2
Notes
(1) Nichia NCSU033X (365 nm)
(2) UVTOP310TO39HS (315 nm)
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The VEML6075 shows a peak sensitivity at 365 nm for the UVA channel and 330 nm for the UVB channel.
The bandwidth (λ0.5) of this UVB peak is in a range of about 320 nm to 340 nm.
The bandwidth (λ0.5) of the UVA channel is within a range of about 350 nm to 375 nm. Its irradiance responsivity is about half
when compared with the UVB channel.
Average Gain 1
10000
UVB
90
80
70
1000
60
UVA
50
40
100
30
20
UVcomp2
UVcomp1
Transient Thermal Impedance
Transient Thermal Impedance
Relative Responsivity (μW/cm2)
Relative Responsivity (μW/cm2)
100
10
0
10
300
320
340
360
380
400
420
440
460
480
500
520
540
560
580
600
Wavelength (nm)
2nd line
Fig. 3 - Relative Responsivity vs. Wavelength Including Response at Higher Wavelength
What does this wavelength mean? To understand this, the diagram below shows that 310 nm is within the so-called UVB
region, and 310 nm to 360 nm covers almost half from each region: UVA and UVB.
Ultraviolet
UVC
100
UVB
280
Visible
Infrared
UVA
320
400
700
Wavelength (nm)
Fig. 4 - Light Spectrum
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Visible light has wavelengths between 400 nm and 750 nm.
UV light has shorter wavelengths, from 200 nm to 400 nm.
UV type A has light with wavelengths between 320 nm and 400 nm.
UV type B has wavelengths between 280 and 320 nm.
UV type C is between 200 nm and 280 nm.
While UVA and UVB reach earth, UVC is blocked by our atmosphere, so it does no harm.
Cosmic- Gamma- XRays
Rays Rays
UVC
UVB
UVA
Infrared
Rays
Visible Light
Microwave
Rays
Ozone Layer
Clouds
Pollution
0.1 %
4.9 %
56 %
39 %
Fig. 5 - Radiation that Reaches Earth Surface
The UVB rays - wavelengths ranging from 280 nm to 320 nm - are extremely energetic and harmful for the skin to the extent
that they are responsible for 65 % of skin tumors. Thankfully, only 0.1 % of the solar energy that arrives on the earth’s surface
is in the shape of UVB radiation.
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The UVA rays - wavelengths ranging from 320 nm to 400 nm - are less powerful than the previous ones, but highly penetrating.
They are capable of reaching the skin, becoming responsible for photoaging and promoting the onset of different forms of skin
cancer. 4.9 % of the solar energy is made up of UVA rays.
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In order to estimate the energy behind this UV radiation and the risk level seen with it, a so-called UV index has been established.
It is a quite complex calculation, weighted according a curve and integrated over the whole spectrum. So, it cannot simply be
related to the irradiance (measured in W/m2). Also see fig. 12.
The calculated index value appears on a scale of 0 to 11+. This index scale is linear and the relation to strength of the irradiance
is shown below.
Ee (W/m2)
UV Index
Strength of Irradiance
0.3
12
Extreme
11
10
Very High
9
8
0.2
7
High
6
5
Moderate
0.1
4
3
2
1
Low
0
0.0
Fig. 6 - Strength of Irradiance and the UV Index
In order to define the energy behind this UV radiation and the risk level seen, the VEML6075 enables the simple reading out
of the irradiance values and calculation of the exact measured UVB and UVA / UVB values. The visible and infrared noise is
also measured and two compensation channels help to remove the solar visible and IR noise outside the UV region. Further on,
a so-called “dummy” channel provides the dark current for accurate compensation.
Setting up and programming the VEML6075 is easily handled by just one I2C-bus command register: command code “0”. All
required functions that need to be set of are located there: power on (SD), integration time (IT), and measurement mode, either
continuous or on-demand (UV_AF and UV_TRIG). The bit “HD” allows for increasing the dynamic. The five following 16-bit-wide
command codes are the read register for the UV, as well as the so-called compensation data, and the last one shows
information about the device ID.
REGISTER UV_CONF DESCRIPTION
REGISTER NAME
COMMAND
COMMAND CODE: 0x00_L (0x00 DATA BYTE LOW) OR 0x00_H (0x00 DATA BYTE HIGH)
BIT
7
REGISTER: UV_CONF
BIT
Reserved
UV_IT
5
7
6:4
HD
4
3
2
1
0
COMMAND CODE: 0x00_L (0x00 DATA BYTE LOW)
Description
0
(0 : 0 : 0) = 50 ms, (0 : 0 : 1) = 100 ms, (0 : 1 : 0) = 200 ms, (0 : 1 : 1) = 400 ms, (1 : 0 : 0) = 800 ms,
(1 : 0 : 1) = reserved, (1 : 1 : 0) = reserved, (1 : 1 : 1) = reserved.
3
0 = normal dynamic setting, 1 = high dynamic setting
UV_TRIG
1
0 = no active force mode trigger, 1 = trigger one measurement
With UV_AF = 1 the VEML60754 conducts one measurement every time the host writes UV_Trig = 1. This bit
returns to “0” automatically.
UV_AF
0
0 = active force mode disable (normal mode), 1 = active force mode enable
SD
0
0 = power on, 1 = shut down
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COMMAND
6
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UVA data is available within command code 7 and UVA / UVB within command code 9.
07h
08h
09h
0Ah
0Bh
L
UVA_Data
R
0x00
UVA LSB output data
H
UVA_Data
R
0x00
UVA MSB output data
UVD
L
Dummy
R
0x00
H
Dummy
R
0x00
UVD
L
UVB_Data
R
0x00
UVB LSB output data
H
UVB_Data
R
0x00
UVB MSB output data
L
UVCOMP1_Data
R
0x00
UVcomp1 LSB output data
UVcomp1 MSB output data
H
UVCOMP1_Data
R
0x00
L
UVCOMP2_Data
R
0x00
UVcomp2 LSB output data
H
UVCOMP2_Data
R
0x00
UVcomp2 MSB output data
In addition, command codes 10 (0x0A) and 11 (0x0B) contain so-called compensation values. These deliver information
about the whole received light within the visible wavelength area (0x0A) and the strength of the infrared content within the
received light (0x0B).
Command code 8 provides information about the dark current value.
The last command code 12 (0x0C) contains information about the device ID:
ID
0x0C_L (0x0C data byte low)
07:00
Default = 0x26, device ID LSB byte
0x0C_H (0x0C data byte high)
07:06
05:04
03:00
Company code = 00,
(0 : 0) Slave address = 0x20
Version code (0 : 0 : 0 : 0) = VEML6075 CS
Device ID MSB byte
Silicon photodiode detectors are known to have good optical response for visible and infrared light. Therefore, the injection of
visible and infrared lights into silicon photodiodes generates undesirable noise current. In order to correct such error sources,
the VEML6075 incorporates UVcomp1 and UVcomp2 noise compensation channels. In addition, there is a UVD dummy channel
for dark current cancellation.
• The UVcomp1 channel allows only visible noise to pass through
• The UVcomp2 channel allows only infrared noise to pass through
• The UVD channel is a dummy channel that allows the other UV channels to cancel out the dark current or any stray light
injection to the silicon substrate.
After reading the raw data through the I2C bus for all five channels (UVB, UVA, UVcomp1, UVcomp2, and UVD), simple UVAcomp
and UVBcomp formulas are used to calculate the UVI signal.
Eq. (1)
UVBcomp = ( UVB - UVD ) - c x ( UV comp1 - UVD ) - d x ( UV comp2 - UVD )
Eq. (2)
( UVB comp x UVB resposivity ) + ( UVA comp x UVA responsivity )
UVI = -------------------------------------------------------------------------------------------------------------------------------------------------------------------2
Eq. (3)
Note
• Based on the actual UVI measurement data under various sunlight conditions, the average UVI from UVB and UVA signals provides a better
UVI tracking with the reference Davis 6490 UVI sensor.
In both Eq. (1) and Eq. (2) formulas, there are four coefficients we need to solve for. For open-air systems with and without
a teflon diffusor over the VEML6075 sensor, the following default VIS and IR coefficients are used:
a = uva_a_coef= 3.33, which is the default value for the UVA VIS coefficient
b = uva_b_coef= 2.5, which is the default value for the UVA IR coefficient
c = uvb_c_coef= 3.66, which is the default value for the UVB VIS coefficient
d = uvb_d_coef= 2.75, which is the default value for the UVB IR coefficient
In the mass production process, each VEML6075 die will be trimmed under reference light sources to ensure a tight sensitivity distribution for UVI calculation.
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UVA comp = ( UVA - UVD ) - a x ( UV comp1 - UVD ) - b x ( UVcomp2 - UVD )
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To extract the visible and IR coefficients (a, b, c, and d), we need two light sources (WLED and incandescent) and set Eq. (1)
and Eq. (2) to zero.
UVA calc = ( UVA - UVD ) - a x ( UV comp1 - UVD ) - b x ( UV comp2 - UVD ) = 0
Eq. (4)
UVBcalc = ( UVB - UVD ) - c x ( UV comp1 - UVD ) - d x ( UVcomp2 - UVD ) = 0
Eq. (5)
Eq. (4) is solved for coefficients a and b using two light sources (WLED and incandescent).
Eq. (5) is solved for coefficients c and d using two light sources (WLED and incandescent).
With all calculations above eliminating the influence within the UVA and UVB response from visible and infrared content, as well
as dark current, the “clean” response of the UVA and UVB channel will look as shown in fig. 7 below.
Average Gain 1
10000
UVB
90
80
70
1000
60
UVA
50
40
100
30
20
UVcomp2
UVcomp1
Transient Thermal Impedance
Transient Thermal Impedance
Relative Responsivity (μW/cm2)
Relative Output (%)
100
10
0
10
300
320
340
360
380
400
420
440
460
480
500
520
540
560
580
600
Wavelength (nm)
2nd line
Fig. 7 - Relative Responsivity vs. Wavelength
(Adjusted with Compensation Channels)
This may help to understand the sensor’s behavior when testing just with lab conditions and defined, calibrated light sources.
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Fig. 8 - VEML6075 Test Set-Up Using a White LED as Light Source
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Fig. 9 - VEML6075 Demo Software View When Light Source Used a White LED
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Fig. 10 - VEML6075 Test Set-Up Using an Incandescent Lamp as Light Source
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Fig. 11 - VEML6075 Demo Software View When Light Source Used an Incandescent Lamp
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To calculate visible light coefficients a and c, use a WLED since UVcomp2 = 0, because a white LED delivers no infrared signal,
so d x (UVcomp2) = 0.
UVBcalc = UVB - ( c x UV comp1 ) - ( d x UV comp2 ) = 0
Eq. (5)
UVB = c x UV comp1
UVB
c = -----------------------UV comp1
UVB data is measured to: 386, UVcomp1 data is measured to: 157.
UVB visible coefficient c = 386/157 = 2.46.
UVA calc = UVA - ( a x UV comp1 ) - ( b x UV comp2 ) = 0
Eq. (4)
UVA = a x UV comp1
UVA
a = -----------------------UV comp1
UVA data is measured to: 301, UVcomp1 data is measured to: 157.
UVA visible coefficient a = 301/157 = 1.92.
To calculate IR coefficients b and d, use a 2700 K, 60 W incandescent light to record the raw data.
UVB - ( c x UV comp1 )
Using c = 2.46 and d = ----------------------------------------------------------UV comp2
Eq. (5)
UVB infrared coefficient, d = (10 791 - (2.46 x 3186))/4673 = 0.63
UVA - ( a x UV comp1 )
Using a = 1.92 and b = ---------------------------------------------------------UV comp2
Eq. (4)
UVA infrared coefficient, b = (8478 - (1.92 x 3186))/4673 = 0.55
Notes
• The typical empirical coefficients b and d for achieving best IR cancellation under a sunlight condition are b = 2.5 and d = 2.75. The open-air
VEML6075 UVI sensitivity is calibrated with the Davis weather station.
• Typical UVB responsivity = 0.00125 UVI/UVBcalc counts.
• Typical UVA responsivity = 0.0011 UVI/UVAcalc counts.
• The VEML6075 UVI sensing resolution is 0.01 UVI.
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CALCULATING THE UV INDEX
Calculation of the UVI is quite complex. Not only does every wavelength between 286.5 nm and 400 nm need to be measured
- in steps of 0.5 nm - but a weighting function also needs to be applied.
400 nm
1
The UV index is defined as: UVI = -------------------mW
25 ---------2
m

I ( λ ) x w ( λ ) x d( λ ) ,
286.5 nm
where the weighting function for erythema is given as:

1
250 < λ ≤ 298

 10 0.094 x ( 298 - λ ) 298 < λ ≤ 328
w( λ) = 
 10 0.015 x ( 139 - λ ) 328 < λ ≤ 400


0
400 < λ
after A. F. McKinlay and B. L. Diffey (1987)
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Fig. 12 - Spectrum of Solar Radiation: Outside the Atmosphere (1) and at Sea Level (2)
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WHAT VALUES MAY BE SEEN WITH THE VEML6075
The defined UV light source also shown within the datasheet (Nichia NCSU033X-365 nm) to check UVA sensitivity and
the 315 nm UVTOP310TO39HS to verify UVB sensitivity will not be available within most labs. In addition, the set-up with the
before-mentioned white LED and incandescent lamp needs accuracy and an alignment to deliver comparable results.
Even “normal” daylight to study and compare the VEML6075’s response may lead to misinterpretation, as the UV power is
strongly dependent on the time of day, season, and location where the measurement will be made.
During winter times, the total skin-affecting irradiance may be so low that even within full sunshine no remarkable values will be
seen.
Axis Title
10000
Summer
160
140
120
1000
1st line
2nd line
2nd line
Skin-Affective Irradiance (mW/m2)
180
100
80
Fall
60
100
40
Spring
20
Winter
0
10
2
4
6
8
10
12
14
16
18
20
22
Time (MEZ)
2nd line
Fig. 13 - Skin-Affecting Irradiance Level vs. Time Seen at the Beginning of the Four Seasons
The correct UV index needs a well-calibrated measurement tool, such as the Davis weather station used to calibrate the
VEML6075.
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WHAT VALUES ARE MEASURED WITH THE VEML6075
Placing the sensor AND the reference measurement tool (e.g. Davis 6490 UVI sensor) in an open area over the whole day shows
the exact performance of the VEML6075.
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Fig. 14 - Measurement Set-Up
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Below, full-day UVI tracking with the Davis UVI sensor and T148 with no diffusor is shown. The maximum UVI is 9.7.
With the empirical findings of the coefficients and comparable measured UV responsivity, the exact UVI is seen.
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Fig. 15 - Test Data Showing Calculated UVI and Comparison to Reference
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UV COEFFICIENTS AND RESPONSIVITY
UV COEFFICIENTS - EMPIRICAL VALUES
UVA coefficient a
3.33
UVA coefficient b
2.5
UVA coefficient c
3.66
UVA coefficient d
2.75
TYPICAL RESPONSIVITY WITHOUT DIFFUSOR
UVA responsivity
0.0011
UVI/count
909
counts for 1 UVI
UVB responsivity
0.00125
UVI/count
800
counts for 1 UVI
TYPICAL RESPONSIVITY WITH 0.125 mm DIFFUSOR AND 2 mm WALL
UVA responsivity
0.002805
UVI/count
356.5
counts for 1 UVI
UVB responsivity
0.002347
UVI/count
426.1
counts for 1 UVI
For responsivity without a diffusor and IT = 100 ms:
UVA sensing resolution of 0.01 UVI = 9 counts
UVB sensing resolution of 0.01 UVI = 8 counts
MECHANICAL CONSIDERATIONS AND WINDOW CALCULATIONS FOR THE VEML6075
As already mentioned, this UVA / UVB sensor will need a well-selected cover that is not only completely transmissive to visible
light (400 nm to 700 nm), but also to UVA and UVB wavelengths (280 nm to 400 nm).
Teflon or polytetrafluoroethylene (PTFE) is a known optical material that allows transmission of UV up to near infrared signals.
A teflon diffusor (PTFE sheet) radiates like Lambert’s cosine law. Thus PTFE enables a cosine angular response for a detector
measuring the optical radiation power at a surface.
Using a 0.4 mm teflon diffusor placed on top of the VEML6075 sensor generates a very close to cosine view angle response.
Compared with the ideal cosine response, the measured view angle response error of a 0.4 mm teflon diffusor is less than 10 %.
Cosine
1000
UV-V
UV-H
100
Transient Thermal Impedance
Transient Thermal Impedance
10000
10
300
320
340
360
380
400
420
440
460
480
500
520
540
560
580
600
Wavelength (nm)
2nd line
Fig. 16 - Relative Radiant Sensitivity vs. Angular Displacement
For more information please also see:
www.berghof.com/en/products/ptfe-products/optical-ptfe/
www.aetnaplastics.com/site_media/media/documents/Acrylite_OP-4_Material_Data_Sheet.pdf
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Relative Responsivity (μW/cm2)
Relative Responsivity (%)
Average Gain 1
110
100
90
80
70
60
50
40
30
20
10
0
-10
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Designing the VEML6075 into an Application
For optimal performance, the window size should be large enough to maximize the light irradiating the sensor. In calculating the
window size, the only dimensions that the design engineer needs to consider are the distance from the top surface of the sensor
to the outside surface of the window and the size of the window. These dimensions will determine the size of the detection zone.
First, the center of the sensor and center of the window should be aligned. The VEML6075 has an angle of half sensitivity of
about ± 55°, as shown in the figure below.
10000
1.0
0.9
40°
1000
0.8
0.7
100
60°
0.6
2nd line
20°
1stDisplacement
line
ϕ - Angular
2nd line
Transmission
(%)
SLight
Sensitivity
rel - Relative
Axis Title
0°
80°
10
0.5 0.4 0.3 0.2 0.1 0
λ - Wavelength (nm)
2nd line
Fig. 17 - Relative Radiant Sensitivity vs. Angular Displacement
Fig. 18 - Angle of Half Sensitivity: Cone
Fig. 19 - Window Above Sensitive Area
Remark:
This wide angle and the placement of the sensor as close as possible to the cover is needed to show good responsivity.
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Designing the VEML6075 into an Application
The size of the window is simply calculated according to triangular rules. The dimensions of the device, as well as the sensitive
area, are shown within the datasheet. For best results, the distance below the window’s upper surface and the specified angle
below the given window diameter (w) are known.
Dimensions (L x W x H in mm): 2.0 x 1.25 x 1.0
w
x
0.5
.
D
d
tan 55° = 1.43 = x/d
x = 1.43 x d
α
1.0
here in drawing α = 55°
dimensions in mm
Fig. 20 - Window Area for an Opening Angle of ± 55°
The calculation is then: tan α = x/d → with α = 55° and tan 55° 1.43 = x/d → x = 1.43 x d.
Then the total width is w = 0.5 mm + 2 x x.
d = 1.0 mm → x = 1.43 mm → w = 0.5 mm + 2.86 mm = 3.36 mm
d = 1.5 mm → x = 2.15 mm → w = 0.5 mm + 4.30 mm = 4.80 mm
d = 2.0 mm → x = 2.86 mm → w = 0.5 mm + 5.72 mm = 6.22 mm
d = 2.5 mm → x = 3.58 mm → w = 0.5 mm + 7.16 mm = 7.66 mm
d = 3.0 mm → x = 4.29 mm → w = 0.5 mm + 8.58 mm = 9.08 mm
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d = 0.5 mm → x = 0.72 mm → w = 0.5 mm + 1.44 mm = 1.94 mm
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Designing the VEML6075 into an Application
A smaller window will also be sufficient, although it will reduce the total sensitivity of the sensor.
Dimensions (L x W x H in mm): 2.0 x 1.25 x 1.0
w
x
0.5
.
D
d
tan 40° = 0.84 = x/d
x = 0.84 x d
α
1.0
here in drawing α = 40°
dimensions in mm
Fig. 21 - Window Area for an Opening Angle of ± 40°
The calculation is then: tan α = x/d → with α = 40° and tan 40° 0.84 = x/d → x = 0.84 x d.
Then the total width is w = 0.5 mm + 2 x x.
d = 0.5 mm → x = 0.42 mm → w = 0.5 mm + 0.84 mm = 1.34 mm
d = 1.0 mm → x = 0.84 mm → w = 0.5 mm + 1.68 mm = 2.18 mm
d = 2.0 mm → x = 1.68 mm → w = 0.5 mm + 3.36 mm = 3.86 mm
d = 2.5 mm → x = 2.10 mm → w = 0.5 mm + 4.20 mm = 4.70 mm
d = 3.0 mm → x = 2.52 mm → w = 0.5 mm + 5.04 mm = 5.54 mm
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APPLICATION NOTE
d = 1.5 mm → x = 1.28 mm → w = 0.5 mm + 2.56 mm = 3.06 mm
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Designing the VEML6075 into an Application
VEML6075 SENSOR BOARD AND DEMO SOFTWARE
The small blue VEML6075 sensor board fits to the so-called sensor starter kit.
Please also see: www.vishay.com/moreinfo/vcnldemokit/.
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Designing the VEML6075 into an Application
With help of the VEML6075 sensor board and the demo software, one can easily test this UVA / UVB sensor. Beside the raw
data of all five channels, the UV index is also given with the calculations shown before. The five integration times are also
selectable. The resulting counts are strictly linear, meaning a factor of 2 in integration time also results in a factor of 2 in output
data counts.
Fig. 22 - Linearity of Integration Times for Small and High Data Values
Beside the raw data read out of command codes 0x07 and 0x09, the corresponding UV index is also shown, as well as the risk
level indicated with changing the color according to fig. 6.
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Fig. 23 - View of the VEML6075 Demo Software Showing Raw Data, UVI, and Risk Level