Sunlight Readability
Contrast Measurements for
the ISD2351 and ISD2353
Serial Input Small
Alphanumeric Display
Appnote 47
by Bob Krause
Introduction
Light emitting diode alphanumeric displays have had a long and
successful relationship with military and avionic equipment.
Applications with very high light ambients now are possible
because of advances in the efficiency of LEDs. Polarized filters
enhance the readability of LEDs by eliminating much of the ambi-
ent reflection from the LED die and the surrounding package.
Readability Criteria
An observer's ability to perceive the information from an alpha-
numeric display depends on two factors. These include font
and size of the message in relationship to the viewing position
and the optical contrast of the message to the surrounding
environment.
Optical contrast is the comparison of the brightness or sterance
(L) of the On/Off/LED to the brightness of the surrounding envi-
ronment. High readability results by optimizing the following
contrast ratios. The first ratio, C
1
, involves the OnLED stear-
ance, L
LEDon
, and the background stearance, L
B
, is optimized
when the OnLED is brighter than the surrounding area. Thus C
1
will be much greater than unity (1).
The next ratio, C
2
, involves the sterance ratio of the On,
L
LEDon
, and Off, L
LEDoff
, LED. C
2
is optimized when the L
LEDon
is much greater than the L
LEDoff
, resulting in being much
greater than unity (1).
The optimal ratio of C3, involving OffLED to the background, is
achieved when the ratio is near zero. The OffLED should have
minimal contrast with the background.
Ratio Equations
1) C
1
—OnLED to Background
L
LEDon
–
L
B
-
C
1
= ------------------------------
L
B
2) C
2
—OnLED to OffLED
L
LEDon
–
L
LEDoff
C
2
= --------------------------------------------
L
LEDoff
3) C
3
—OffLED to Background
L
LEDoff
–
L
B
C
3
= ---------------------------------------
L
B
The equations for these three ratios are given below.
The U.S. military has established contrast ratios limits to satisfy
sunlight readability criteria. These criteria are published in,
”Night
Vision Goggle Lighting Specification.”
The specifications
are shown below:
C1—OnLED to Background
C2—OnLED to OffLED
C3—OffLED to Background
Optical Filtering
An LED, regardless of its brightness, has a difficult time com-
peting with the sterance of the sun. An LED display’s readabil-
ity can be greatly improved by using contrast enhancement
filters. The filter of choice is one that eliminates the interfer-
ence of the sun with the background of the display. A number
of filter vendors offer anti-reflection coated, circular polarized
(AR/CP), optically tinted bandpass and neutral density filters
which have proven very helpful in satisfying the sunlight read-
ability contrast requirements.
The display front surface and the areas surrounding the LEDs
have specular reflector characteristics. This reflective property
allows optimum contrast when used with a circular polarized fil-
ter with anti-reflective coating. See Table 1 for Filter Selection
Guide.
≥
2.0:1 Minimum
≥
2.0:1 Minimum
≤
0.25:1 Maximum
2000 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA
www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
OSRAM Opto Semiconductors GmbH & Co. OHG • Regensburg, Germany
www.osram-os.com • +49-941-202-7178
1
May 31, 2000-14
Table 1. Contrast enhancement filter selection guide
Display P/N
or Condition
ISD2010
ISD2012
ISD2352*
ISD2310
ISD2312
ISD2011
ISD2351*
ISD2311
ISD2013
ISD2353*
ISD2313
High Ambient Light
High Ambient Light
ISD2010
ISD2012
ISD2352*
ISD2310
ISD2312
ISD2011
ISD2351*
ISD2311
ISD2013
ISD2353*
ISD2313
High Ambient Light
Display Color
Red/
Hi Eff. Red
Transmission
25% at 635 nm
Filter Color
Red
Filter Model No.
Filter Manufacturer
MPC 20-15C
Marks
Yellow
25% at 585 nm
Amber
MPC 30-25C
Marks
MPC 50-22C
Marks
MPC 80-10C
Marks
MPC 80-37C
Marks
HLF-608-5R
Hoya
Hi Eff. Green
22% at 565 nm
Yellow/Green
All Colors
All Colors
Red/
Hi Eff. Red
10% Neutral
37% Neutral
14%
Neutral Gray
Neutral Gray
Reddish
Orange
Yellow
14%
Yellowish
Orange
Yellow/Green
HLF-608-3Y
Hoya
HLF-608-1G
Hoya
HNCP10
Polaroid
Hi Eff. Green
14%
All Colors
10% Neutral
Neutral Gray
* Sunlight viewable displays. All other part numbers represent all the standard Military Small Alphanumeric Displays.
Manufacturers
Marks Polarized Corporation
25B Jefryn Blvd. West
Deer Park, NY 1179-5715
(516) 242-1300
HOYA Optics, Inc.
3400 Edison Way
Fremont, CA 94538-6138
(415) 490-1880
Polaroid Corp.
Polarizer Division
1Upland Road
Norwood, MA 02062
(617) 577-2000
2000 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA
www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
OSRAM Opto Semiconductors GmbH & Co. OHG • Regensburg, Germany
www.osram-os.com • +49-941-202-7178
Appnote 47
2
May 31, 2000-14
Contrast Measurements
The ability to read a display in direct sunlight used to be deter-
mined by using a series of standard observers and irradiating
the display with one sun. Recent technical studies have estab-
lished contrast criteria permitting laboratory measurements
that verify sunlight readability.
A yellow ISD2351 and a high efficiency green ISD2353 Small
Alphanumeric Display were evaluated for sunlight readability
under a simulated sun with an incident of 4200 fc. These two
displays were tested with both pass-band and neutral density
AR/CP filters. The measurement technique and results follow.
Figure 1. Sunlight readability contrast measurement setup
Contrast Measurement Setup
Contrast measurements require the use of a spot photometer,
which measures the luminance of the surface within a specific
spot size. A Photo Research Spectra Pritchard spot photometer
Model 1980B with a Macro Spectar MS-80 Lens was calibrated
and set to read out in foot Lamberts, fL. The display and filter
were mounted on a micro adjustable X/Y/Z stage. This stage,
the spot photometer, and a 500 W Unimat LX80 light source
were mounted on a optical bench. The light source was ori-
ented 30° from the normal of the display. See Figure 1.
2000 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA
www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
OSRAM Opto Semiconductors GmbH & Co. OHG • Regensburg, Germany
www.osram-os.com • +49-941-202-7178
Appnote 47
3
May 31, 2000-14
Figure 2. LEDs selected for measurement
Figure 3. Points for luminous stearance measurement
See Figure 3
for exploded view
La
Lb
Ld
Lc
The sterance measurements were made using the photometer
with an angle setting of 2 minutes resulting in a spot size of
.004 in. The incidence flux was determined by using a Kodak
6080 standard lambertian reflector painted slide.
The X/Y/Z stage supported the display, the AR/CP filter, and the
display drive electronics. Figure 3 shows the asterisk (*) pro-
grammed on the display. Each LED had a duty factor of ON
17.6%. The center of the asterisk was used as the measure-
ment LED. The stage was used to position the display at the
four contrast measurement points as shown in Figure 2.
Contrast Calculations
The data derived from the spot photometer was used to calcu-
late the three contrast ratios, C
1
, C
2
, C
3
. For best accuracy, L
B
was the average of three spot locations. Figure 3 shows these
as L
a
, L
b
, L
c
. L
a
is the substrate sterance between two LED die,
L
b
is the substrate sterance of the area between four LED die;
and L
c
is the sterance of a gold trace connecting the LEDs.
Measurements were made using a yellow ISD2351 and a
Marks MPC80-10C neutral density gray filter. This display had a
typical intensity of 2450 µcd/LED with an average wavelength
of 585 nm.
Measurements were also made using a green ISD2353 with a
Marks MPC50-22C yellow/green bandpass filter and a Marks
MPC80-10C neutral density gray filter. This display had a typi-
cal intensity of 3470 µcd/LED with an average wavelength of
572 nm.
The data and results of the experiment are shown in Table 2.
Conclusion
From the data, the most readable combination is the green
ISD2353 display and a green bandpass AR/CP filter, followed by
the green display and the 10% transmissive neutral density
gray AR/CP filter. In both cases these combinations exceeded
the military limit by almost 2.5 times. The yellow display has
optimum contrast with a neutral density AR/CP gray filter.
Table 2. Luminous contrast at 4200 fc
Display Color
Filter Model #
Status
L
a
Green
MPC50-22C
Green
MPC80-10C
Green
MPC50-22C
Yellow
MPC80-10C
Yellow
No Filter
LED-On
LED-Off
LED-On
LED-Off
LED-On
LED-Off
LED-On
LED-Off
LED-On
LED-Off
12.00
11.00
9.70
8.80
12.60
11.10
9.30
7.80
208.00
198.00
L
b
10.20
9.40
8.60
8.60
11.30
9.80
8.20
7.20
198.00
171.00
L
c
27.10
20.60
17.80
11.40
37.30
17.70
19.40
10.30
650.00
853.00
L
d
101.60
12.60
69.80
10.10
111.00
14.40
46.50
7.70
480.00
171.00
Footlamberts
L
B
16.43
13.67
12.03
9.60
15.30
12.87
12.30
8.43
352.00
407.33
C
1
5.18
4.80
6.25
2.78
0.36
C
2
7.06
5.91
6.71
5.04
1.81
C
3
0.08
0.05
0.12
0.09
0.58
2000 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA
www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
OSRAM Opto Semiconductors GmbH & Co. OHG • Regensburg, Germany
www.osram-os.com • +49-941-202-7178
Appnote 47
4
May 31, 2000-14
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