Back lighting using Argus LEDs

PETER KI%ISTUFEK

Argus LEDs (shown in Figure 1) were developed by Siemens for applications requiring homogeneous light over large areas, since their light emission covers a fairly large solid angle. With this special feature, Argus LEDs further extend the wide range of standard LEDs which are mainly suitable for switching or operating status indication, because their radiation is concentrated in the axial direction.

The benefits of Argus LEDs can be fully utilized if an external reflector and a diffuser are used. In this arrange- ment, an even level of light from areas, symbols and characters is achieved.

DIFFERENCE IS IN THE CONSTRUCTION

Argus diodes are fabricated on the same production line and with the same design concepts as Siemens' standard LEDs. This is also true for the well-known LED chips. As the radiation from standard LEDs is concentrated in the axial direction (Figure 2a), the chip is mounted in a special reflector and molded into an epoxy resin body in the form of a concave lens.

The Argus LED, however, is designed to obtain an enlarged viewing angle (Figure 2b), and this is achieved by a carrier without a reflector and a concave shape of epoxy resin body. To avoid hot spots within the illuminated area, the light emitted in the axial direction is reduced to about 20% of the maximum luminous intensity.

BENEFITS OF ARGUS LEDS RELATIVE TO CONVENTIONAL LIGHT SOURCES

There is no difference between driving standard LEDs and Argus diodes. Commercially available driver stages can be used for the standard Argus LEDs, type designa- tion L. K380-LO.

In addition, LSTTL and CMOS logic outputs are

Siemens A G, Bereich Bauelemente, Vertrieb Lumineszenz- dioden, Miinchen, FRG

Figure 1. Argus LEDs from Siemens

capable of supplying the required current for the low- current versions (L. K389-FO). These can be operated in the forward direction with 7.5 mA at maximum (45 mA for the standard Argus version) and give a luminous flux of typically 2.5 mlm* at 2 mA forward current.

The installation height of the Argus diodes is reduced by more than 1 mm relative to comparable standard LEDs. This and the wide viewing angle allow evenly illuminated areas to be built with low display height.

One assembly of Argus LED, external reflector and diffuser has an installation height similar to that of a standard 7-segment display. Hence panels can be con- structed with mechanically and electro-optically matched

*The unit lumen (lm) indicates that with the Argus LED, the luminous flux is measured while with the standard LED the luminous intensity in candela (cd) in the axial direction is given.

DISPLAYS, APRIL 1 9 8 8 0141--9382/88/020085--03 $03.00 © 1988 Butterworth & Co (Publishers) Ltd 85

Pmat t pml'ile

30 ° 20 ° 10 °

40 ° ~. 1.0

50 °

600 I

70° ~ i 2 800

0.8

\

b 0 o

80°

180 °

0.6 0.4 02 0 20 ° 40 ° 60 ° 80 ° 100o120 ° 1 .8 .8 .4

/ ' \ J \ ] "

20 ° 40" 60 ° 80 ° 100 ° 120'

Figure 2. Viewing angles of (a) conventional LEDs (b) Argus LEDs. The conical emission beam of the standard LED makes it ideal for use as a signalling lamp. Argus LEDs emit their light over an enlarged solid angle. (Both illustrations show polar coordinates on the left and rectangular coordinates on the right)

features. One Argus diode lights an area up to 1 cm 2. Large areas entail cascading a number of diodes.

Siemens provides application notes and customer assistance as required.

ARGUS I.~TI'~S A S SU'J~' I ' I ' I 'U ' I '~dS FOR LAMPS In many cases, light bulbs are easily replaced by Argus diodes. An optimum substitute, however, may require an appropriately shed reflector and a diffuser-type front panel.

The reflector material should have a fairly high diffuse reflection (above 90%). Pocan B7375 and Pocan B7376 thermoplastic polyester have successfully been used in many of the applications described. The outline dimen- sions in Figure 3 are an example of such an arrangement. Requirements differ for the individual applications, and reflectors and diffusers must be matched for optimum results.

14.00 max. ~i

Di f f use r --I-- I ! I I

in mm

Figure 3. Section through a reflector suitable for Argus LEDs

TECHNICAL DESCRIPTION

The Argus LED is similar to the 3-mm standard LED type. For outline dimensions, see Figure 4. Available emission colours, super-red, yellow and green corres- pond to those of the standard version. Maximum ratings

°.8_.= ~=- - -H- -~ . ~ Cathode 0.4 | H ~

H Spaciog measured

0.4

0.40"6 - - -1~ ~__l~-

t

3 mm above package r- base to DIN 41 868

Spacing 2.54

Figure 4. Outline dimensions of Argus LED

86 DISPLAYS, APRIL 1988

I: duct

Table 1. Important limit values and characteristics (typical values) of the Argus LEDs; for both the standard versions and L. K380 and the low-current versions L. K389

LS K380-LO LS K389-FO LY K380-LO LY K389-FO LG K380-LO LG K389-FO super-red yellow green

Operating and storage temperature - 55 to 100*C - 55 to 100*C Forward current 45 mA 7.5 mA 45 mA 7.5 mA Surge current 1 A 25 mA IA 25 mA Power dissipation (0A = 25"C) 150 mW 20 mW 150 mW 20 mW Thermal resistance junction/air 500 K/W 500 K/W Wavelength at peak emission 635 nm 590 nm (IF = 20 mA with L. K380) (Iv = 2 mA with L. K389) Dominant wavelength 628 nm 592 nm (It = 20 mA with L. K380) (IF = 2 mA with L. K389) Luminous flux 20 mlm 2.5 mlm (IF = 10 mA with L. K380) (IF = 2 mA with L. K389) Forward voltage 2.0 V 1.9 V 2.0 V 1.9 V (It = 10 mA with L. K380) (IF = 2 mA with L. K389)

-55 to 100*C 45 mA 7.5 mA

1A 25mA 150 mW 20 mW

500 K/W 565 nm

564 nm

20 mira 2.5 mlm 20 mira 2.5 mlm

2.0 V 1.9 V

and characteristics differ f rom standard and low-current Argus versions (see Table 1). The term standard refers to types with a maximum forward current o f 45 mA (typ. 10 mA). For low-current Argus diodes, the maxi- m u m forward current is 7.5 mA (typ. 2 mA).

BACKGROUND LIGHTING FOR DISPLAY PANELS

Argus LEDs are ideal for background lighting in many applications, as there are virtually no restrictions in display area size and installation height.

Relative to conventional light sources (bulbs), the Argus diodes give benefits such as reduced current con- sumption, essentially longer service life and suitability

for automatic assembly (available on tape). Applications of the Argus diodes have already been found in entertainment electronics. Here they provide illuminated symbols for video equipment and TV sets. They can also be used in car radios and dashboards. Manufac- turers of large electrical equipment benefit f rom the Argus LEDs as they can now build virtually main- tenance-free operator panels, thus relieving maintenance staff from the simple but time consuming work of making replacements.

Another field of application is opened up in measure- ment and control engineering, particularly in centralized control systems. Here, for example, they light up displays in signalling systems and automated production plants to show the status of equipment in a most reliable way for safety purposes.

DISPLAYS, APRIL 1988 87

JOURNAL PUBLISHING Principles and Practice

Gillian Page Robert Campbell

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