ISD2351 Datasheet, PDF (1/4 Pages) OSRAM GmbH – Sunlight Readability Contrast Measurements

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, C₁, 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 C₁
will be much greater than unity (1).
The next ratio, C₂, involves the sterance ratio of the On,
L_LEDon, and Off, L_LEDoff, LED. C₂ 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.
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 ≥ 2.0:1 Minimum
C2—OnLED to OffLED
≥ 2.0:1 Minimum
C3—OffLED to Background ≤ 0.25:1 Maximum
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.
Ratio Equations
1) C₁—OnLED to Background
C₁ = ^L----^L---^E---^D---_L-^o---_Bn----–-----^L---^B--
2) C₂—OnLED to OffLED
C₂ = ^L----^L---^E---^D---_L-^o---_Ln---_E-–--_D---^L_o---^Lf--_f-^E---^D----^o---^f--^f
3) C₃—OffLED to Background
C₃ = --^L----^L--^E----^D---^o--_L-^f--^f-_B----–----^L----^B------
 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 • [phone]
1
May 31, 2000-14