 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

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

will be much greater than unity (1).

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.

Sunlight Readability

Contrast Measurements for

the ISD2351 and ISD2353

Serial Input Small

Alphanumeric Display

Appnote 47

by Bob Krause

Ratio Equations

C

1

L

LEDon

L

B

–

L

B

-------------------------------

=

C

2

L

LEDon

L

LEDoff

–

L

LEDoff

--------------------------------------------

=

C

3

L

LEDoff

L

B

–

L

B

---------------------------------------

=